CHAPTER XXVI.
CHEMISTRY AND ALCHEMY—CHEMICAL COMBINATIONS—THE ATMOSPHERIC AIR.
We have in the foregoing pages given some experiments, and considered several of the metals, but there are numerous very interesting subjects still remaining; indeed, the number is so great that we can only pick and choose. All people are desirous to hear something of the atmosphere, of water, and the earth; and as we proceed to speak of crystals and minerals, and so on to geology, we shall learn a good deal respecting our globe—its conformation and constituents. But the atmospheric air must be treated of first. This will lead us to speak of oxygen and nitrogen. Water will serve to introduce hydrogen with a few experiments, and thus we shall have covered a good deal of ground on our way towards various other elements in daily use and appreciation. Now let us begin with a few words concerning Chemistry itself.
At the very outset we are obliged to grope in the dark after the origin of this fascinating science. Shem, or “Chem,” the son of Noah, has been credited with its introduction, and, at any rate, magicians were in Egypt in the time of Moses, and the lawgiver is stated by ancient writers to have gained his knowledge from the Egyptians. But we need not pursue that line of argument. In more modern times the search for the Philosopher’s Stone and the Elixir of Life, which respectively turned everything to gold, and bestowed long life upon the fortunate finder, occupied many people, who in their researches no doubt discovered the germs of the popular science of Chemistry in Alchemy, while the pursuit took a firm hold of the popular imagination for centuries; and even now chemistry is the most favoured science, because of its adaptability to all minds, for it holds plain and simple truths for our every-day experience to confirm, while it leads us step by step into the infinite, pleasing us with experiments as we proceed.
Alchemy was practised by numerous quacks in ancient times and the Middle Ages, but all its professors were not quacks. Astrology and alchemy were associated by the Arabians. Geber was a philosopher who devoted himself entirely to alchemy, and who lived in the year 730 A.D. He fancied gold would cure all disease, and he did actually discover corrosive sublimate, nitric acid, and nitrate of silver. To give even a list of the noted alchemists and magicians would fill too much space. Raymond Sully, Paracelsus, Friar Bacon, Albertus Magnus, Thomas Aquinas, Flamel, Bernard of Treves, Doctor Dee, with his assistant Kelly, and in later times Jean Delisle, and Joseph Balsamo (Cagliostro), who was one of the most notorious persons in Europe about one hundred years ago (1765-1789), are names taken at random; and with the older philosophers chemistry was an all-absorbing occupation—not for gold, but knowledge.
The revelation was slow. On the temperature of bodies the old arts of healing were based—for chemistry and medicine were allies. The elements, we read, existed on the supposition “that bodies were hot or cold, dry or moist”; and on this distinction for a long time “was based the practice of medicine.” The doctrine of the “three principles” of existence superseded this,—the principles being salt, mercury, and sulphur. Metals had been regarded as living bodies, gases as souls or spirits. The idea remained that the form of the substance gave it its character. Acid was pointed; sweet things were round.
Chemistry, then, has had a great deal to contend against. From the time of the Egyptians and Chinese, who were evidently acquainted with various processes,—dyeing, etc.,—the science filtered through the alchemists to Beecher and Stahl, and then the principle of affinity—a disposition to combine—was promulgated, supplemented in 1674 by Mayow, by the theory of divorce or analysis. He concluded that where union could be effected, separation was equally possible. In 1718 the first “Table of Affinities” was produced. Affinity had been shown to be elective, for Mayow pointed out that fixed salts chose one acid rather than another. Richter and Dalton made great advances. Before them Hales, Black, Priestley, Scheele, Lavoisier, and numerous others penetrated the mysteries of the science whose history has been pleasantly written by more than one author who we have not been able to consult, and have no space to do more than indicate. In later days Faraday, De la Rive, Roscoe, and many others have rendered chemistry much more popular, while they have added to its treasures. The story of the progress of chemistry would fill a large volume, and we have regretfully to put aside the introduction and pass on.
Before proceeding to investigate the elements, a few words concerning the general terms used in chemistry will be beneficial to the reader. If we look at the list of the elements, pp. 308-9, we shall see various terminations. Some are apparently named from places, some from their characteristics. Metals lately discovered by the spectroscope (and recently) end in ium; some end in “ine,” some in “on.” As far as possible in late years a certain system of nomenclature has been adhered to, but the old popular names have not been interfered with.
When elements combine together in certain proportions of each they receive certain names. The following table will explain the terms used ; for instance, we find that—
| Compounds of | Oxygen | are termed | Oxides, as oxide of copper. |
| ” | Hydrogen | ” | Hydrides, as hydride of potassium. |
| ” | Chlorine | ” | Chlorides, as chloride of sodium. |
| ” | Nitrogen | ” | Nitrides, as nitride of boron. |
| ” | Bromine | ” | Bromides, as bromide of potassium. |
| ” | Iodine | ” | Iodides, as iodide of potassium. |
| ” | Sulphur | ” | Sulphides, or Sulphurets, as sulphuret of lead. |
| ” | Selenium | ” | Selenides, as selenide of mercury. |
| ” | Carbon | ” | Carbides, or Carburets, as carbide of nitrogen, and so on. |
The above examples refer to the union in single proportion of each, and are called Binary Compounds. When more than one atom of each element exists in different proportions we have different terms to express such union. If one atom of oxygen be in the compound it is called a “monoxide” or “protoxide”; two atoms of oxygen in combination is termed “dioxide” or “binoxide”; three, “trioxide,” or “tritoxide”; four is the “tetroxide” or “per-oxide,” etc. When more than one atom, but not two atoms is involved, we speak of the sesqui-oxide (one-and-a-half),—“oxide” being interchangeable for “sulphide” or “chloride,” according to the element.
There are other distinctions adopted when metals form two series of combinations, such as ous and ic, which apply, as will be seen, to acids. Sulphuric and sulphurous acids, nitric and nitrous acid are familiar examples. In these cases we shall find that in the acids ending in “ous” oxygen is present in less quantity than in the acids ending in ic. The symbolic form will prove this directly, the number of atoms of oxygen being written below,
Sulphurous Acid = H2 SO3
Nitrous Acid = HNO2.
Sulphuric Acid = H2SO4.
Nitric Acid = HNO3.
Whenever a stronger compound of oxygen is discovered than that denominated by ic, chemists adopt the plan of dubbing it the per (ὑπέρ over), as per-chloric acid, which possesses four atoms of oxygen (HClO4), chloric acid being HClO3. The opposite Greek term, ὑπὸ (hupo, below), is used for an acid with less than two atoms of oxygen, and in books is written “hypo”-chlorous (for instance). Care has been taken to distinguish between the higher and lower; for “hyper” is used in English to denote excess, as hyper-critical; and hypo might to a reader unacquainted with the derivation convey just the opposite meaning to what is intended.
While speaking of these terminations we may show how these distinctive endings are carried out. We shall find, if we pursue the subject, that when wehave a salt of any acid ending in ic the salt terminates in “ate.” Similarly the salts of acids ending in ous, end in “ite.” To continue the same example we have—
Sulphurous Acid, which forms salts called Sulphites.
Sulphuric Acid, which forms salts called Sulphates.
Besides these are sulphides, which are results of the unions or compounds of elementary bodies. Sulphites are more complicated unions of the compounds. Sulphates are the salts formed by the union of sulphuric acid with bases. Sulphides or sulphurets are compounds in which sulphur forms the electro-negative element, and sulphites are salts formed by the union of sulphurous acids with bases, or by their action upon them.
Fig. 322.—Combinations of elements.
The symbolical nomenclature of the chemist is worse than Greek to the uninitiated. We frequently see in so-called popular chemical books a number of hieroglyphics and combinations of letters with figures very difficult to decipher, much less to interpret. These symbols take the place of the names of the chemical compounds. Thus water is made up of oxygen and hydrogen in certain proportions; that is, two of hydrogen to one of oxygen. The symbolic reading is simple, H2O, = the oxide of hydrogen. Potassium again mingles with oxygen. Potassium is K in our list; KO is oxide of potassium (potash). Let us look into this a little closer.
The union of one particle of a simple body with a particle of another simple body can be easily understood; but, as we have seen, it is possible to have substances consisting of four or five different particles, though the greater number of chemical combinations consist of two or three dissimilar ones. In the diagram (fig. 322) we have some possible combinations.
In these combinations we may have one particle of a in combination with one, two, three, four, or five of b, and many particles of a can unite with various molecules of b. Suppose we have oxygen and sulphur compounds as follows:—
Thus there are three different compounds of these two elements—SO, SO2, SO3 (without water).
Fig. 323.-(1) Hydrosulphurous Acid. (2) Sulphurous Acid. (3) Sulphuric Acid.
A compound body may combine with another compound body, and this makes a complicated compound. Suppose we have a mixture of sulphuric acid and potash. We have a sulphate of potassium (K2SO4) and combinations of these combinations may likewise be formed. We must read these symbols by the light of the combining weights given in the table, and then we shall find the weight of oxygen or other elements in combination. Thus when we see a certain symbol (Hg.S for instance), we understand that they form a compound including so many parts of mercury and so many of sulphur, which is known as vermilion. Hg.O is oxide of mercury, and by reference to the table of Atomic Weights, we find mercury is Hg., and its combining weight is 200; while oxygen is O, and its weight is 16. Thus we see at once how much of each element is contained in oxide of mercury, and this proportion never varies; there must be 200 of one and 16 of the other, by weight, to produce the oxide. So if the oxygen has to be separated from it, the sum of 216 parts must be taken to procure the 16 parts of oxygen. When we see, as above, O2 or O3, we know that the weight must be calculated twice or three times, O being 16; O2 is therefore 32 parts by weight. So when we have found what the compounds consist of, we can write them symbolically with ease.
Composition of the Atmospheric Air.
We have already communicated a variety of facts concerning the air. We have seen that it possesses pressure and weight. We call the gaseous envelope of the earth the atmosphere, and we are justified in concluding that other planets possess an atmosphere also, though of a different nature to ours. We have seen how easy it is to weigh the air, but we may repeat the experiment. (See illustration, fig. 45, [page 50].) We shall find that a perfectly empty glass globe will balance the weights in the scale-pan; admit the air, and the glass globe will sink. So air possesses weight. We have mentioned the Magdeburg hemispheres, the barometer, the air-pump, and the height and the pressure of the atmosphere have been indicated. The density of the atmosphere decreases as we ascend; for the first seven miles the density diminishes one-fourth that of the air at the sea-level, and so on for every succeeding seven.
In consequence of the equal, if enormous, pressure exercised in every direction, we do not perceive the inconvenience, but if the air were removed from inside of a drum, the parchment would quickly collapse. We feel the air when we move rapidly. We breathe the air, and that statement brings us to consider the composition of the atmosphere, which, chemically speaking, may vary a little (as compared with the whole mass) in consequence of changes which are continually taking place, but to all intents and purposes the air is composed as follows, in 100 parts:
| Nitrogen | 79 parts. |
| Oxygen | 20 parts. |
| Carbonic Acid | 04 parts. |
with minute quantities of other ingredients, such as ammonia, iodine, carbonetted hydrogen, hydrochloric acid, sulphuretted hydrogen, nitric acid, carbonic oxide, and dust particles, as visible in the sunbeams, added.
The true composition of the atmosphere was not known till Lavoisier demonstrated that it consisted of two gases, one of which was the vital fluid, or oxygen, discovered by Priestley. To the other gas Lavoisier gave the name of Azote,—an enemy of life,—because it caused death if inhaled alone. The carbonic acid in the air varies very much, and in close, heated, and crowded rooms increases to a large quantity, which causes lassitude and headache.
We can easily prove the existence of carbonic acid gas as exhaled from the lungs. Suppose we take a glass and fill it partly with clear lime-water; breathe through a glass tube into the water in the glass, and very quickly you will perceive that the lime-water is becoming cloudy and turbid. This cloudiness is due to the presence of chalk, which has been produced by the action of the carbonic acid gas in the lime-water. This is a well known and always interesting experiment, because it leads up to the vital question of our existence, and the functions of breathing and living.
A popular writer once wrote a book entitled, “Is Life Worth Living?” and a witty commentator replied to the implied question by saying, “It depends upon the liver.” This was felt to be true by many people who suffer, but the scientific man will go farther, and tell you it depends upon the air you breathe, and on the carbonic acid you can raise to create heat,—animal heat,—which is so essential to our well-being. We are always burning; a furnace is within us, never ceasing to burn without visible combustion. We are generating heat by means of the blood. We know that we inhale air into the lungs, and probably are aware that the air so received parts with the oxygen to renew the blood. The nitrogen dilutes the oxygen, for if we inhaled a less-mixed air we should either be burnt up or become lunatics, as light-headed as when inhaling “laughing-gas.” This beautifully graduated mixture is taken into our bodies, the oxygen renews the blood and gives it its bright red colour; the carbon which exists in all our bodies is cold and dead when not so vivified by oxygen. The carbonic acid given off produces heat, and our bodies are warm. But when the action ceases we become cold, we die away, and cease to live. Man’s life exemplifies a taper burning; the carbon waste is consumed as the wax is, and when the candle burns away—it dies! It is a beautiful study, full of suggestiveness to all who care to study the great facts of Nature, which works by the same means in all matter. We will refer to plants presently, after having proved by experiment the existence of nitrogen in the air.
Rutherford experimented very cruelly upon a bird, which he placed beneath a glass shade, and there let it remain in the carbonic acid exhaled from its lungs, till the oxygen being at length all consumed by the bird, it died. When the atmosphere had been chemically purified by a solution of caustic potash, another bird was introduced, but though it lived for some time, it did not exist so long as the first. Again the air was deprived of the carbonic acid, and a third bird was introduced. The experiment was thus repeated, till at length a bird was placed beneath the receiver, and it perished at once. This is at once a cruel and clumsy method of making an experiment, which can be more pleasantly and satisfactorily practised by burning some substance in the air beneath the glass. Phosphorus, having a great affinity for oxygen, is usually chosen. The experiment can be performed as follows with a taper, but the phosphorus is a better exponent.
Let us take a shallow basin with some water in it, a cork or small plate floating upon the water, and in the plate a piece of phosphorus. We must be careful how we handle phosphorus, for it has a habit, well known, but sometimes forgotten by amateur chemists, of suddenly taking fire. Light this piece of phosphorus,—a small piece will do if the jar be of average “shade” size,—and place the glass over it, as in the illustration (fig. 325). The smoke will quickly spread in the jar, and the entry of air being prevented, because the jar is resting under water, phosphoric acid will be formed, and the oxygen thereby consumed. The water, meanwhile, will rise in the jar, the pressure of the air being removed. The burning phosphorus will soon go out, and when the glass is cool, you will be able to ascertain what is inside the jar. Put a lighted taper underneath, and it will go out. The taper would not go out before the phosphorus was burnt in the glass, and so now we perceive we have azote in the receptacle—that is, nitrogen. The other, the constituent of our atmosphere, carbonic acid, as we have seen, is very injurious to the life of animals, and as every animal breathes it out into the air, what becomes of it? Where does all this enormous volume of carbonic acid, the quantities of this poison which are daily and nightly exhaled, where do they all go to? We may be sure nature has provided for the safe disposal of it all. Not only because we live and move about still,—and of course that is a proof,—but because nature always has a compensating law. Remember nothing is wasted; not even the refuse, poisonous air we get rid of from our lungs. Where does it go?
Fig. 324.—Rutherford’s experiment.
It goes to nourish the plants and trees and vegetables that we delight to look upon and to eat the fruit of. Thus the vegetable world forms a link between the animals and the minerals. Vegetables obtain food, so to speak, and nourishment from water, ammonia, and carbonic acid, all compound bodies, but inorganic.
Water consists of oxygen and hydrogen, carbonic acid of carbon and oxygen, and ammonia of hydrogen and nitrogen. Water and ammonia are present in the air; so are oxygen and nitrogen. Water falls in the form of rain, dew, etc. So in the atmosphere around us we find nearly every necessary for plant-life; and in the ground, which supplies some metallic oxides for their use, we find the remainder. From the air, then, the plant derives its life.
Fig. 325—Drawing the oxygen from air by combustion.
The vegetable kingdom in turn gives all animals their food. This you will see at a glance is true. Certainly animals live on animals. Man and wilder animals live on the beasts of the field in a measure, but those beasts derive their nourishment from vegetables—the vegetable kingdom. So we live on the vegetable kingdom, and it separates the carbonic acid from the air, and absorbs it. What we do not want it takes. What we want it gives. Vegetables give out oxygen, and we consume it gladly. We throw away carbonic acid, and the plants take it greedily; and thus the atmosphere is retained pure for our use. We can, if desirable, prove that plants absorb carbonic acid and give out oxygen by placing leaves of a plant in water, holding the acid in solution, and let the sun shine upon them. Before long we shall find that the carbonic acid has disappeared, and that oxygen has come into the water.
Carbonic acid is sufficiently heavy to be poured from one vessel to another; and if we have obtained some in a glass, we can extinguish a taper by pouring the invisible gas on to the lighted taper, when it will be immediately extinguished.
From the foregoing observations it will be perceived how very desirable it is that ventilation should be attended to. People close up windows and doors and fireplaces, and go to bed and sleep. In the morning they complain of headache and lassitude; they wonder what is the matter, and why the children are not well. Simply because they have been rebreathing the carbonic acid. Go into a closed railway carriage which is nearly filled (and it is astonishing to us how people can be so foolish as to close every outlet), and you will recoil in disgust. These travellers shut the ventilators and windows “because of the cold.” A very small aperture will ventilate a railway carriage; but a close carriage is sickening and enervating, as these kind of travellers find out by the time they reach their journey’s end. Air was given us to breathe at night as well as by day; and though from man’s acts or omissions there may be circumstances in which “night” air may affect the health, we maintain that air is no more injurious naturally than “day” air. Colder it may be, but any air at night is “night” air, in or out of doors at night; and we are certain that night air in itself never hurt any healthy person. It is not nature’s plan to destroy, but to save. If a person delicate in constitution gets hot, and comes out into a colder atmosphere, and defy nature in that way, he (or she) must take the consequences. But air and ventilation (not draught) are necessaries of health, and to say they injure is to accuse nature falsely. There are many impurities in the air in cities, and in country places sometimes, but such impurities are owing to man’s acts and omissions. With average sanitary arrangements and appliances in a neighbourhood no one need be afraid to breathe fresh air night or day; and while many invalids have, we believe, been retarded in recovery from being kept in a close room, hundreds will be benefited by plenty of fresh air. We should not so insist upon these plain and simple truths were there not so many individuals who think it beneficial to close up every avenue by which air can enter, and who then feel ill and out of spirits, blaming everything but their own short-sightedness for the effect of their own acts. An inch or two of a window may be open at night in a room, as the chimney register should be always fully up in bedrooms. When there are fires the draught supplies fresh air to the room with sufficient rapidity. But many seaside journeys might be avoided if fresh air were insisted on at home.
There is another and an important constituent of the atmosphere called Ozone, which is very superior oxygen, or oxygen in what is termed the “Allotropic” state, and is distantly related to electricity, inasmuch as it can be produced by an electrical discharge. This partly accounts for the freshness in the air after a thunderstorm, for we are all conscious that the storm has “cleared the air.” The fresh, crisp ozone in the atmosphere is evident. Ozone differs from oxygen in possessing taste and smell, and it is heavier by one-half than the oxygen gas. There is a good deal of ozone in the sea breeze, and we can, though not infallibly, detect its presence by test-paper prepared with iodide of potassium, which, when ozone is present, will turn blue. We have still something to learn about ozone, which may be considered as “condensed oxygen.”
Fig. 326. Development of gas by combustion. Fig. 327.
We have frequently mentioned “combustion,” and as under ordinary circumstances such effects cannot take place without atmospheric air, we will consider it. Combustion is chemical action accompanied by light and heat. Chemical union is always attended by the development of heat, not always by light, because the union varies in intensity and quickness. But when a candle is burning we can study all the interesting phenomena of combustion. We have already spoken of Heat and Light, so we need only refer the readers to those subjects in the former parts of this volume. Heat is referable to chemical action, and varies according to the energy of union. Heat is always present, remember, in a greater or less degree; and when visible combustion takes place we see light. Invisible combustion goes on in our bodies, and we feel heat; when we get cold we feed the fire by eating, or blow it by exercise and air in our lungs.
Fig. 328.—Gas evolved from flame.
We shall speak, however, of combustion now as it affects us in daily life; our fires, our candles, gas, etc., and under these ordinary circumstances hydrogen and carbon are present. (We shall hear more about carbon presently.) These unite with the oxygen to form water and carbonic acid; the water being visible as we first put the cold shade upon the lighted lamp, and the carbonic acid renders the air impure.
In the case of a common candle, or lamp, combustion takes place in the same way. The wick is the intermediary. The oil mounts in the lamp wick, where it is converted into a gas by heat; it then “takes fire,” and gives us light and heat. The candle-flame is just the same with one exception: the burning material is solid, not liquid, though the difference is only apparent, for the wax is melted and goes up as gas. The burning part of the wick has a centre where there is no combustion, and contains carbon. We can prove this by placing a bent tube, as in the illustration (fig. 326), one end in the unburning part of the flame. We shall soon see a dark vapour come over into the receiver. This is combustible, for if we raise the tube without the glass we can light the gas (fig. 327). If we insert the end of the tube into the brilliant portion of the flame we shall perceive a black vapour, which will extinguish the combustion, for it is a mixture of carbonic acid gas and aqueous vapour, in which (fig. 328) particles of carbon are floating.
Fig. 329.—Davy’s safety lamp.
Fig. 330.—Davy lamp (section).
When we proceed to light our lamps to read or to write by, we find some difficulty in making the wick burn at first. We present to it a lighted taper, and it has no immediate effect. Here we have oil and cotton, two things which would speedily set a warehouse in flames from top to bottom, but we cannot even ignite them, try all we can. Why?—Because we must first obtain a gas, oil will not burn liquid; it must be heated to a gaseous point before it will burn, as all combustion depends upon that,—so flames mount high in air. Now in a candle-flame, as will be seen in the diagram (fig. 331.), there are three portions,—the inner dark core, which consists of unburnt gas; the outer flame, which gives light; and the outside rim of perfect combustion non-luminous. In the centre, A, there is no heat. If we place a piece of gauze wire over the flame at a little distance the flame will not penetrate it. It will remain underneath, because the wire, being of metal, quickly absorbs the heat, and consequently there is no flame. This idea led to the invention of the “safety” lamp by Sir Humphrey Davy, which, although it is not infallible, is the only lamp in general use in mines (figs. 329, 330).
Fig. 331.—Construction of a candle flame.
Mines must have light, but there is a gas in mines, a “marsh” gas, which becomes very explosive when it mixes with oxygen. Of course the gas will be harmless till it meets oxygen, but, in its efforts to meet, it explodes the moment the union takes place; instead of burning slowly like a candle it goes off all at once. This gas, called “fire damp,” is carburetted hydrogen, and when it explodes it develops into carbonic acid gas, which suffocates the miners.
Fig. 332.—Pouring carbonic acid on a lighted taper.
CHAPTER XXVII.
NON-METALLIC ELEMENTS.
OXYGEN—SYMBOL O; ATOMIC WEIGHT 16.
Oxygen is certainly the most abundant element in nature. It exists all around us, and the animal and vegetable worlds are dependent upon it. It constitutes in combination about one-half of the crust of the earth, and composes eight-ninths of its weight of water. It is a gas without taste or colour. Oxygen was discovered by Priestley and Scheele, in 1774, independently of each other.
Fig. 333.—Oxygen from oxide of mercury.
Oxygen can be procured from the oxides of the metals, particularly from gold, silver, and platinum. The noble metals are reducible from their oxides by heat, and this fact assists us at once. If we heat chlorate of potash, mixed with binoxide of manganese, in a retort in a furnace, the gas will be given off. There are many other ways of obtaining oxygen, and we illustrate two (figs. 333, 335).
The red oxide of mercury will very readily evolve oxygen, and if we heat a small quantity of the compound in a retort as per illustration (fig. 333) we shall get the gas. In a basin of water we place a tube test-glass, and the gas from the retort will pass over and collect in the test tube, driving out the water.
Fig. 334.—Showing retort placed in furnace.
The other method mentioned above,—viz., by heating chlorate of potash, etc., in a furnace, is shown in the following illustration. Oxygen, as we have said, is a colourless and inodorous gas, and for a long time it could not be obtained in any other form; but lately both oxygen and hydrogen have been liquified under tremendous pressure at a very low temperature. Oxygen causes any red-hot substance plunged into it to burn brightly; a match will readily inflame if a spark be remaining, while phosphorus is exceedingly brilliant, and these appearances, with many others equally striking, are caused by the affinity for those substances possessed by the gas. Combustion is merely oxidation, just as the process of rusting is, only in the latter case the action is so slow that no sensible heat is produced. But when an aggregate of slowly oxidising masses are heaped together, heat is generated, and at length bursts into flame. This phenomenon is called “spontaneous combustion.” Cases have been known in which the gases developed in the human body by the abuse of alcoholic drinks have ended fatally; in like manner the body being completely charred. (Combustion must not be confounded with ignition, as in the electric light.) Oxygen then, we see, is a great supporter of combustion, though not a combustible itself as coal is. When the chemical union of oxygen with another substance is very rapid an explosion takes place.
Fig. 335.—The generation of oxygen from oxide of manganese and potash.
Oxidation occurs in various ways. Besides those already mentioned, all verdigris produced on copper, all decays of whatever kind, disintegration, and respiration, are the effects of oxygen. The following experiment for the extraction of oxygen directly from the air was made by M. Boussingault, who passed the gas upon a substance at a certain temperature, and released it at a higher. The illustration on page 351 will show the way in which the experiment was performed.
Boussingault permitted a thin stream of water to flow into a large empty flask, and by this water the air was gradually driven out into a flask containing chloride of calcium and sulphuric acid, which effectually dried it. This dry air then passed into a large tube inside the reverberatory furnace, in which tube were pieces of caustic baryta. Heated to a dull redness this absorbs oxygen, and when the heat is increased to a bright red the superabundant gas is given off. Thus the oxygen was permitted to pass from the furnace-tube into the receiving glass, and so pure oxygen was obtained from the air which had been in the glass bottle at first (fig. 338).
Fig. 336.—Phosphorus burning in oxygen.
HYDROGEN—SYMBOL H; ATOMIC WEIGHT 1.
Hydrogen is abundant in nature, but never free. United with oxygen it forms water, hence its name, “water-former.” It is to Parcelcus that its discovery is due, for he found that oil of vitriol in contact with iron disengaged a gas which was a constituent of water. This gas was subsequently found to be inflammable, but it is to Cavendish that the real explanation of hydrogen is owing. He explained his views in 1766.
Hydrogen is obtained in the manner illustrated in the cut, by means of a furnace, as in fig. 339, or by the bottle method, as per fig. 340. The first method is less convenient than the second. A gun-barrel or fire-proof tube is passed through the furnace, and filled with iron nails or filings; a delivery tube is at the farther end, and a flask of water boiling at the other. The oxygen combines with the iron in the tube, and the hydrogen passes over. The second method is easily arranged. A flask, as in the cut, is provided, and in it some zinc shavings are put. Diluted sulphuric acid is then poured upon the metal. Sulphate of zinc is formed in the flask, and the hydrogen passes off.
Hydrogen being the lightest of all known bodies, its weight is put as 1, and thus we are relatively with it enabled to write down the weights of all the other elements. Hydrogen is fourteen-and-a-half times lighter than atmospheric air, and would do admirably for the inflation of balloons were it not so expensive to procure in such large quantities as would be necessary. Ordinary coal gas, however, contains a great deal of hydrogen, and answers the same purpose.
Fig. 337.—Magnesium wire burning in oxygen.
A very pretty experiment may be made with a bladder full of hydrogen gas. If a tube be fitted to the bladder already provided with a stop-cock, and a basin of ordinary soap-suds be at hand, by dipping the end of the tube in the solution and gently expressing the gas, bubbles will be formed which are of exceeding lightness (fig. 341). They can also be fired with a taper.
Another experiment may be made with hydrogen as follows:—If we permit the gas to escape from the flask, and light it, as in the illustration, and put a glass over it, we shall obtain a musical note, higher or lower, according to the length, breadth, and thickness of the open glass-tube (fig. 342). If a number of different tubes be employed, we can obtain a musical instrument—a gas harmonium.
Fig. 338.—Extraction of oxygen from air.
Hydrogen burns with a blue flame, and is very inflammable. Even water sprinkled upon a fire will increase its fierceness, because the hydrogen burns with great heat, and the oxygen is liberated. Being very light, H can be transferred from one vessel to another if both be held upside down. Some mixtures of H and O are very explosive. The oxyhydrogen blow-pipe is used with a mixture of O and H, which is forcibly blown through a tube and then ignited. The flame thus produced has a most intense heating-power.
A very easy method of producing hydrogen is to put a piece of sodium into an inverted cylinder full of water, standing in a basin of water. The sodium liberates the hydrogen by removing the oxygen from the liquid.
WATER—SYMBOL H2O; ATOMIC WEIGHT 18.
At page 59 of this volume we said something about water, and remarked (as we have since perceived by experiment) that “water is composed of oxygen and hydrogen in proportions, by weight, of eight of the former to one of the latter gas; in volume, hydrogen is two to one”; and we saw that “volume and weight were very different things.” This we will do well to bear in mind, and that, to quote Professor Roscoe, “Water is always made up of sixteen parts of oxygen to two parts of hydrogen by weight”; sixteen and two being eighteen, the combining weight of water is eighteen.
Fig. 339.—Preparation of hydrogen with furnace.
Fig. 340.—Apparatus for generating hydrogen by flask.
We can prove by the Eudiometer that hydrogen when burnt with oxygen forms water; and here we must remark that water is not a mere mechanical mixture of gases, as air is. Water is the product of chemical combination, and as we have before said, is really an oxide of hydrogen, and therefore combustion, or electricity, must be called to our assistance before we can form water, which is the result of an explosion, the mixture meeting with an ignited body—the aqueous vapour being expanded by heat.
The ancients supposed water to be a simple body, but Lavoisier and Cavendish demonstrated its true character. Pure water, at ordinary temperatures, is devoid of taste and smell, and is a transparent, nearly colourless, liquid. When viewed in masses it is blue, as visible in a marked degree in the Rhone and Rhine, at Geneva, and Bâle respectively. Its specific gravity is 1, and it is taken as the standard for Sp. Gravity, as hydrogen is taken as the standard for Atomic Weight. The uses of water and the very important part it plays in the arrangements of nature as a mechanical agent, geology can attest, and meteorology confirm. It composes the greater portions of animals and plants; without water the world would be a desert—a dead planet.
Fig. 341.—Blowing bubbles with hydrogen gas.
We sometimes speak of “pure” spring water, but such a fluid absolutely pure can scarcely be obtained; and though we can filter water there will always remain some foreign substance or substances in solution. It is well known that the action of water wears away and rounds off hard rocks, and this power of disintegration is supplemented by its strength as a solvent, which is very great. Rain-water is purest in the country as it falls from the clouds. In smoky towns it becomes sooty and dirty. It is owing to the solvent properties of water, therefore, that we have such difficulty in obtaining a pure supply. There is hard water and soft water. The former is derived from the calcareous formations, and contains lime, like the Kent water. This can be ascertained by noticing the incrustations of the vessels wherein the water is boiled. But water rising from hard rocks, such as granite, can do little to disintegrate them at the moment, and therefore the water rises purer. Springs from a great depth are warm, and are known as “thermal springs”; and when they come in contact with carbonic acid and some salts in their passage to the surface, they are known as “mineral waters.” These waters hold in solution salts of lime and magnesia, or carbonates of soda with those of lime and magnesia; salts of iron, and compounds of iodine and bromine are found in the natural mineral waters also, as well as sulphurous impregnations, instances of which will occur to every reader.
Fig. 342.—Experiment with hydrogen.
Fig. 343.—The composition of water.
We mentioned the Eudiometer just now, and we give an illustration of it. This instrument is used to ascertain the proportions in which the elements of water are composed by synthesis, or a putting together of the constituents of a body to make it up. This is distinguished from analysis, which means separating the compound body into its elements, as we do when we pass the electric current through water.
The Eudiometer consists of a stout glass tube sealed hermetically at one end; two platinum wires are pushed in through the glass just before the end is sealed. The tube is now filled with mercury, and inverted in a bowl of the same metal. Hydrogen, and then oxygen, are admitted through the mercury in the recognised proportions of two to one. By the time the mercury is somewhat more than half displaced, the tube should be held upon a sheet of india-rubber at the bottom of the vessel to keep the metal in the tube, for when the necessary explosion takes place the mercury might also be driven out. A spark from the electrophorus or from a Leyden jar may now be passed through the gases in the tube. The explosion occurs, and water is formed inside. If the mercury be again admitted it will rise nearly to the very top of the tube, driving the bubble up. Thus we find we have formed water from the two gases.
The decomposition of water is easily affected by electricity, and if a little sulphuric acid be added to the water, the experiment will be thereby facilitated. Two wires from a battery should be inserted through a glass filled with the water, and into two test tubes also filled. The wires terminate in platinum strips, and are fastened at the other end to the positive and negative poles of the galvanic battery. The gases will collect in the test tubes, and will be found in proper proportions when the current passes.
Fig. 344.—The Eudiometer.
Fig. 345.—Decomposition of water.
So much for water in its liquid state. The solid condition of water (ice) is equally interesting. When we apply heat to water, we get a vapour called “steam”; when we cool water to 32° Fahr., we get a solid mass which weighs just the same as the liquid we have congealed, or the steam we have raised from an equal amount of water. But water expands while in the process of solidification, just as it does when it becomes gaseous, and as we have remarked before, our water-pipes bear full testimony to this scientific fact. When ice forms it has a tendency to crystallize, and some of these ice crystals are, as we see, very beautiful. Snow is only water in a nearly solid form, and the crystals are extremely elegant, appearing more like flowers than congealed water, in tiny six-pointed ice crystals. Many philosophers of late years have written concerning these tiny crystals, which, in common with all crystals, have their own certain form, from which they never depart. Snowflakes are regular six-sided prisms grouped around a centre forming angles of 60° and 120°. There are a number of forms, as will be seen from the accompanying illustrations, and at least ninety-six varieties have been observed. One snowflake, apparently so like all other flakes that fall, can thus be viewed with much interest, and yet, while so very various, snowflakes never get away from their proper hexagonal structure. It has been remarked that snowflakes falling at the same time have generally the same form.
Of the latent heat of ice, etc., we have already spoken in our article upon Heat, and therefore it will be sufficient to state that the latent heat of water is 79 thermal units, because when passing from the liquid to the solid state a certain amount of water absorbs sufficient heat to raise an equal quantity of the liquid 79°. This can be proved by taking a measured quantity (say a pint) of water at 79° and adding ice of the same weight to the water. The mixture will be found to be at zero. Therefore the ice has absorbed or rendered latent 79° of heat which the water possessed. If we melt ice until only a trace of it is left, we shall still find the water as cold as the ice was; all the latent heat is employed in melting the ice. So it will take as much heat to bring a pound of ice at zero to a pound of water at zero, as it would to raise 79 pounds of water 1°. The same law applies to steam.
Fig. 346.—Snow crystals.
Water can be distilled in small quantities by an apparatus, as figured in the illustration, and by these means we get rid of all impurities which are inseparable from the liquid otherwise. When it is desirable to distil large quantities of water a larger apparatus is used, called an “Alembic.” The principle is simply to convert the liquid by heat into vapour, then cool it, by condensation, in another vessel.
Fig. 347.—Distilling water.
The evaporation of water, with its effects upon our globe, belong more to the study of Meteorology.
Fig. 348.—Distillation.
Rain-water is the purest, as we have said, because it goes through the process of distillation by nature. The sun takes it up, by evaporation, into the air, where it is condensed, and falls as rain-water. Water containing carbonate of lime will petrify or harden, as in stalactite caverns. The carbonic acid escapes from the dripping water, the carbonate in solution is deposited as a stalactite, and finally forms pillars in the cave. Sea-water contains many salts; its composition is as follows, according to Dr. Schwertzer, of Brighton:—
| Water | 964·74372 grains. |
| Chloride of sodium (salt) | 28·05948 ” |
| Chloride of potassium | 0·76552 ” |
| Chloride of magnesium | 3·66658 ” |
| Bromide of magnesium | 0·02929 ” |
| Sulphate of magnesia | 2·29578 ” |
| Sulphate of lime | 0·40662 ” |
| Carbonate of lime | 0·03301 ” |
| (With traces of iodine and ammonia). | |
| 1000·00000 grains. |
Fig. 349.—Stalactite Cavern.
There is much more oxygen in water than in air, as can be ascertained by analysis of these compounds. This great proportion in favour of water enables fish to breathe by passing the water through the gills. Marine animals (not fishes), like the whale,—which is a warm-blooded creature, and therefore not suited to exist without air,—are obliged to come to the surface to breathe. The density of salt water is much greater than that of fresh water, and therefore swimming and flotation is easier in the sea than in a river. We shall have more to say of water by-and-by.
NITROGEN—SYMBOL N; ATOMIC WEIGHT 14.
We have already made some reference to this gas when speaking of the atmosphere and its constituents, of which nitrogen is the principal. From its life-destroying properties it is called “azote” by French chemists, and when we wish to obtain a supply of nitrogen all we have to do is to take away the oxygen from the air by burning phosphorus on water under a glass. Nitrogen is not found frequently in solid portions of the globe. It is abundant in animals. It is without colour or smell, and can be breathed in air without danger. It is heavy and sluggish; but if we put a taper into a jar of nitrogen it will go out, and animals die in the gas for want of oxygen, as nitrogen alone cannot support life.
Fig. 350.—Obtaining nitrogen.
The affinity of nitrogen for other substances is not great, but it gives rise to five compounds, which are as below, in the order they are combined with oxygen:—
| Nitrous oxide (“laughing gas”) | (Monoxide) | N2O. |
| Nitric oxide | Dioxide | N2O2. |
| Nitrous acid | Trioxide | N2O3. |
| Nitric peroxide | Tetroxide | N2O4. |
| Nitric acid | Pentoxide | N2O5. |
These compounds are usually taken as representative examples of combining weight, and as explanatory of the symbolic nomenclature of chemistry, as they advance in such regular proportions of oxygen with nitrogen. The combining weight of nitrogen is 14, and when two parts combine with five of oxygen it makes nitric acid, and we put it down as N2O5; on adding water, HNO3, as we can see by eliminating the constituents and putting in the proportions. Actually it is H2N2O6, or, by division, HNO3.
Nitrogen plays a very important part in nature, particularly in the vegetable kingdom. Nitric acid has been known for centuries. Geber, the alchemist, was acquainted with a substance called “nitric,” which he found would yield a dissolvent under certain circumstances. He called it “dissolving fluid.” At the end of the twelfth century Albert Magnus investigated the properties of this acid, and in 1235 Raymond Lully prepared nitre with clay, and gave the liquid the name of “aqua-fortis.” But till 1849 nitric acid was only known as a hydrate,—that is, in combination with water,—but now we have the anhydrous acid.
Fig. 351.—Apparatus for obtaining nitrogen by using metal to absorb the oxygen of the air.
Oxygen and nitrogen combine under the influence of electricity, as shown by Cavendish, who passed a current through an atmospheric mixture of oxygen and nitrogen, in a tube terminating in a solution of potash, lime, and soda. Every time the spark passed, the volume of gas diminished, and nitric acid was formed, as it is in thunderstorms, when it does not remain free, but unites with ammonia, and forms a highly useful salt, which promotes vegetable growth. Here is another instance of the usefulness of thunderstorms, and of the grand provisions of nature for our benefit. Nitric acid is obtained by distilling nitre with sulphuric acid. The liquid is, when pure, colourless, and is a powerful oxidizer. It dissolves most metals, and destroys vegetable and animal substances. By an addition of a little sulphuric acid the water is taken from the nitric acid, and a very powerful form of it is the result. The acid is of great use in medicine, and as an application to bites of rabid animals or serpents. It converts cotton waste into “gun-cotton” by a very simple process of steeping, washing, and pressing. From the hydraulic press it comes in discs like “quoits,” which will burn harmlessly and smoulder away, but if detonated they explode with great violence. As a rule, when damp, it is not dangerous, but it can be fired even when wet. It will explode at a less temperature than gunpowder, and, moreover, yields no smoke, nor does it foul a gun. Gun-cotton, when dissolved in ether, gives us collodion for photographic purposes.
Fig. 352.—Nitric acid obtained from nitre and sulphuric acid.
In speaking farther of the compounds of nitrogen with oxygen, we will limit ourselves to the monoxide, or laughing gas. This is now used as an anæsthetic in dentistry, etc., and is quite successful, as a rule. People afflicted with heart disease should not use it without advice, however. When inhaled into the lungs it makes the subject very hilarious, and the effect is rather noisy. It is obtained from the nitrate of ammonia, which, on the application of heat, decomposes into nitrous oxide and vapour. Warm water should be used for the trough. The gas is a powerful supporter of combustion.
Fig. 353.—Cavendish’s experiment.
Binoxide of nitrogen is of importance in the manufacture of sulphuric acid.
Fig. 354.—Experiment to obtain nitric acid.
Nitrogen combines with hydrogen, forming various compounds. These are the “amines,” also ammonia, and ammonium. Ammonia possesses the properties of a base. Its name is derived from Jupiter Ammon, near whose temple it was prepared, from camels’ dung. But bodies containing nitrogen give off ammonia in course of distilling, and hartshorn is the term applied to horn-cuttings, which yield ammonia, which is a colourless gas of strong odour and taste now obtained from gas-works.
Fig. 355.—Apparatus for obtaining laughing-gas.
Fig. 356.—Inhaling laughing gas.
Fig. 357.—Generation of ammonia.
To obtain ammonia heat equal parts of chloride of ammonia (sal ammoniac) and quick-lime powdered (see fig. 357). The gas must be collected over mercury, because it is very soluble in water. Ammonia is useful to restore tipsy people and fainting ladies. A solution of ammonia is used for cauteries. Ammoniacal gas is remarkable for its solubility in water. To prepare the solution the gas is forced through a series of flasks. The tubes carrying the gas should be continued to the bottoms of the flasks, else the solution, being lighter than water, the upper portion alone would be saturated. The tubes carrying away the solution are raised a little, so that the renewal is continually proceeding. The gas liquifies under a pressure of six atmospheres, at a temperature of 10° Cent. This experiment can be artificially performed by heating chloride of silver saturated with ammonia, and the silver will part with the gas at a temperature of 40° C. The gas will then condense in a liquid form in the tube. The experiment may be facilitated by placing the other extremity of the tube in snow and salt, and by the liquid we can obtain intense cold. This experiment has been made use of by M. Carré in his refrigerator (which was described in the Physics’ section), by which he freezes water. We may, however, just refer to the process. Whenever the condition of a body is changed from that of liquid to a gas, the temperature is greatly lowered, because the heat becomes “latent.” The latest freezing machine consists of an apparatus as shown in the illustrations herewith (figs. 359 and 360). The machine is of wrought iron, and contains, when ready for action, a saturated solution of ammonia at zero. This is in communication with another and an air-tight vessel, of which the centre is hollow. The first process is to heat the solution, and the gas escapes into the second “vase,” which is surrounded by cold water, and quite unable to escape. A tremendous pressure is soon obtained, and this, added to the cold water, before long liquifies the ammonia, and when the temperature indicates 130° the hot vessel is suddenly cooled by being put into the water. The gas is thus suddenly converted into a liquid, the water in the second hollow vase is taken out, and the bottle to be frozen is put into the cavity. The cold is so great, in consequence of the transformation of the liquid ammonia into a gas, that it freezes the water in any vessel put into the receiver. The ammonia can be reconverted into liquid and back again, so no loss is occasioned by the process, which is rapid and simple. This is how great blocks of ice are produced in water-bottles.
Fig. 358.—Liquefaction of ammonia.
The one important point upon which care is necessary is the raising of the temperature. If it be elevated beyond 130° C., the pressure will be too great, and an explosion will occur.
Fig. 359.—Carré’s refrigerator (first action).
The abundant formation of ammonia from decaying animal matter is evident to everyone, and depends upon the presence of moisture to a great extent. Chloride of ammonia is called sal-ammoniac, and the carbonate of ammonia crystallizes from the alkaline liquid produced by the distillation of certain animal matter. The compounds of ammonia are easily recognized by a certain sharp taste. They are highly valuable remedial agents, acting particularly upon the cutaneous system, and when taken internally, produce the effect of powerful sudorifics. Their volatility, and the facility with which they are expelled from other substances, render them of great importance in chemistry, and peculiarly fit them for the purposes of many chemical analyses. The ammonia compounds display a remarkable analogy to the corresponding combinations of potash and soda. The compounds of ammonia are highly important in their relation to the vegetable kingdom. It may be assumed that all the nitrogen of plants is derived from the ammonia which they absorb from the soil, and from the surrounding atmosphere.
Fig. 360.—Carré’s refrigerator (second action).
The similarity of ammonia to the metallic oxides has led to the conjecture that all its combinations contain a compound metallic body, which has received the name ammonium (NH4); but no one has yet succeeded in its preparation, although by peculiar processes it may be obtained in the form of an amalgam.
Ammonias, in which one or more atoms of hydrogen are replaced by basic radicals, are termed Amides, or Amines.
CHAPTER XXVIII.
NON-METALLIC ELEMENTS (continued).
CHLORINE—BROMINE—IODINE—FLUORINE—CARBON—SULPHUR—PHOSPHORUS—SILICON—BORON—TELLURIUM—ARSENIC.
Chlorine (Cl.) is usually found with sodium in the mineral kingdom, and this chloride of sodium is our common salt. Chlorine can be obtained by heating hydrochloric acid with binoxide of manganese. (Atomic weight 35·5.)
Fig. 361.—Generation of chlorine.
Chlorine possesses a greenish-yellow colour, hence its name “Chloros,” green. It should be handled carefully, for it is highly injurious and suffocating. It possesses a great affinity for other substances, and attacks the metals. For hydrogen it has a great affection, and when hydrogen is combined with any other substances chlorine immediately attacks them, and in time destroys them. But even this destructive and apparently objectionable quality makes chlorine very valuable; for if we carry the idea to its conclusion, we shall find that it also destroys offensive and putrid matter, and purifies the atmosphere very much. Most colouring matters include hydrogen, and therefore they are destroyed by chlorine, which is a great “bleacher” as well as a purifier. If we dip any vegetable dyes into a jar of chlorine, they will become white if the dyed substances are damp.
Hydrochloric acid is known as muriatic acid and spirits of salt. It is obtained when salt is treated with sulphuric acid and the gas comes off into water. Equal parts of the acid and the salt are put into a flask as in the cut (fig. 362), and diluted with water. The mixture is then heated. The gas is condensed in the bottles half-full of water. The result gives sulphate of soda and hydrochloric acid. This acid is procured in soda manufactories, and with nitric acid is called “aqua regia,” a solvent for gold. When chlorine and hydrogen are mixed in equal proportions they explode in sunlight. In the dark or by candle-light they are harmless. Dry chlorine gas can be obtained by interposing a glass filled with some chloride of calcium. The gas being heavier than air (about 2½ times), displaces it in the flask, and when it is filled another can be placed in position. This mode causes a little waste of gas, which should not be breathed.
Fig. 362.—Production of hydrochloric acid.
Chlorine possesses a great affinity for certain bodies. If the gas be thrown upon phosphorus, the latter will burn brilliantly. Arsenic, tin, and antimony when powdered and poured from a shoot into a vase of chlorine will burst into brilliant sparks, and other metals will glow when introduced to this gas. Chlorine forms many unstable combinations with oxygen. Its combination with hydrogen has already been referred to.
Bromine is a rare element. (Symbol Br. Atomic weight 80.)
It is deep brownish red, very volatile, and of a peculiar odour. Bromine unites with the elementary bodies, and forms some oxygen compounds. It resembles chlorine in its properties, and is used in medicine and in photography. It is found in saline springs and in salt water, combined with soda and magnesium. The presence of bromine may easily be detected in the strong smell of seaweed. Its combinations with metals are termed bromides. It is a powerful poison.
Iodine is another relative of chlorine. It is found in seaweed, which by burning is reduced to kelp. When iodine is heated a beautiful violet vapour comes off, and this characteristic has given it its name (“iodes,” violet). Iodine was discovered by Courtois, of Paris, and in 1813, Gay Lussac made it a special study. It is solid at ordinary temperatures, and assumes crystallized forms in plates of metallic lustre. It is an excellent remedy in “goitre” and such affections. (Symbol I. Atomic weight 127.)
Fluorine is very difficult to prepare. Fluor spar is a compound of fluorine and calcium. This element is gaseous, and combines so rapidly that it is very difficult to obtain in a free state. Etching on glass is accomplished by means of hydrofluoric acid, for fluorine has a great affinity for silicic acid, which is contained in glass. The glass is covered with wax, and the design is traced with a needle. The acid attacks the glass and leaves the wax, so the design is eaten in. (Symbol F. Atomic weight 19.)
Fig. 363.—Apparatus for obtaining dry chlorine gas.
Chlorine, fluorine, bromine, and iodine are termed “Halogens” (producers of salts). They appear, as we have seen, in a gaseous, liquid, and solid form respectively.
Carbon is the most, or one of the most, largely diffused elements in nature, and claims more than a passing notice at our hands, though even that must be brief. We may put down carbon next to oxygen as the most important element in the world. The forms assumed by carbon are very variable, and pervade nature in all its phases. We have carbon in crystals, in the animal and vegetable kingdoms, and amongst the chief minerals a solid, odourless, tasteless, infusible, and almost insoluble body. In various combinations carbon meets us at every turn; united with oxygen it forms carbonic acid, which we exhale for the plants to imbibe. We have it in coal, with hydrogen and oxygen. We have it building up animal tissues, and it is never absent in two out of the three great divisions of nature—the plants and the animals (Symbol C; Atomic W. 12).
Fig. 364.—Facets of a brilliant.
Fig. 365.—Facets of a rose diamond.
We have carbon in three different and well-known conditions; as the diamond, as graphite, or black-lead, and as charcoal. The properties of the diamond are well known, and we shall, when we get to Crystallography, learn the forms of diamond or crystals of carbon. At present we give an illustration or two, reserving all explanation for the present. Diamond cutting is a matter of some difficulty, and it requires skill to cut in the proper direction. Diamonds are found in India, Brazil, and at the Cape of Good Hope, in alluvial soil. The identity of diamond and charcoal was discovered accidentally. An experiment to fuse a few small diamonds resulted in their disappearance, and when the residue was examined it was found that the diamonds had been burned, that they had combined with oxygen and formed carbonic acid, just as when coal burns. The diamond is the hardest of all substances, the most valuable of gems, and the purest condition in which carbon appears.
Graphite (Plumbago) is termed “black-lead,” and is the next purest form of carbon. It crystallizes and belongs to the primitive formations. In Cumberland it is dug up and used to make pencils; the operations can be seen at Keswick. It has other uses of a domestic character.
Charcoal is the third form of carbon, and as it possesses no definite form, is said to be amorphous. Charcoal is prepared in air-tight ovens, so that no oxygen can enter and burn the wood thus treated. Coke is the result of the same process applied to coal. The gas manufactories are the chief depôts for this article, and it is used in locomotive engines. The various smokeless coals and prepared fuels, however, are frequently substituted.
Fig. 366.—Coke ovens.
Coke ovens were formerly much resorted to by the railway companies, who found the ordinary coal too smoky for locomotive purposes, and apt to give rise to complaints by passengers and residents near the line.
The origin of wood charcoal we have seen. All vegetable substances contain carbon. When we burn wood, in the absence of air as far as possible, oxygen and hydrogen are expelled. The wood is piled in layers as in the illustration (fig. 368), covered over with turf and mould, with occasional apertures for air. This mass is ignited, the oxygen and hydrogen are driven off, and carbon remains. (Animal charcoal is obtained from calcining bones). Wood charcoal attracts vapours, and water, if impure, can be purified by charcoal, and any impure or tainted animal matter can be rendered inoffensive by reason of charcoal absorbing the gases, while the process of decay goes on just the same. Housekeepers should therefore not always decide that meat is good because it is not offensive to the olfactory nerves. Charcoal will remove the aroma, but the meat may be nevertheless bad. The use of charcoal in filters is acknowledged universally, and as a constituent of gunpowder it is important.
Fig. 367.—Charcoal burning.
Carbon is not easily affected by the atmospheric air, or in the earth; so in many instances wood is charred before being driven into the ground; and casks for water are prepared so. Soot is carbon in a pulverised condition, and Indian ink is manufactured with its assistance.
Fig. 368.—Wood piles of charcoal burners.
The preparation of wood charcoal gives occupation to men who are frequently wild and untutored, but the results of their labour are very beneficial. Care should be taken not to sleep in a room with a charcoal stove burning, unless there is ample vent for the carbonic acid gas, for it will cause suffocation. Lampblack is obtained by holding a plate over the flame of some resinous substance, which deposits the black upon it. There is a special apparatus for this purpose.
Fig. 369.—Seltzer-water manufactory.
Carbon combines with oxygen to make carbonic acid gas, as we have already mentioned, and in other proportions to form a more deadly compound than the other. The former is the dioxide (CO2), the latter the monoxide, or carbonic oxide (CO). The dioxide is the more important, being held in the atmosphere, and combined with lime in chalk. All sparkling beverages contain carbonic acid, to which their effervescence is due. The soda and other mineral waters owe their sparkle to this gas. Soda-water consists of a weak solution of carbonate of soda and the acid. There is a vessel holding chalk and water, and another containing some sulphuric acid. When the sulphuric acid is permitted to unite with the chalk and water, carbonic acid is liberated. A boy turning a wheel forces the gas into the water in the bottles, or the water and carbonate of soda is drawn off thus impregnated into bottles and corked down, in the manner so familiar to all. The bottles are made of the shape depicted, so that the bubble of air shall be at the top when the bottle lies down. If it be not kept so, the air will eventually escape, no matter how tightly the cork be put in. The ordinary “soda-water” contains scarcely any soda. It is merely water, chalk, and carbonic acid. The “Gazogene” is made useful for small quantities of soda-water, and is arranged in the following manner. The appearance of it is familiar to all. It consists of a double vessel, into the upper part of which a solution of any kind—wine and water, or even plain water—is put, to be saturated with carbonic acid, or “aerated,” and into the lower one some carbonate of soda and tartaric acid. A tube leads from this lower to the top of the upper vessel, which screws on and off. By shaking the apparatus when thus charged and screwed together, some of the liquid descends through the tube into the lower vessel and moistens the soda and acid, which therefore act on each other, and cause carbonic acid to be disengaged; this, rising up through the tube (which is perforated with small holes at the upper part), disperses itself through the liquid in small bubbles, and causes sufficient pressure to enable the liquid to absorb it, which therefore effervesces when drawn off by the tap.
Fig. 370.—Gazogene.
Carbonic acid can be liquified, and then it is colourless. In a solid form it resembles snow, and if pressed with the fingers it will blister them. Being very heavy the gas can be poured into a vase, and if there be a light in the receptacle the flame will be immediately extinguished.
Fig. 371.—Soda-water apparatus.
That even the gas introduced into seltzer-water is capable of destroying life, the following experiment will prove. Let us place a bird within a glass case as in the illustration (fig. 373), and connect the glass with a bottle of seltzer-water or a siphon. As soon as the liquid enters, the carbonic acid will ascend, and this, if continued for a long time, would suffocate the bird, which soon begins to develop an appearance of restlessness.
Fig. 372.—Pouring out the carbonic acid gas.
We have already remarked upon the important part taken by this gas in nature, so we need only mention its existence in pits and caves. There are many places in which the vapour is so strong as to render the localities uninhabitable. In the Middle Ages the vapours were attributed to the presence of evil spirits, who were supposed to extinguish miners’ lamps, and suffocate people who ventured into the caves. In the Grotto Del Cane there is still an example, and certain caves of Montrouge are often filled with the gas. A lighted taper held in the hand will, by its extinction, give the necessary warning. Oxygen and carbon are condensed in carbonic acid, for the gas contains a volume of oxygen equal to its own. If we fill a glass globe, as per illustration (fig. 374), with pure oxygen, and in the globe insert two carbon points, through which we pass a current of electricity, we shall find, after the experiment, that if the stop-cock be opened, there is no escape of gas, and yet the mercury does not rise in the tube, so the oxygen absorbed has been replaced by an equal volume of carbonic acid.
Fig. 373.—Experiment with carbonic acid.
The other combination of carbon with oxygen is the carbonic oxide (CO), and when a small quantity of oxygen is burnt with it it gives a blue flame, as on the top of the fire in our ordinary grates. This gas is present in lime kilns, and is a very deadly one. We must now pass rapidly through the compounds of carbon with hydrogen, merely referring to coal for a moment as we go on.
Coal, of which we shall learn more in Mineralogy and Geology, is a combination, mechanical or otherwise, and is the result of the decomposition of vegetable matter in remote ages,—the so-called “forests,” which were more like the jungles than the woods of the present day. Moss and fern played prominent parts in this great transformation, as we can see in the Irish peat-bogs, where the first steps to the coal measures are taken.
Fig. 374.—Experiment showing that carbonic acid contains oxygen and carbon.
The compounds of carbon with hydrogen are important. There is the “light” carburetted hydrogen (CH4), which is usually known as fire-damp in coal mines. It is highly inflammable and dangerous. The safety-lamp invented by Davy is a great protection against it, for as the gas enters it is cooled by the wire, and burns within harmlessly. The explosion warns the miner. “Heavy” carburetted hydrogen possesses double the quantity of carbon (C2H4). It is also explosive when mixed with oxygen.
Fig. 375.—Temperature reduced by contact with wire.
The most useful compound is coal-gas, and though its principal function appears to be in some manner superseded by electricity, “gas” is still too important to be put aside. It can easily be obtained by putting small fragments of coal in the bowl of a tobacco-pipe, closing the bowl with clay, and putting it in the fire. Before long the gas will issue from the stem of the pipe, and may either be lighted or collected in a bladder. For the use of the “million,” however, gas is prepared upon a very large scale, and is divided into three processes—its “formation,” “purification,” and its “collection” for distribution to consumers. The first process is carried on by means of retorts shown in the illustration (fig. 376). The first portion of the next figure is a section of a furnace, the other part shows two furnaces from the front. The following is the mode employed. The coal is put into retorts fitted to the furnace, so that they are surrounded by the flames, and terminating in a horizontal tube called the hydraulic main, E, which is in its turn connected with a pit or opening for the reception of the tar and ammoniacal liquor, etc., which condenses from the gas. It then passes up and down a series of tubes in water, called a “condenser,” and in this are reservoirs or receptacles for any tar and ammonia that remain. But sulphur is still present, so the gas is carried to the purifying apparatus (D in fig. 378), which consists of a large cylindrical vessel air-tight, with an inverted funnel, nearly filled with a mixture of lime and water. The gas bubbles in, and the sulphur unites with the lime, while the gas rises to the top (trays of lime are used when the gas enters from the bottom). The Gasometer, a large vessel closed at the top and open below, dips into a large trough of circular shape. The gasometer is balanced by weights and chains, and may be raised (See fig. 379). When quite empty the top rests upon the ground, and when the gas enters it is raised to the top of the frame which supports it. We have now our Gasometer full. When the time comes to fill the pipes for lighting purposes, some of the weights are removed, the Gasometer falls down slowly, and forces the gas through the tubes into the main supply to be distributed. About four cubic feet of gas is obtained from every pound of coal. When gas and air become mixed, the mixture is very explosive. In a house where an escape of gas is detected let the windows be opened at the top, and no light introduced for several minutes.
Fig. 376.—Retorts.
Fig. 377.—Section. Front view.
Fig. 378.—Condenser. Purifier. Gasometer.
Fig. 379.—Gasometer.
It has been calculated that one ton of good coal produces the following:—
| 1 | Chaldron of coke | weighing | 1,494 lbs. |
| 12 | Gallons of tar | “ | 135 lbs. |
| 12 | Gallons of ammoniacal liquor | “ | 100 lbs. |
| 5,900 | Cubic feet of gas | “ | 291 lbs. |
| Loss (water) | “ | 220 lbs. | |
| Total | 2,240 lbs. |
Fig. 380.—Gasometer.
We can thus estimate the profits of our gas companies at leisure. The analysis of gas made by Professor Bunsen is as under, in 100 parts.
| Hydrogen | 45·58 |
| Marsh gas | 34·90 |
| Carbonic oxide | 6·64 |
| Olefiant gas | 4·08 |
| Butyline | 2·38 |
| Sulphide of hydrogen | 0·29 |
| Nitrogen | 2·46 |
| Carbonic acid | 3·67 |
| 100·00 |
Gas, therefore, is very injurious, for it rapidly vitiates the atmosphere it burns in, and is very trying to the eyes, as well as destructive to gilt ornaments.
Tar is familiar to all readers, and though unpleasant to handle or to smell, it produces the beautiful aniline dyes. Tar pills are very efficacious for some blood disorders, and will remove pimples, etc., from the face, and cure “boils” effectually. If a dose of five be taken first, in a day or two four, and so on, no second remedy need be applied. We have known cases finally cured, and no recurrence of boils ever ensued after this simple remedy.
Fig. 381.—Tar manufactory
Tar is one of the results left in the distillation both of wood and coal: in places where wood is plentiful and tar in request, it is produced by burning the wood for that purpose; and in some of the pits in which charcoal is produced, an arrangement is made to collect the tar also. Coal-tar and wood-tar are different in some respects, and are both distilled to procure the napthas which bear their respective names. From wood-tar creosote is also extracted, and it is this substance which gives the peculiar tarry flavours to provisions, such as ham, bacon, or herrings, cured or preserved by being smoked over wood fires. Tar is used as a sort of paint for covering wood-work and cordage when much exposed to wet, which it resists better than anything else at the same price; but the tar chiefly used for these purposes is that produced by burning fir or deal wood and condensing the tar in a pit below the stack of wood; it is called Stockholm tar, as it comes chiefly from that place.
Carbon only combines with nitrogen under peculiar circumstances. This indirect combination is termed cyanogen (CN). It was discovered by Gay-Lussac, and is used for the production of Prussian blue. Hydrocyanide of potassium (Prussic acid) is prepared by heating cyanide of potassium with sulphuric acid. It is a deadly poison, and found in peach-stones. Free cyanogen is a gas. The bisulphide of carbon is a colourless, transparent liquid. It will easily dissolve sulphur and phosphorus and several resins. When phosphorus is dissolved in it, it makes a very dangerous “fire,” and one difficult to extinguish. We must now leave carbon and its combinations, and come to sulphur.
Fig. 382.—Sulphur furnace.
Sulphur is found in a native state in Sicily and many other localities which are volcanic. It is a yellow, solid body, and as it is never perfectly free from earthy matter, it must be purified before it can be used. It possesses neither taste nor smell, and is insoluble in water. Sulphur is purified in a retort, C D, which communicates with a brick chamber, A. The retort is placed over a furnace, K, and the vapour passes into the chimney through the tube, D, where it condenses into fine powder called “flowers of sulphur” (brimstone). A valve permits the heated air to pass off, while no exterior air can pass in, for explosions would take place were the heated vapour to meet the atmospheric air. The danger is avoided by putting an air reservoir outside the chimney which is heated by the furnace. The sulphur is drawn out through the aperture, r, when deposited on the floor of the chamber. The sulphur is cast into cylinders and sold. Sulphur is soluble in bisulphide of carbon, and is used as a medical agent.
The compounds of sulphurs with oxygen form an interesting series. There are two anhydrous oxides (anhydrides),—viz., sulphurous and sulphuric anhydride (SO2 and SO3). There are two notable acids formed by the combination with water, sulphurous and sulphuric, and some others, which, as in the case of nitrogen, form a series of multiple proportions, the oxygen being present in an increasing regularity of progression, as follows:—
| Name of Acid. | Chemical Formula. |
|---|---|
| Hypo-sulphurous acid | H2SO2 |
| Sulphurous acid | H2SO3 |
| Sulphuric acid | H2SO4 |
| Thio-sulphuric, or hypo-sulphuric acid | H2S2O3 |
| Dithionic acid | H2S2O6 |
| Trithionic acid | H2S3O6 |
| Tetrathionic acid | H2S4O6 |
| Pentathionic acid | H2S5O6 |
The last four are termed “polythionic,” because the proportions of sulphur vary with constant proportions of the other constituents.
Fig. 383.—Liquefaction of sulphuric acid.
The sulphurous anhydride mentioned above is produced when we burn sulphur in the air, or in oxygen; it may be obtained in other ways. It is a colourless gas, and when subjected to pressure may be liquified, and crystallized at very low temperature. It was formerly called sulphuric acid. It is a powerful “reducing agent,” and a good antiseptic. It dissolves in water, and forms the H2SO3, now known as sulphurous acid.
Fig. 384.—Retorts and receivers for acid.
Sulphuric acid is a most dangerous agent in wicked or inexperienced hands, and amateurs should be very careful when dealing with it. It takes the water from the moist air, and from vegetable and animal substances. It carbonizes and destroys all animal tissues. Its discovery is due to Basil Valentine, in 1440. He distilled sulphate of iron, or green vitriol, and the result was “oil of vitriol.” It is still manufactured in this way in the Hartz district, and the acid passes by retorts into receivers. The earthen retorts, A, are arranged in the furnace as in the illustration, and the receivers, B, containing a little sulphuric acid, are firmly fixed to them. The oily brown product fumes in the air, and is called “fuming sulphuric acid,” or Nordhausen acid. Sulphuric acid is very much used in chemical manufactures, and the prices of many necessaries, such as soap, soda, calico, stearin, paper, etc., are in close relationship with the cost and production of sulphur, which also plays an important part in the making of gunpowder. The manufacture of the acid is carried on in platinum stills.
Fig. 385.—Experiment to show the existence of gases in solution.
Sulphuretted hydrogen, or the hydric sulphide (H2S), is a colourless and horribly-smelling gas, and arises from putrefying vegetable and animal matter which contains sulphur. The odour of rotten eggs is due to this gas, which is very dangerous when breathed in a pure state in drains, etc. It can be made by treating a sulphide with sulphuric acid. It is capable of precipitating the metals when in solution, and so by its aid we can discover the metallic ingredient if it be present. The gas is soluble in water, and makes its presence known in certain sulphur springs. The colour imparted to egg-spoons and fish-knives and forks sometimes is due to the presence of metallic sulphides. The solution is called hydro-sulphuric acid.
Phosphorus occurs in very small quantities, though in the form of phosphates we are acquainted with it pretty generally, and as such it is absorbed by plants, and is useful in agricultural operations. In our organization—in the brain, the nerves, flesh, and particularly in bones—phosphorus is present, and likewise in all animals. Nevertheless it is highly poisonous. It is usually obtained from the calcined bones of mammalia by obtaining phosphoric acid by means of acting upon the bone-ash with sulphuric acid. Phosphorus when pure is colourless, nearly transparent, soft, and easily cut. It has a strong affinity for oxygen. It evolves white vapour in atmospheric air, and is luminous; to this element is attributable the luminosity of bones of decaying animal matter. It should be kept in water, and handled—or indeed not handled—but grasped with a proper instrument.
Phosphorus is much used in the manufacture of lucifer matches, and we are all aware of the ghastly appearance and ghostly presentment it gives when rubbed upon the face and hands in the dark. In the ripples of the waves and under the counter of ships at sea, the phosphorescence of the ocean is very marked. In Calais harbour we have frequently noticed it of a very brilliant appearance as the mail steamer slowly came to her moorings. This appearance is due to the presence of phosphorus in the tiny animalculæ of the sea. It is also observable in the female glow-worm, and the “fire-fly.” Phosphorus was discovered by Brandt in 1669.
Fig. 386.—Manufacture of sulphuric acid.
It forms two compounds with oxygen-phosphorous acid, H2PO4, and phosphoric acid, H3PO4. The compound with hydrogen is well marked as phosphuretted hydrogen, and is a product of animal and vegetable decomposition. It may frequently be observed in stagnant pools, for when emitted it becomes luminous by contact with atmospheric air. There is a very pretty but not altogether safe experiment to be performed when phosphuretted hydrogen has been prepared in the following manner. Heat small pieces of phosphorus with milk of lime or a solution of caustic potash; or make a paste of quick-lime and phosphorus, and put into the flask with some quick-lime powdered. Fix a tube to the neck, and let the other end be inserted in a basin of water. (See illustration, fig. 388.) Apply heat; the phosphuretted hydrogen will be given off, and will emerge from the water in the basin in luminous rings of a very beautiful appearance. The greatest care should be taken in the performance of this very simple experiment. No water must on any account come in contact with the mixture in the flask. If even a drop or two find its way in through the bent tube a tremendous explosion will result, and then the fire generated will surely prove disastrous. The experiment can be performed in a cheaper and less dangerous fashion by dropping phosphate of lime into the basin. We strongly recommend the latter course to the student unless he has had some practice in the handling of these inflammable substances, and learnt caution by experience. The effect when the experiment is properly performed is very good, the smoke rising in a succession of coloured rings.
Fig. 387.—(Phosphuretted hydrogen and marsh gas) Will-o’-the-Wisp.
Silicon is not found in a free state in nature, but, combined with oxygen, as Silica it constitutes the major portion of our earth, and even occurs in wheat stalks and bones of animals. As flint or quartz (see Mineralogy) it is very plentiful, and in its purest form is known as rock crystal, and approaches the form of carbon known as diamond. When separated from oxygen, silicon is a powder of greyish-brown appearance, and when heated in an atmosphere of oxygen forms silicic “acid” again, which, however, is not acid to the taste, and is also termed “silica,” or “silex.” It is fused with great difficulty, but enters into the manufacture of glass in the form of sand. The chemical composition of glass is mixed silicate of potassium or sodium, with silicates of calcium, lead, etc. Ordinary window-glass is a mixture of silicates of sodium and calcium; crown glass contains calcium and silicate of potassium. Crystal glass is a mixture of the same silicate and lead. Flint glass is of a heavier composition. Glass can be coloured by copper to a gold tinge, blue by cobalt, green by chromium, etc. Glass made on a large scale is composed of the following materials, according to the kind of glass that is required.
Flint glass (“crystal”) is very heavy and moderately soft, very white and bright. It is essentially a table-glass, and was used in the construction of the Crystal Palace. Its composition is—pure white sea-sand, 52 parts, potash 14 parts, oxide of lead, 34 parts = 100.
| Plate Glass. | Crown Glass. | Green (Bottle) Glass. | |||
|---|---|---|---|---|---|
| Pure white sand | 55 parts. | Fine sand | 63 parts. | Sea sand | 80 parts. |
| Soda | 35 ” | Chalk | 7 ” | Salt | 10 ” |
| Nitre | 8 ” | Soda | 30 ” | Lime | 10 ” |
| Lime | 2 ” | ||||
| 100 ” | 100 ” | 100 ” | |||
The ingredients to be made into glass (of whatever kind it may be) are thoroughly mixed together and thrown from time to time into large crucibles placed in a circle, A A (fig. 389), in a furnace resting on buttresses, B B, and heated to whiteness by means of a fire in the centre, C, blown by a blowing machine, the tube of which is seen at D. This furnace is shown in prospective in fig. 390. The ingredients melt and sink down into a clear fluid, throwing up a scum, which is removed. This clear glass in the fused state is kept at a white heat till all air-bubbles have disappeared; the heat is then lowered to a bright redness, when the glass assumes a consistence and ductility suitable to the purposes of the “blower.”
Fig. 388.—Experiment with phosphuretted hydrogen.
Glass blowing requires great care and dexterity, and is done by twirling a hollow rod of iron on one end of which is a globe of melted glass, the workman blowing into the other end all the time. By reheating and twirling a sheet of glass is produced. Plate glass is formed by pouring the molten glass upon a table with raised edges. When cold it is ground with emery powder, and then polished by machinery.
Fig. 389.—Crucibles.
Many glass articles are cast, or “struck-up,” by compression in moulds, and are made to resemble cut-glass, but they are much inferior in appearance. The best are first blown, and afterwards cut and polished. Of whatever kind of glass the article may be, it is so brittle that the slightest blow would break it, a bad quality which is got rid of by a process called “annealing,” that is, placing it while quite hot on the floor of an oven, which is allowed to cool very gradually. This slow cooling takes off the brittleness, consequently articles of glass well annealed are very much tougher than others, and will scarcely break in boiling water.
Fig. 390.—Plate-glass casting—bringing out the pot.
The kind generally used for ornamental cutting is flint-glass. Decanters and wine-glasses are therefore made of it; it is very bright, white, and easily cut. The cutting is performed by means of wheels of different sizes and materials, turned by a treadle, as in a common lathe, or by steam power; some wheels are made of fine sandstone, some of iron, others of tin or copper; the edges of some are square, or round, or sharp. They are used with sand and water, or emery and water, stone wheels with water only.
Fig. 391.—Glass furnace. (See also fig. 390 for detail.)
Fig. 392.—Glass-cutting.
In a soluble form silicic acid is found in springs, and thus enters into the composition of most plants and grasses, while the shells and scales of “infusoria” consist of silica. As silicate of alumina,—i.e., clay,—it plays a very important rôle in our porcelain and pottery works.
Boron is found in volcanic districts, in lakes as boracic acid, in combination with oxygen. It is a brownish-green, insoluble powder, in a free state, but as boracic acid it is white. It is used to colour fireworks with the beautiful green tints we see. Soda and boracic acid combine to make borax (or biborate of soda). Another and inferior quality of this combination is tinkal, found in Thibet. Borax is much used in art and manufactures, and in glazing porcelain. (Symbol B, Atomic Weight 11).
Selenium is a very rare element. It was found by Berzilius in a sulphuric-acid factory. It is not found in a free state in nature. It closely resembles sulphur in its properties. Its union with hydrogen produces a gas, seleniuretted hydrogen, which is even more offensive than sulphuretted hydrogen. (Symbol Se, Atomic Weight 79).
Tellurium is also a rare substance generally found in combination with gold and silver. It is like bismuth, and is lustrous in appearance. Telluretted hydrogen is horrible as a gas. Tellurium, like selenium, sulphur, and oxygen, combines with two atoms of hydrogen. (Symbol Te, Atomic Weight 129).
Fig. 393.—Casting plate-glass.
Arsenic, like tellurium, possesses many attributes of a metal, and on the other hand has some resemblance to phosphorus. Arsenic is sometimes found free, but usually combined with metals, and is reduced from the ores by roasting; and uniting with oxygen in the air, is known as “white arsenic.” The brilliant greens on papers, etc., contain arsenic, and are poisonous on that account. Arsenic and hydrogen unite (as do sulphur and hydrogen, etc.), and produce a foetid gas of a most deadly quality. This element also unites with sulphur. If poured into a glass containing chlorine it will sparkle and scintillate as in the illustration (fig. 395). (Symbol As, Atomic Weight 75).
Before closing this division, and passing on to a brief review of the Metals, we would call attention to a few facts connected with the metalloids we have been considering. Some, we have seen, unite with hydrogen only, as chlorine; some with two atoms of hydrogen, as oxygen, sulphur, etc., and some with three, as nitrogen and phosphorus; some again with four, as carbon and silicon. It has been impossible in the pages we have been able to devote to the Metalloids to do more than mention each briefly and incompletely, but the student will find sufficient, we trust, to interest him, and to induce him to search farther, while the general reader will have gathered some few facts to add to his store of interesting knowledge. We now pass on to the Metals.
Fig. 394—The manufacture of porcelain in China.
Fig. 395.—Experiment showing affinity between arsenic and chlorine.
CHAPTER XXIX.
THE METALS.
WHAT METALS ARE—CHARACTERISTICS AND GENERAL PROPERTIES OF METALS—CLASSIFICATION—SPECIFIC GRAVITY—DESCRIPTIONS.
We have learnt that the elements are divided into metalloids and metals, but the line of demarcation is very faint. It is very difficult to define what a metal is, though we can say what it is not. It is indeed impossible to give any absolute definition of a metal, except as “an element which does not unite with hydrogen, or with another metal to form a chemical compound.” This definition has been lately given by Mr. Spencer, and we may accept it as the nearest affirmative definition of a metal, though obviously not quite accurate.
Fig. 396.—Laminater.
A metal is usually supposed to be solid, heavy, opaque, ductile, malleable, and tenacious; to possess good conducting powers for heat and electricity, and to exhibit a certain shiny appearance known as “metallic lustre.” These are all the conditions, but they are by no means necessary, for very few metals possess them all, and many non-metallic elements possess several. The “alkali” metals are lighter than water; mercury is a fluid. The opacity of a mass is only in relation to its thickness, for Faraday beat out metals into plates so thin that they became transparent. All metals are not malleable, nor are they ductile. Tin and lead, for example, have very little ductility or tenacity, while bismuth and antimony have none at all. Carbon is a much better conductor of electricity than many metals in which such power is extremely varied. Lustre, again, though possessed by metals, is a characteristic of some non-metals. So we see that while we can easily say what is not a metal, we can scarcely define an actual metal, nor depend upon unvarying properties to guide us in our determination.
The affinity of metals for oxygen is in an inverse ratio to their specific gravity, as can be ascertained by experiment, when the heaviest metal will be the least ready to oxidise. Metals differ in other respects, and thus classification and division become easier. The fusibility of metals is of a very wide range, rising from a temperature below zero to the highest heat obtainable in the blow-pipe, and even then in the case of osmium there is a difficulty. While there can be no question that certain elements, iron, copper, gold, silver, etc., are metals proper, there are many which border upon the line of demarcation very closely, and as in the case of arsenic even occupy the debatable land.
Specific Gravity is the relation which the weight of substance bears to the weight of an equal volume of water, as already pointed out in Physics. The specific gravities of the metals vary very much, as will be seen from the table following—water being, as usual, taken as 1:—
| Aluminium | 2·56 | Lead | 11·3 | Rubidium | 1·5 |
| Antimony | 6·7 | Lithium | ·593 | Ruthenium | 11·4 |
| Arsenic | 6· | Magnesium | 1·74 | Silver | 105 |
| Bismuth | 9·7 | Manganese | 8· | Sodium | ·972 |
| Cadmium | 8·6 | Mercury | 13·5 | Strontium | 2·5 |
| Calcium | 1·5 | Molybdenum | 8·6 | Thallium | 11·8 |
| Chromium | 6·8 | Nickel | 8·8 | Tin | 7·2 |
| Cobalt | 8·9 | Osmium | 21·4 | Titanium | 5·3 |
| Copper | 8·9 | Palladium | 11·8 | Tungsten | 17·6 |
| Gold | 19·3 | Platinum | 21·5 | Uranium | 18·4 |
| Indium | 7·3 | Potassium | ·865 | Zinc | 7·1 |
| Iridium | 21·1 | Rhodium | 12·1 | Zircon | 4·3 |
Some metals are therefore lighter and some heavier than water.
The table underneath gives the approximate fusing points of some of the metals (Centigrade Scale)—
| (Ice melts at 0°.) | |||||
| Platinum[21] | about 1500° | Zinc | ” | 400° | |
| Gold | ” 1200° | Lead | ” | 330° | |
| Silver | ” 1000° | Bismuth | ” | 265° | |
| Cast iron | 1000-1200° | Tin | ” | 235° | |
| Wrought iron | ” 1500° | Sodium | ” | 97° | |
| Copper | ” 1100° | Potassium | ” | 60° | |
| Antimony | ” 432° | Mercury | ” | 40° | |
There are some metals which, instead of fusing,—that is, passing from the solid to the liquid state,—go away in vapour. These are volatile metals. Mercury, potassium, and sodium, can be thus distilled. Some do not expand with heat, but contract (like ice), antimony and bismuth, for instance, while air pressure has a considerable effect upon the fusing point. Some vaporise at once without liquefying; others, such as iron, become soft before melting.
Alloys are combinations of metals which are used for many purposes, and become harder in union. Amalgams are alloys in which mercury is one constituent. Some of the most useful alloys are under-stated:—
| Name of Alloy. | Composition. |
|---|---|
| Aluminium bronze | Copper and aluminium. |
| Bell metal | Copper and tin. |
| Bronze | ” |
| Gun metal | ” |
| Brass | Copper and zinc. |
| Dutch metal | ” |
| Mosaic gold | ” |
| Ormulu | ” |
| Tombac | ” |
| German silver | Copper, nickel, and zinc. |
| Britannia metal | Antimony and tin. |
| Solder | ” |
| Pewter | Tin and lead. |
| Type metal | Lead and antimony (also copper at times). |
| Shot | Lead and arsenic. |
| Gold currency | Gold and copper. |
| Silver currency | Silver and copper. |
| Stereotype metal | Lead, antimony, and bismuth. |
Metals combine with chlorine, and produce chlorides,
Metals combine with sulphu, and produce sulphides,
Metals combine with oxygen, and produce oxides, and so on.
The metals may be classed as follows in divisions:—
| Metals of the alkalies | as Potassium, Sodium, Lithium, Ammonium. |
| Metals of the alkaline earths | as Barium, Calcium, Magnesium, Strontium. |
| Metals of the earths | as Aluminium, Cerium, Didymium, Erbium, Glucinium, Lanthanum, Terbium, Thorium, Yttrium, Zirconium. |
| Metals proper— | |
| Common Metals | as {Iron, Manganese, Cobalt, Nickel, Copper, Bismuth, Lead, Tin, Zinc, Chromium, Antimony. |
| Noble Metals | as {Mercury, Silver, Gold, Platinum, Palladium, Rhodium, Ruthenium, Osmium, Iridium. |
We cannot attempt an elaborate description of all the metals, but we will endeavour to give a few particulars concerning the important ones, leaving many parts for Mineralogy to supplement and enlarge upon. We shall therefore mention only the most useful of the metals in this place. We will commence with Potassium.
Metals of the Alkalies.
Potassium has a bright, almost silvery, appearance, and is so greatly attracted by oxygen that it cannot be kept anywhere if that element be present—not even in water, for combustion will immediately ensue on water; and in air it is rapidly tarnished. It burns with a beautiful violet colour, and a very pretty experiment may easily be performed by throwing a piece upon a basin of water. The fragment combines with the oxygen of the water, the hydrogen is evolved, and burns, and the potassium vapour gives the gas its purple or violet colour. The metal can be procured by pulverizing carbonate of potassium and charcoal, and heating them in an iron retort. The vapour condenses into globules in the receiver, which is surrounded by ice in a wire basket. It must be collected and kept in naphtha, or it would be oxidised. Potassium was first obtained by Sir Humphrey Davy in 1807. Potash is the oxide of potassium, and comes from the “ashes” of wood.
Fig. 397.—Preparation of potassium.
The compounds of potassium are numerous, and exist in nature, and by burning plants we can obtain potash (“pearlash”). Nitrate of potassium, or nitre (saltpetre), (KNO3), is a very important salt. It is found in the East Indies. It is a constituent of gunpowder, which consists of seventy-five parts of nitre, fifteen of charcoal, and ten of sulphur. The hydrated oxide of potassium, or “caustic potash” (obtained from the carbonate), is much used in soap manufactories. It is called “caustic” from its property of cauterizing the tissues. Iodide, bromide, and cyanide of potassium, are used in medicine and photography.
Fig. 398.—Machine for cutting soap in bars.
Soap is made by combining soda (for hard soap), or potash (for soft soap), with oil or tallow. Yellow soap has turpentine, and occasionally palm oil, added. Oils and fats combine with metallic oxides, and oxide of lead with olive oil and resin forms the adhesive plaister with which we are all familiar when the mixture is spread upon linen. Fats boiled with potash or soda make soaps; the glycerine is sometimes set free and purified as we have it. Sometimes it is retained for glycerine soap. Fancy soap is only common soap coloured. White and brown Windsor are the same soap—in the latter case browned to imitate age! Soap is quite soluble in spirits, but in ordinary water it is not so greatly soluble, and produces a lather, owing to the lime in the water being present in more or less quantity, to make the water more or less “hard.”
Fig. 399.—Soap-boiling house.
Sodium is not unlike potassium, not only in appearance, but in its attributes; it can be obtained from the carbonate, as potassium is obtained from its carbonate. Soda is the oxide of sodium, but the most common and useful compound of sodium is the chloride, or common salt, which is found in mines in England, Poland, and elsewhere. Salt may also be obtained by the evaporation of sea water. Rock salt is got at Salzburg, and the German salt mines and works produce a large quantity. The Carbonate of Soda is manufactured from the chloride of sodium, although it can be procured from the salsoda plants by burning. The chloride of sodium is converted into sulphate, and then ignited with carbonate of lime and charcoal. The soluble carbonate is extracted in warm water, and sold in crystals as soda, or (anhydrous) “soda ash.” The large quantity of hydrochloric acid produced in the first part of the process is used in the process of making chloride of lime. A few years back, soda was got from Hungary and various other countries where it exists as a natural efflorescence on the shores of some lakes, also by burning sea-weeds, especially the common bladder wrack (Fucus vesiculosus), the ashes of which were melted into masses, and came to market in various states of purity. The bi-carbonate of soda is obtained by passing carbonic acid gas over the carbonate crystals. Soda does not attract moisture from the air. It is used in washing, in glass manufactories, in dyeing, soap-making, etc.
Sulphate of Soda is “Glauber’s Salt”; it is also employed in glass-making. Mixed with sulphuric acid and water, it forms a freezing mixture. Glass, as we have seen, is made with silicic acid (sand), soda, potassa, oxide of lead, and lime, and is an artificial silicate of soda.
Fig. 400.—Mottled soap-frames.
Lithium is the lightest of metals, and forms the link between alkaline and the alkaline earth metals. The salts are found in many places in solution. The chloride when decomposed by electricity yields the metal.
Cæsium and Rubidium require no detailed notice from us. They were first found in the solar spectrum, and resemble potassium.
Ammonium is only a conjectural metal. Ammonia, of which we have already treated, is so like a metallic oxide that chemists have come to the conclusion that its compounds contain a metallic body, which they have named hypothetically Ammonium. It is usually classed amongst the alkaline metals. The salts of ammonia are important, and have already been mentioned. Muriate (chloride) of ammonia, or sal-ammoniac, is analogous to chloride of sodium and chloride of potassium. It is decomposed by heating it with slaked lime, and then gaseous ammonia is given off.
The Metals of the Alkaline Earths.
Fig. 401.—Soda furnace.
Barium is the first of the four metals we have to notice in this group, and will not detain us long, for it is little known in a free condition. Its most important compound is heavy spar (sulphate of baryta), which, when powdered, is employed as a white paint. The oxide of barium, BaO, is termed baryta.
Nitrate of Baryta is used for “green fire,” which is made as follows:—Sulphur, twenty parts; chlorate of potassium, thirty-three parts; and nitrate of baryta, eighty parts (by weight).
Calcium forms a considerable quantity of our earth’s crust. It is the metal of lime, which is the oxide of calcium. In a metallic state it possesses no great interest, but its combinations are very important to us. Lime is, of course, familiar to all. It is obtained by evolving the carbonic acid from carbonate of lime (CaO).
The properties of this lime are its white appearance, and it develops a considerable amount of heat when mixed with water, combining to make hydrate of lime, or “slaked lime.” This soon crumbles into powder, and as a mortar attracts the carbonic acid from the air, by which means it assumes the carbonate and very solid form, which renders it valuable for cement and mortar, which, when mixed with sand, hardens. Caustic lime is used in whitewashing, etc.
Carbonate of Lime (CaCO3) occurs in nature in various forms, as limestone, chalk, marble, etc. Calc-spar (arragonite) is colourless, and occurs as crystals. Marble is white (sometimes coloured by metallic oxides), hard, and granular. Chalk is soft and pulverizing. It occurs in mountainous masses, and in the tiniest shells, for carbonate of lime is the main component of the shells of the crustacea, of corals, and of the shell of the egg; it enters likewise into the composition of bones, and hence we must regard it as one of the necessary constituents of the food of animals. It is an almost invariable constituent of the waters we meet with in Nature, containing, as they always do, a portion of carbonic acid, which has the power of dissolving carbonate of lime. But when gently warmed, the volatile gas is expelled, and the carbonate of lime deposited in the form of white incrustations upon the bottom of the vessel, which are particularly observed on the bottoms of tea-kettles, and if the water contains a large quantity of calcareous matter, even our water-bottles and drinking-glasses become covered with a thin film of carbonate of lime. These depositions may readily be removed by pouring into the vessels a little dilute hydrochloric acid, or some strong vinegar, which in a short time dissolves the carbonate of lime.
Sulphate of Lime (CaSO4) is found in considerable masses, and is commonly known under the name of Gypsum. It occurs either crystallized or granulated, and is of dazzling whiteness; in the latter form it is termed Alabaster, which is so soft as to admit of being cut with a chisel, and is admirably adapted for various kinds of works of art. Gypsum contains water of crystallization, which is expelled at a gentle heat. But when ignited, ground, and mixed into a paste with water, it acquires the property of entering into chemical combination with it, and forming the original hydrate, which in a short time becomes perfectly solid. Thus it offers to the artist a highly valuable material for preparing the well-known plaster of Paris figures, and by its use the noblest statues of ancient and modern art have now been placed within the reach of all. Gypsum, moreover, has received a valuable application as manure. In water it is slightly soluble, and imparts to it a disagreeable and somewhat bitterish, earthy taste. It is called “selenite” when transparent.
Phosphate of Lime constitutes the principal mass of the bones of animals, and is extensively employed in the preparation of phosphorus; in the form of ground bones it is likewise used as a manure. It appears to belong to those mineral constituents which are essential to the nutrition of animals. It is found in corn and cereals, and used in making bread; so we derive the phosphorus which is so useful to our system.
Chloride of Lime is a white powder smelling of chlorine, and is produced by passing the gas over the hydrate of lime spread on trays for the purpose. It is the well-known “bleaching powder.” It is also used as a disinfectant. The Fluoride of Calcium is Derbyshire spar, or “Blue John.” Fluor spar is generally of a purple hue. We may add that hard water can be softened by adding a little powdered lime to it.
Magnesium sometimes finds a place with the other metals, for it bears a resemblance to zinc. Magnesium may be prepared by heating its chloride with sodium. Salt is formed, and the metal is procured. It burns very brightly, and forms an oxide of magnesia (MgO). Magnesium appears in the formation of mountains occasionally. It is ductile and malleable, and may be easily melted.
Carbonate of Magnesia, combining with carbonate of lime, form the Dolomite Hills. When pure, the carbonate is a light powder, and when the carbonic acid is taken from it by burning it is called Calcined Magnesia.
The Sulphate of Magnesia occurs in sea-water, and in saline springs such as Epsom. It is called “Epsom Salts.” Magnesium wire burns brightly, and may be used as an illuminating agent for final scenes in private theatricals. Magnesite will be mentioned among Minerals.
Strontium is a rare metal, and is particularly useful in the composition of “red-fire.” There are the carbonate and sulphate of strontium; the latter is known as Celestine. The red fire above referred to can be made as follows, in a dry mixture. Ten parts nitrate of strontia, 1½ parts chlorate of potassium, 3½ parts of sulphur, 1 part sulphide of antimony, and ½ part charcoal. Mix well without moisture, enclose in touch paper, and burn. A gorgeous crimson fire will result.
Metals of the Earths.
Aluminium (Aluminum) is like gold in appearance when in alloy with copper, and can be procured from its chloride by decomposition with electricity. It occurs largely in nature in composition with clays and slates. Its oxide, alumina (Al2O3), composes a number of minerals, and accordingly forms a great mass of the earth. Alumina is present in various forms (see [Minerals]) in the earth, all of which will be mentioned under Crystallography and Mineralogy. The other nine metals in this class do not call for special notice.
Heavy Metals
Iron, which is the most valuable of all our metals, may fitly head our list. So many useful articles are made of it, that without consideration any one can name twenty. The arts of peace and the glories of war are all produced with the assistance of iron, and its occurrence with coal has rendered us the greatest service, and placed us at the head of nations. It occurs native in meteoric stones.
Iron is obtained from certain ores in England and Sweden, and these contain oxygen and iron (see [Mineralogy]). We have thus to drive away the former to obtain the latter. This is done by putting the ores in small pieces into a blast furnace (fig. 402) mixed with limestone and coal. The process of severing the metal from its ores is termed smelting, the air supplied to the furnace being warmed, and termed the “hot blast.” The “cold blast” is sometimes used. The ores when dug from the mine are generally stamped into powder, then “roasted,”—that is, made hot, and kept so for some time to drive off water, sulphur, or arsenic, which would prevent the “fluxes” acting properly. The fluxes are substances which will mix with, melt, and separate the matters to be got rid of, the chief being charcoal, coke, and limestone. The ore is then mixed with the flux, and the whole raised to a great heat; as the metal is separated it melts, runs to the bottom of the “smelting furnace,” and is drawn off into moulds made of sand; it is thus cast into short thick bars called “pigs,” so we hear of pig-iron, and pig-lead. Iron is smelted from “ironstone,” which is mixed with coke and limestone. The heat required to smelt iron is so very great, that a steam-engine is now generally employed to blow the furnace. (Before the invention of the steam-engine, water-mills were used for the same purpose.) The smelting is conducted in what is called a blast furnace. When the metal has all been “reduced,” or melted, and run down to the bottom of the furnace, a hole is made, out of which it runs into the moulds; this is called “tapping the furnace.”
Fig. 402.—Blast furnace.
Smelting is often confounded with melting, as the names are somewhat alike, but the processes are entirely different; in melting, the metal is simply liquefied, in smelting, the metal has to be produced from ores which often have no appearance of containing any, as in the case of ironstone, which looks like brown clay.
The cone of the furnace, A, is lined with fire-bricks, i i, which is encased by a lining, l l; outside are more fire-bricks, and then masonry, m n; C is the throat of the furnace; D the chimney. The lower part, B, is called the boshes. As soon as the ore in the furnace has become ignited the carbon and oxygen unite and form carbonic acid, which escapes, and the metal fuses at last and runs away. The coal and ore are continually added year after year. The glassy scum called “slag “ protects the molten iron from oxidation.
Fig. 403.—General foundry, Woolwich Arsenal.
The metal drawn from the blast furnace is “pig iron,” or “cast” iron, and contains carbon. This kind of iron is used for casting operations, and runs into sand-moulds. It contracts very little when cooling. It is hard and brittle.
Fig. 404.—Wire rollers.
Fig. 405.—Cutting edges.
Bar Iron is the almost pure metal. It is remarkably tenacious, and may be drawn into very fine wire or rolled. But it is not hard enough for tools. It is difficult to fuse, and must be welded by hammering at a red heat. Wire-drawing is performed by taking the metal as a bar, and passing it between rollers (fig. 404), which flattens it, and then between a new set, which form cutting edges on the rolled plate (fig. 405), the projections of one set fitting into the hollows of the other closely as in the illustration. The strips of metal come out at the aperture seen at A in the next illustration. These rods are drawn through a series of diminishing holes in a steel plate, occasionally being heated to keep it soft and ductile. When the wire has got to a certain fineness it is attached to a cylinder and drawn away, at the same time being wound round the cylinder over a small fire. Some metals can be drawn much finer than others. Gold wire can be obtained of a “thickness” (or thinness) of only the 5,000th part of an inch, 550 feet weighing one grain! But platinum has exceeded this marvellous thinness, and wire the 30,000th part of an inch has been produced. Ductility and malleability are not always found in the same metal in proportion. The sizes of wires are gauged by the instrument shown in the margin. The farther the wire will go into the groove the smaller its “size.”
Fig. 406.—Rollers.
Fig. 407.—Wire size.
Steel contains a certain amount of carbon, generally about 1 to 2 per cent. Cast steel is prepared from cast iron. Steel from bar-iron has carbon added, and is termed bar-steel. The process is called “cementation,” and is carried on by packing the bars of iron in brick-work boxes, with a mixture of salt and soot, or with charcoal, which is termed “cement.” Steel is really a carbide of iron, and Mr. Bessemer founded his process of making steel by blowing out the excess of carbon from the iron, so that the proper amount—1·5 per cent.—should remain.
Fig. 408.—Coarse wire-drawing.
A brief summary of the Bessemer process may be interesting. If a bar of steel as soft as iron be made red-hot and plunged into cold water, it will become very hard. If it be then gently heated it will become less hard, and is then fitted for surgical instruments. The various shades of steel are carefully watched,—the change of colour being due to the varying thickness of the oxide; for we know that when light falls upon very thin films of a substance,—soap-bubbles, for instance,—the light reflected from the under and upper surfaces interfere, and cause colour, which varies with the thickness of the film. These colours in steel correspond to different temperatures, and the “temper” of the steel depends upon the temperature it has reached. The following table extracted from Haydn’s “Dictionary of Science” gives the “temper,” the colour, and the uses of the various kinds of steel.
Fig. 409.—Fine wire-drawing.
| Temperature Cent. Fahr. | Colour. | Uses of Steel. |
|---|---|---|
| 220° = 430° | Faint yellow | Lancets. |
| 232° = 450° | Pale straw | Best razors and surgical instruments. |
| 243° = 470° | Yellow | Ordinary razors, pen-knives, etc. |
| 254° = 490° | Brown | Small shears, scissors, cold chisels, etc. |
| 265° = 510° | Brown and purple spots | Axes, pocket-knives, plane-irons, etc. |
| 277° = 530° | Purple | Table-knives, etc. |
| 288° = 550° | Light blue | Swords, watch-springs, etc. |
| 293° = 560° | Full blue | Fine saws, daggers, etc. |
| 316° = 600° | Dark blue | Hand and pit saws. |
The Bessemer process transfers the metal into a vessel in which there are tubes, through which air is forced, which produces a much greater heat than a bellows does. Thus in the process the carbon of the iron acts as fuel to maintain the fusion, and at the same time by the bubbling of the carbonic acid mixes the molten iron thoroughly.
During the bubbling up of the whole mass of iron, and the extreme elevation of temperature caused by the union of the carbon of the impure iron with the oxygen of the air, the oxide of iron is formed, and as fast as it forms fuses into a sort of glass; this unites with the earthy matters of the “impure” iron, and floats on the upper part as a flux, thus ridding the “cast iron” of all its impurities, with no other fuel than that contained in the metal itself, and in the air used. When the flame issuing from the “converter” contracts and changes its colour, then the time is known to have arrived when the iron is “ de-carbonized.” The amount of carbon necessary is artificially added, ebullition takes place, a flame of carbonic oxide comes out, and the metal is then run into ingots.
The compounds of iron which are soluble in water have a peculiar taste called chalybeate (like ink). Many mineral springs are so flavoured, and taste, as the immortal Samuel Weller put it, “like warm flat-irons.” Iron is frequently used as a medicine to renew the blood globules.
Protoxide of Iron is known only in combination.
Sesqui-Oxide of Iron is “red ironstone.” Powdered it is called English rouge, a pigment not altogether foreign to our use. In a pure state it is a remedy for arsenical poisoning, and is really the “rust” upon iron.
Fig. 410.—Bessemer’s process.
Bisulphide of Iron is iron pyrites, and is crystalline.
Chloride of Iron is dissolved from iron with hydrochloric acid. It is used in medicine.
Cyanide of Iron makes, with cyanide of potassium, the well-known prussiate of potash (ferro-cyanide of potassium), which, when heated, precipitates Prussian blue (cyanogen and iron).
The Sulphate of the Protoxide is known as copperas, or green vitriol, and is applied to the preparation of Prussian blue.
Manganese is found extensively, but not in any large quantities, in one place; iron ore contains it. It is very hard to fuse, and is easily oxidised. The binoxide is used to obtain oxygen, and when treated with potassium and diluted, it becomes the permanganate of potassium, and is used as “Condy’s fluid.” It readily oxides organic matters, and is thus a disinfectant. It crystallizes in long, deep, red needles, which are dissolved in water. It is a standard laboratory test. There are other compounds, but in these pages we need not detail them.
Cobalt and Nickel occur together. They are hard, brittle, and fusible. The salts of cobalt produce beautiful colours, and the chloride yields an “invisible” or sympathetic ink. The oxide of cobalt forms a blue pigment for staining glass which is called “smalt.” Nickel is chiefly used in the preparation of German silver and electro-plating. The salts of nickel are green. Nickel is difficult to melt, and always is one of the constituents of meteoric iron, which falls from the sky in aërolites. It is magnetic like cobalt, and is extracted from the ore called kupfer-nickel. A small United-States coin is termed a “nickel.”
Fig. 411.—Native copper.
Copper is the next metal we have to notice. It has been known for centuries. It is encountered native in many places. The Cornish copper ore is the copper pyrites. The fumes of the smelting works are very injurious, containing, as they do, arsenic and sulphur. The ground near the mines is usually bare of vegetation in consequence of the “smoke.” Sheet copper is worked into many domestic utensils, and the alloy with zinc, termed Brass, is both useful and ornamental. Red brass is beaten into thin leaves, and is by some supposed to be “gold leaf”; it is used in decorative work. Bronze is also an alloy of copper, as are gun-metal, bell-metal, etc.
Next to silver, copper is the best conductor of electricity we have. It is very hard and tough, yet elastic, and possesses malleability and ductility in a high degree. It forms two oxides, and there are several sulphides; the principal of the latter are found native, and worked as ores. The sulphate of copper is termed blue vitriol, and is used in calico-printing, and from it all the (copper) pigments are derived. It is also used in solution by agriculturists to protect wheat from insects. When copper or its alloys are exposed to air and water, a carbonate of copper forms, which is termed verdigris. All copper salts are poisonous; white of eggs is an excellent remedy in such cases of poisoning.
Lead is obtained from galena, a sulphide of lead. It is a soft and easily-worked metal. When freshly cut it has quite a bright appearance, which is quickly tarnished. Silver is often present in lead ore, and is extracted by Pattison’s process, which consists in the adaptation of the knowledge that lead containing silver becomes solid, after melting, at a lower temperature than lead does when pure. Pure lead therefore solidifies sooner.
One great use of lead is for our domestic water-pipes, which remind us in winter of their presence so disagreeably. Shot is made from lead, and bullets are cast from the same metal. Shot-making is very simple, and before the days of breech-loading guns and cartridges, no doubt many readers have cast bullets in the kitchen and run them into the mould over a basin of water or a box of sand. For sporting purposes lead is mixed with arsenic, and when it is melted it is poured through a sort of sieve (as in the cut) at the top of a high tower. (See figs. 413 and 414). The latter illustration gives the section of the shot tower; A is the furnace, B is the tank for melting the lead, and the metal is permitted by the workman at C to run through the sieve in fine streams. As the lead falls it congeals into drops, which are received in water below to cool them. They are, of course, not all round, and must be sorted. This operation is performed by placing them on a board tilted up, and under which are two boxes. The round shot rush over the first holes and drop into the second box, but the uneven ones are caught lagging, and drop into box No. 1. They are accordingly sent to the furnace again.
Fig. 412.—Shot tower.
The next process is to sort the good shot for size. This is done by sieves—one having holes a little larger than the size of shot required. This sieve passes through it all of the right size and smaller, and keeps the bigger ones. Those that have passed this examination are then put into another sieve, which has holes in it a little smaller than the size of shot wanted. This sieve retains the right shot, and lets the smaller sizes pass, and so on. The shot are sized and numbered, glazed by rolling them in a barrel with graphite, and then they are ready for use. Bullets are made by machinery by the thousand, and made up into cartridges with great speed.
Fig. 413.—Sieve for making shot.
The compounds of lead are also poisonous, and produce “colic,” to which painters are subject. Red lead, or minium, is a compound of the protoxide and the binoxide, and may be found native. The former oxide is litharge; white lead, or the carbonate of lead, is a paint, and is easily obtained by passing a stream of carbonic acid into a solution of acetate of lead. It is used as a basis of many paints.
Fig. 414.—Section of shot tower.
Tin is another well-known metal. It is mentioned by Moses. It possesses a silver-like lustre, and is not liable to be oxidised. The only really important ore is called Tinstone, from which the oxygen is separated, and the metal remains. Cornwall has extensive tin mines. Tin is malleable and ductile, and can be beaten into foil or “silver leaf,” or drawn into wire. It prevents oxidation of iron if the latter be covered with it, and for tinning copper vessels for culinary purposes. The Romans found tin in Cornwall, and the term “Stanneries” was applied to the courts of justice among the tin miners in Edward the First’s time. We have already mentioned the alloys of tin. The oxides of tin, “Stannous” and “Stannic,” are useful to dyers. The latter is the tinstone (SnO2). Sulphide of tin is called “Mosaic gold,” and is much used for decorative purposes.
Fig. 415.—Preparing lead for bullets.
Zinc is procured from calamine, or carbonate of zinc, and blende, or sulphide of zinc. It has for some years been used for many purposes for which lead was once employed, as it is cheap and light. Zinc is a hard metal of a greyish colour, not easily bent, and rather brittle; but when made nearly red-hot, it can be rolled out into sheets or beaten into form by the hammer. Zinc is about six-and-three-quarter times heavier than water. Like many other metals, it is volatile (when heated to a certain extent it passes off into vapour), and the probable reason that it was not known or used of old is that it was lost in the attempt to smelt its ores. Zinc is now obtained by a sort of distillation; the ores are mixed with the flux in a large earthen crucible or pot.
We have already noticed the alloy of zinc with copper (brass), and the use of zinc to galvanize iron by covering the latter with a coating of zinc in a bath is somewhat analogous to electro-plating. The metal is largely used as the positive element in galvanic batteries, and for the production of hydrogen in the laboratory. Zinc forms one oxide (ZnO), used for zinc-white. The sulphate of zinc is white vitriol, and the chloride of zinc is an “antiseptic.” Certain preparations of the metal are used in medicine as “ointments” or “washes,” and are of use in inflammation of the eyelids.
Chromium. This “metallic element” is almost unknown in the metallic state. But although little known, the beautiful colours of its compounds make it a very interesting study. The very name leads one to expect something different to the other metals—chroma, colour. The metal is procured from what is known as chrome-ironstone, a combination of protoxide of iron and sesqui-oxide of chromium (FeOCr2O3). By ignition with potassium we get chromic acid and chromate of potassium, a yellow salt which is used to make the other compounds of chromium. The metal is by no means easy to fuse.
Sesqui-Oxide of Chromium is a fine green powder employed in painting porcelain.
Chromate of Lead is termed “chrome yellow,” and in its varieties is employed as a paint.
Chromate of Mercury is a beautiful vermilion. There are numerous other combinations which need not be mentioned here.
Fig. 416.—Type-casting.
Antimony was discovered by Basil Valentine. The Latin term is Stibium, hence its symbol, Sb. It is very crystalline, and of a peculiar bluish-white tint. It will take fire at a certain high temperature, and can be used for the manufacture of “Bengal Lights,” with nitre and sulphur in the proportions of antimony “one,” the others two and three respectively.
The compounds of antimony are used in medicine, and are dangerous when taken without advice. They act as emetics if taken in large quantities. Our “tartar emetic” is well known.
Antimony, in alloy with lead and a little tin, form the type metal to which we are indebted for our printing. Type-casting is done by hand, and requires much dexterity. A ladle is dipped into the molten metal, and the mould jerked in to fill it properly, and then the type is removed and the mould shut ready for another type; and a skilful workman can perform these operations five hundred times in an hour,—rather more than eight times a minute,—producing a type each time; this has afterwards to be finished off by others. The metal of which type is made consists of lead and antimony; the antimony hardens it and makes it take a sharper impression. The letters are first cut in steel, and from these “dies” the moulds are made in brass, by stamping, and in these the types are cast.
Stereotype consists of plates of metal taken, by casting, from a forme of type set up for the purpose: an impression was formerly carried on by plaster-of-Paris moulds, but lately what is termed the papier-maché process is adopted. The paper used is now made in England, and the prepared sheet is placed upon the type and beaten upon it. Paste is then filled in where there are blanks, and another and thicker sheet of the prepared paper is placed over all, dried, and pressed. When this is properly done the paper is hardened, and preserves an impression of the type set up. The paper mould is then put into an iron box, and molten metal run in. In a very short time a “stereotype” plate is prepared from the paper, which can be used again if necessary. The metal plate is put on the machine.
There are several compounds of antimony, which, though valuable to chemists, would not be very interesting to the majority of readers. We will therefore at once pass to the Noble Metals.
The Noble Metals.
There are nine metals which rank under the above denomination:—Mercury, Silver, Gold, Platinum, Palladium, Rhodium, Ruthenium, Osmium, Iridium. We will confine ourselves chiefly to the first four on the list.
Mercury, or Quicksilver, is the first of the metals which remain unaltered by exposure to atmospheric air, and thus are supposed to earn their title of nobility. Mercury is familiar to us in our barometers, etc., and is fluid in ordinary temperatures, though one of the heaviest metals we possess. It is principally obtained from native cinnabar, or sulphide of mercury (vermilion), and the process of extraction is very easy. Mercury was known to the ancients, and is sometimes found native. In the mines the evil effects of the contact with mercury are apparent.
This metal forms two oxides,—the black (mercurous) oxide, or suboxide (Hg2O), and the red (mercuric) oxide, or red precipitate. The chlorides are two,—the subchloride, or calomel, and the perchloride, or corrosive sublimate. The sulphides correspond with the oxides; the mercuric sulphide has been mentioned. Its crimson colour is apparent in nature, but the Chinese prepare it in a particularly beautiful form. Many amalgams are made with mercury, which is useful in various ways that will at once occur to the reader.
Silver is the whitest and most beautiful of metals, and its use for our plate and ornaments is general. It is malleable and ductile, and the best conductor of electricity and heat that we have. It is not unfrequently met with in its native state, but more generally it is found in combination with gold and mercury, or in lead, copper, and antimony ores. The mines of Peru and Mexico, with other Western States of America, are celebrated—Nevada, Colorado, and Utah in particular. The story of the silver mine would be as interesting as any narrative ever printed. The slavery and the death-roll would equal in horror and in its length the terrible records of war or pestilence. We have no opportunity here to follow it, or its kindred metals with which it unites, on the sentimental side; but were the story of silver production written in full, it would be most instructive.
Fig. 417.—Native silver.
Silver is found with lead (galena), which is then smelted. The lead is volatilized, and the silver remains. It is also extracted by the following process, wherein the silver and golden ore is crushed and washed, and quicksilver, salt, and sulphate of copper added, while heat is applied to the mass. From tank to tank the slime flows, and deposits the metals, which are put into retorts and heated. The mercury flies off; the silver and gold remain in bars.
In some countries, as in Saxony and South America, recourse is had to another process, that of amalgamation, which depends on the easy solubility of silver and other metals in mercury. The ore, after being reduced to a fine powder, is mixed with common salt, and roasted at a low red heat, whereby any sulphide of silver the ore may contain is converted into chloride. The mixture is then placed, with some water and iron filings, in a barrel which revolves round its axis, and the whole agitated for some time, during which process the chloride of silver becomes reduced to the metallic state. A portion of mercury is then introduced, and the agitation continued. The mercury combines with the silver, and the amalgam is then separated by washing. It is afterwards pressed in woollen bags to free it from the greater part of the mercury, and then heated, when the last trace of mercury volatilizes and leaves the silver behind.
Nitrate of Silver is obtained when metallic silver is dissolved in nitric acid. It is known popularly as lunar caustic, and forms the base of “marking inks.” Chloride of silver is altered by light, but the iodide of silver is even more rapidly acted on, and is employed in photography. Fulminating silver is oxide of silver digested in ammonia. It is very dangerous in inexperienced hands. It is also prepared by dissolving silver in nitric acid, and adding alcohol. It cools in crystals. Fulminating mercury is prepared in the same way.
Gold is the most valuable of all metals,—the “king of metals,” as it was termed by the ancients. It is always found “native,” frequently with silver and copper. Quartz is the rock wherein it occurs. From the disintegration of these rocks the gold sands of rivers are formed, and separated from the sands by “washing.” In Australia and California “nuggets” are picked up of considerable size.
It is a rather soft metal, and, being likewise costly, is never used in an absolutely pure state. Coins and jewellery are all alloyed with copper and silver to give them the requisite hardness and durability. Gold is extremely ductile, and very malleable. One grain of gold may be drawn into a wire five hundred feet in length, and the metal may be beaten into almost transparent leaves 1/200000 of an inch in thickness!
Fig. 418.—Native gold.
Aqua-regia, a mixture of hydrochloric and nitric acids, is used to dissolve gold, which is solved only by selenic acid, though the free chlorine will dissolve it. Faraday made many experiments as to the relation of gold to light. (See “Phil. Trans.,” 1857, p. 145.) The various uses of gold are so well known that we need not occupy time and space in recording them. Gilding can be accomplished by immersing the articles in a hot solution of chloride of gold and bicarbonate of potash mixed; but the electro process is that now in use, by which the gold precipitates on the article to be plated.
We have already described the process of electro-plating in the case of silvered articles, and we need only mention that electro-gilding is performed very much in the same way. But gilding is also performed in other ways; one of which, the so-called water gilding, is managed as follows. Gilding with the gold-leaf is merely a mechanical operation, but water-gilding is effected by chemistry.
Water-gilding is a process (in which, however, no water is used) for covering the surface of metal with a thin coating of gold; the best metal for water-gilding is either brass, or a mixture of brass and copper. A mixture of gold and mercury, in the proportion of one part of gold to eight of mercury, is made hot over a fire till they have united; it is then put into a bag of chamois-leather, and the superfluous mercury pressed out. What remains is called an “amalgam”; it is soft, and of a greasy nature, so that it can be smeared over any surface with the fingers. The articles to be gilt are made perfectly clean on the surface, and a liquid, made by dissolving mercury in nitric acid (aqua-fortis), is passed over them with a brush made of fine brass wire, called a “scratch-brush.” The mercury immediately adheres to the surface of the metal, making it look like silver; when this is done, a little of the amalgam is rubbed on, and the article evenly covered with it. It is then heated in a charcoal fire till all the mercury evaporates, and the brass is left with a coating of gold, which is very dull, but may be burnished with a steel burnisher and made bright if necessary. In former times articles were inlaid with thin plates of gold, which were placed in hollows made with a graver, and melted in, a little borax being applied between.
When a solution of “chloride of gold” is mixed with ether, the ether takes the gold away from the solution, and may be poured off the top charged with it. This solution, if applied to polished steel by means of a camel-hair pencil, rapidly evaporates, leaving a film of gold adhering to the steel, which, when burnished with any hard substance, has a very elegant appearance. In this way any ornamental design in gold may be produced, but it is not very durable. The gilt ornaments, scrolls, and mottoes on sword-blades, are sometimes done in this way.
Platinum is the heaviest of all metals, gold being next. Platinum is practically infusible, and quite indifferent to reagents. It is therefore very useful in certain manufactories, and in the laboratory. It can be dissolved by aqua-regia. The stills for sulphuric acid are made of platinum, and the metal is used for Russian coinage, but must be very difficult to work on account of its infusible property.
Fig. 419.—Döbereiner’s lamp.
In the finely-divided state it forms a gray and very porous mass, which is known as spongy platinum, and possesses the remarkable property of condensing gases within its pores. Hence, when a jet of hydrogen is directed upon a piece of spongy platinum, the heat caused by its condensation suffices to inflame the gas. This singular power has been applied to the construction of a very beautiful apparatus, known as Döbereiner’s lamp, which consists of a glass jar, a, covered by a brass lid, e, which is furnished with a suitable stop-cock, c, and in connection with a small bell jar, f, in which is suspended, by means of a wire, a cylinder of metallic zinc, z. When required for use, the outer jar is two-thirds filled with a mixture of one part sulphuric acid and four parts water, and the stop-cock opened to allow the escape of atmospheric air, the spongy platinum contained in the small brass cylinder, d, being covered by a piece of paper. The stop-cock is then closed, and the bell jar, f, allowed to fill with hydrogen, and after it has been filled and emptied several times, the paper is removed from the platinum and the cock is again opened, when the gas, which escapes first, makes the metal red-hot and finally inflames. This property of platinum is also used in the “Davy” lamp.
The remaining metals do not call for detailed notice.
In conclusion, we may refer to the following statement, which in general terms gives the properties of the metals, their oxides and sulphides for ordinary readers.
General Classification of the Metals.
The metals admit of being really distinguished by the following table, in which they are presented in several groups, according to their peculiar properties, and each distinguished by a particular name:—
| Metals. | Properties of the | |
|---|---|---|
| Oxides. | Sulphides. | |
| (A.) Light Metals. Specific gravity from 0·8 to 1; never occur in the uncombined state. | Powerful bases; possessing a strong affinity for water, and form with it hydrates. They yield their oxygen to carbon only at a white heat. | Powerful bases, which oxidize in the air, and form sulphates; when treated with acids evolve hydrosulphuric acid. |
| (a.) Alkaline Metals. 1. Potassium. 2. Sodium. (Ammonium.) | Highly caustic; powerful bases, separate all other oxides from their combinations with acids; are very soluble in water, and do not lose their water of hydration at the highest temperatures; attract carbonic acid rapidly from the air. | Caustic; strong bases; very soluble in water, and dissolve a large quantity of sulphur, which is separated on addition of an acid as a white powder, termed milk of sulphur; they were formerly termed liver of sulphur. |
| (b.) Metals of the Alkaline Earths. 3. Calcium. 4. Barium. 5. Strontium. | Caustic; strong bases; slightly soluble in water; lose their water of hydration at a moderate heat, and powerfully absorb carbonic acid. | Caustic; strong bases; dissolve sulphur, and are partly soluble in water, and partly insoluble. |
| (c.) Metals of the Earths proper. 6. Magnesium. 7. Aluminium. | Weak bases, Feebly caustic, or not caustic. insoluble in water. | Insoluble in water. |
| (B.) Heavy Metals. Specific gravity from 5 to 21; are found chiefly in combination with oxygen, and frequently with sulphur and arsenic; some are native. | Feebler bases than the foregoing, some are acids; insoluble in water, and lose their water of hydration at a moderate heat. | Neutral compounds; insoluble in water; antimony and several of the rarer metals produce compounds with sulphur, which deport themselves as acids. |
| (a.) Common Metals. Become oxidized in the air. 8. Iron. 9. Manganese. 10. Cobalt. 11. Nickel. 12. Copper. 13. Bismuth. 14. Lead. 15. Tin. 16. Zinc. 17. Chromium. 18. Antimony. | With few exceptions, are soluble in acids, and, when ignited with carbon at a red heat, yield their oxygen; are, for the most part, fusible and non-volatile. | Those occurring in nature are somewhat brass-like in appearance, and are termed pyrites and blendes. Those which are artificially prepared have peculiarcolours; by heat they are converted into sulphates. |
| (b.) Noble Metals. Unchangeable in the air. 19. Mercury. 20. Silver. 21. Gold. 22. Platinum. | Have more the properties of acids than of bases; are decomposed by ignition into oxygen and metal. | With the exception of sulphide of mercury, they leave the pure metal when ignited. |
CHAPTER XXX.
ORGANIC CHEMISTRY.
RADICALS—ACIDS—BASES—NEUTRALS.
In the introduction to these brief chapters upon Chemistry, we said that the science was divided into two sections, the first section consisting of the simple combinations, and the other of compound combinations. The latter being met with chiefly in animal and vegetable matter, as distinguished from dead or inert matter, was termed Organic. This distinction will be seen below.
| COMBINATIONS OF SIMPLE GROUPS. | COMBINATIONS OF COMPOUND GROUPS. |
|---|---|
| Inorganic. | Organic. |
| I. Elements and their Combinations. (1) Non-Metallic. (2) Metallic. | I. Compound Radicals and their Combinations. |
| II. Peculiar Decompositions of the above. (1) By Electricity. (2) By Light. | II. Peculiar Decompositions of the above. (1) Spontaneous. (2) Dry Distillation. |
We have already placed before our readers the elements and their simple combinations, and have incidentally mentioned the decomposition by electricity and by light. In the section upon Electricity the positive and negative poles are explained. Oxygen appears always at the positive pole, potassium at the negative. The other simple bodies vary. Chlorine, in combination with oxygen, is evolved at the negative pole, but when with hydrogen at the positive pole. In the series below each element behaves electro-negatively to those following it, and electro-positively to those above it; and the farther they are apart the stronger their opposite affinities are.
Electrical Relation of the Elements.
- Oxygen.
- Sulphur.
- Nitrogen.
- Chlorine.
- Bromine.
- Iodine.
- Fluorine.
- Phosphorus.
- Arsenic.
- Carbon.
- Chromium.
- Boron.
- Antimony.
- Silicium.
- Gold.
- Platinum.
- Mercury.
- Silver.
- Copper.
- Bismuth.
- Lead.
- Cobalt.
- Nickel.
- Iron.
- Zinc.
- Hydrogen.
- Manganese.
- Aluminium.
- Magnesium.
- Calcium.
- Strontium.
- Barium.
- Sodium.
- Potassium.
The importance of these facts to science is unmistakable, and, indeed, many attempts have been made to explain, from the electrical condition of the elements, the nature of chemical affinity, and of chemical phenomena in general.
Electrotyping is another instance of decomposition by means of electricity, and respecting decomposition by light we know how powerful the action of the sun’s rays are upon plants, and for the evolution of oxygen. The daguerreotype and photographic processes are also instances which we have commented upon. So we can pass directly to the consideration of the compound groups.
In nearly every complex organic compound we have a relatively simple one of great stability, which is termed the radical, which forms, with other bodies, a compound radical.[22] In these complex groups we find certain elements generally,—viz., carbon, hydrogen, nitrogen, sulphur, and phosphorus. Some compounds may consist of two of these, but the majority contain three (hydrogen, oxygen, and carbon). Many have four (carbon, oxygen, hydrogen, and nitrogen), and some more than four, including phosphorus and sulphur. Others, again, may contain chlorine and its relatives, arsenic, etc., in addition. Now we will all admit that in any case in which carbon is present in composition with other simple bodies forming an organic body, and if that body be ignited in the air, it burns and leaves (generally) a black mass. This is a sure test of the presence of carbon, and forms an organic compound. Similarly in decomposition nitrogen and sulphur in combination inform us they are present by the odour they give off. We need not go farther into this question of radicals and compound radicals than to state that a compound radical plays the part of an element in combination. We find in alcohol and ether a certain combination termed Ethyl. This “compound radical” occurs in same proportions in ether, chloride of ethyl, iodide of ethyl, etc., as C2H5; so it really acts as a simple body or element, though it is a compound of carbon and hydrogen. A simple radical is easily understood; it is an element, like potassium, for instance. We may now pass to the organic combinations classified into Acids, Bases, and Indifferent, or Neutral, Bodies.
I. Acids.
There are several well-known organic acids, which we find in fruits and in plants. They are volatile and non-volatile; acids are sometimes known as “Salts of Hydrogen.” We have a number of acids whose names are familiar to us,—viz., acetic, tartaric, citric, malic, oxalic, tannic, formic, lactic, etc.
Acetic acid (HC2H4O2) is a very important one, and is easily found when vegetable juices, which ferment, are exposed to the air, or when wood and other vegetable matter is subjected to the process of “dry distillation.” Vinegar contains acetic acid, which is distilled from wood, as we shall see presently. Vinegar is made abroad by merely permitting wine to get sour; hence the term Vin-aigre. In England vinegar is made from “wort,” of malt which is fermented for a few days, and then put into casks, the bung-holes of which are left open for several weeks, until the contents have become quite sour. The liquid is then cleared by isinglass. The vinegar of commerce contains about 6 per cent. of pure acetic acid, and some spirit, some colouring matter, and, of course, water. Wood vinegar (pyroligneous acid) is used for pickles. The ordinary vinegar when distilled is called white vinegar, and it may also be obtained from fruits, such as gooseberries or raspberries.
Fig. 420.—Vinegar ground.
Fig. 421.—Boiler or copper.
Acetic acid, or “wood vinegar,” is prepared as follows:—There are some large iron cylinders set in brickwork over furnaces, and these cylinders have each a tube leading to a main pipe in which the liquid is received for condensation. The cylinders, which contain about seven or eight hundredweight, are filled with logs of wood, either oak, beech, birch, or ash, the door is closely fastened, and the joints smeared with clay; the fires are now lighted and kept up all day, till the cylinders are red-hot; at night they are allowed to cool. In the morning, the charcoal, into which the wood is now converted, is withdrawn, and a fresh charge supplied; it is then found that about thirty or forty gallons of liquid has condensed in the main tube from each cylinder, the remainder being charcoal and gases which pass off; the liquid is acid, brown, and very offensive, and contains acetic acid, tar, and several other ingredients, among which may be named creosote; it is from this source all the creosote, for the cure of toothache, is obtained. To purify this liquid it is first distilled, and this separates much of the tar; it is then mixed with lime, evaporated to dryness, and heated to expel the remaining tar and other impurities; it is next mixed with sulphate of soda and water, and the whole stirred together; the soda, now in union with the acetic acid, is washed out from the lime and strained quite clear; it is afterwards evaporated till it crystallises, and vitriol (sulphuric acid) then added; finally the acetic acid is distilled over, and the sulphuric acid left in union with the soda, forming sulphate of soda, to be used in a similar process for the next batch of acid. The acetic acid is now quite colourless, transparent, and very sour, possessing a fragrant smell. This is not pure acetic acid, but contains a considerable quantity of water. The acetic acid of commerce, mixed with seven times its bulk of water, forms an acid of about the strength of malt vinegar, perfectly wholesome, and agreeable as a condiment.
Fig. 422.—Vinegar-cooling process.
Pure acetic acid may be made by mixing dry acetate of potash with oil of vitriol in a retort, and distilling the acetic acid into a very cold receiver; this, when flavoured with various volatile oils, forms the aromatic vinegar sold by druggists. It is a very strong acid, and if applied to the skin will quickly blister it.
Fig. 423.—Tan-yard and pits.
Acetate of lead, or sugar of lead, is obtained by dissolving oxide of lead in vinegar. A solution of this salt makes the goulard water so familiar to all. Acetate of lead is highly poisonous.
Acetate of copper is verdigris, and poisonous. Other acetates are used in medicine.
We may pass quickly over some other acids. They are as follows:—
Tartaric Acid (C4H6O6) is contained in grape juice, and crystallizes in tabular form. The purified powdered salt is cream of Tartar.
Citric Acid (C6H8O7) is found native in citrons and lemons, as well as in currants and other fruits. It is an excellent anti-scorbutic.
Malic Acid (C4H6O5) is found chiefly in apples, as its name denotes (malum, an apple). It is prepared from mountain-ash berries.
Oxalic Acid (C2H2O4). If we heat sugar with nitric acid we shall procure this acid. It is found in sorrel plants.
Tannic Acid (C27H22O17). It is assumed that all vegetables with an astringent taste contain this acid. Tannin is known for its astringent qualities. The name given to this acid is derived from the fact that it possesses a property of forming an insoluble compound with water, known as leather. Tanning is the term employed. Tannin is found in many vegetable substances, but oak bark is usually employed, being the cheapest. The “pelts,” hides, or skins, have first to be freed from all fat or hair by scraping, and afterwards soaking them in lime and water. Then they are placed in the tan-pit between layers of the bark, water is pumped in, and the hides remain for weeks, occasionally being moved from pit to pit, or relaid, so as to give all an equal proportion of pressure, etc. The longer the leather is tanned—it may be a year—the better it wears.
Skins for gloves and binding are tanned with “sumach,” or alum and salt. Sometimes the leather is split by machinery for fine working. Parchment is prepared from the skins of asses, sheep, goats, and calves, which are cleaned, and rubbed smooth with pumice stone.
Tannic acid, with oxide of iron, produces Ink, for the gall-nut contains a quantity of the acid. All the black inks in use generally are composed of green vitriol (sulphate of iron) in union with some astringent vegetable matter; the best is the gall-nut, although, for cheapness, logwood and oak bark have each been used. An excellent black ink may be made by putting into a gallon stone bottle twelve ounces of bruised galls, six ounces of green vitriol, and six of common gum, and filling up the bottle with rain water; this should be kept three or four weeks before using, shaking the bottle from time to time.
Blue ink has lately been much used; it is made by dissolving newly-formed Prussian blue in a solution of oxalic acid. To make it, dissolve some yellow prussiate of potash in water in one vessel, and some sulphate of iron in another, adding a few drops of nitric acid to the sulphate of iron; now mix the two liquids, and a magnificent blue colour will appear, in the form of a light sediment; this is to be put upon a paper filter, and well washed by pouring over it warm water, and allowing it to run through; a warm solution of oxalic acid should now be mixed with it, and the Prussian blue will dissolve into a bright blue ink.
Fig. 424.—Unhairing the hide.
Red ink is made by boiling chips or raspings of Brazil wood in vinegar, and adding a little alum and gum; it keeps well, and is of a good colour. A red ink of more beautiful appearance, but not so durable, may be made by dissolving a few grains of carmine in two or three teaspoonfuls of spirit of hartshorn.
Marking ink is made by dissolving nitrate of silver in water, and then adding some solution of ammonia, a little gum water, and some Indian ink to colour it. Printers’ ink is made by grinding drying oil with lamp-black.
The powdered gall-nut is an excellent test for iron in water. It will turn violet if any iron be present.
Fig. 425.—Drying rooms for hides.
Formic Acid (CH2O2) is the caustic means of defence employed by ants, hence the term formic. It can be artificially prepared by distilling a mixture of sugar, binoxide of manganese, and sulphuric acid. On the skin it will raise blisters.
Lactic Acid (C3H6O3) is present in vegetable and animal substances. Sour whey contains it, and the presence of the acid in the whey accounts for its power of removing from table-linen stains. When what is called “lactic fermentation” occurs, milk is said to be “turned.”
II.
Bases.
The definition of a base is not easy. We have described bases as substances which, combining with acids, form salts, but the definition of a base is as unsatisfactory as that of acid or salt. All vegetable bases contain nitrogen, are usually very bitter, possess no smell or colour, and are insoluble in water. They are usually strong poisons, but very useful in medicine.
The most important are the following bases:—
Quinine is contained in the cinchona (yellow) bark. One hundred parts of the bark have been calculated to yield three of quinine.
Morphine is the poisonous base of opium, which is the juice of the poppy, and is prepared chiefly in India and China.
Nicotine is the active principle of tobacco, and varies in quantity in different tobaccos. Havannah tobacco possesses the least. It is a powerful poison, very oily, volatile, and inflammable.
Conia is prepared from the hemlock. It is fluid and volatile. It is also a deadly poison, and paralyses the spine directly.
Fig. 426.—Hemlock.
Strychnine is found in poisonous trees, particularly in the nux-vomica seeds of Coromandel. It produces lock-jaw and paralysis. There is no antidote for strychnine; emetics are the only remedy.
The above are chiefly remarkable for their uses in medicine, and in consequence of their highly poisonous character are best left alone by unpractised hands.
A German chemist, named Serturner, was the first to extract the active principle from Opium. The question of opium importation has lately been attracting much attention, and the opinions concerning its use are divided. Probably in moderation, and when used by ordinary people (not demoralized creatures), it does little harm.
Fig. 427.—The Poppy.
Opium is the juice of the “common” poppy, and derives its name from the Greek opos, juice. The plant is cultivated in India, Persia, and Turkey. After the poppy has flowered the natives go round, and with a sharp instrument wound, or puncture, every poppy head. This is done very early in the morning, and under the influence of the sun during the day the juice oozes out. Next morning the drops are scraped off. The juice is then placed in pots, dried, and sent for export. The “construction” of opium is very complicated, for it contains a number of ingredients, the most important being morphia, narcotine, meconic acid, and codeia. It is to the first named constituent that the somnolent effect of opium is due.
III.
Indifferent Substances.
There are a great number of so-called “indifferent” substances to which we cannot be indifferent. Such bodies as these have neither acid nor basic properties, and stand no comparison with salts. They are of great importance, forming, as they do, the principal nutriment of animals. Some contain nitrogen, some do not; they may therefore be divided into nitrogenous and non-nitrogenous substances; the former for solid portions of the body, the latter for warmth.
We will take the latter first, and speak of some of them—such as starch, gum, sugar, etc.
Starch is found in the roots of grain, in the potato, dahlia, artichoke, etc., and by crushing the parts of the plant, and washing them, the starch can be collected as a sediment. In cold water and in spirits of wine starch is insoluble. The various kinds of starch usually take their names from the plants whence they come. Arrowroot is obtained from the West Indian plant Maranta Arundinacea. Cassava and tapioca are from the manioc; sago, from the sago palm; wheat starch, and potato starch are other examples.
Fig. 428.—Plantation of sugar-canes.
If starch be baked in an oven at a temperature of about 300° it becomes, to a great extent, soluble in cold water, forming what is called “British gum”; this is largely used for calico printing and other purposes; if boiled in water under great pressure, so that the temperature can be raised to the same degree, it is also changed into an adhesive sort of gum, “mucilage”; this is the substance made use of by the government officials to spread over the backs of postage and receipt stamps to make them adhere. The starch of grain, during germination, or growth, contains diastase, which converts the starch into gum and sugar; the same effect can be produced by heating starch with diluted sulphuric acid.
Gum found in plants is chiefly procured from the Mimosa trees, from which it flows in drops, and is called Gum Arabic. There are other so-called “gums,” but this is the one generally referred to.
Sugar exists in fruits, roots, and in the stalks of plants, in the juice of the cane, maple, and beet-root particularly. The canes are crushed, the juice is clarified with lime to prevent fermentation, and the liquid is evaporated. It is then granulated and cleared from the molasses. Sugar, when heated, becomes dark, and is called “caramel.” It is used for colouring brandy, and gives much difficulty to the sugar refiners.
Fig. 429.—Refining vacuum pan.
Fig. 430.—Sugar moulds.
Fig. 431.—Turning the loaves.
Sugar refining is conducted as follows. The raw (brown) sugar is mixed into a paste with water, and allowed to drain. The sugar thus becomes white. It is then dissolved in water, with animal charcoal and bullocks’ blood. The liquid is boiled, and put into a dark cistern with holes at the bottom, and cotton fibres being fastened in the holes, are hung into another dark cistern, into which the liquid runs pure and white. It is then pumped into a copper vessel,—vacuum pan,—and condensed to the proper consistence. Subsequently it is poured into conical moulds, and pure syrup poured upon the crystal shapes. The caramel is then removed through a hole at the end. The moulds or loaves are then dried, and if not even or elegant they are turned in a lathe. Finally they are packed up as “loaf sugar.” Sugar undergoes no decomposition, and is the cause of non-decomposition in other substances. For this reason it is employed in “preserving” fruit, etc. Sugar is obtained from beet by crushing and rasping the roots, as the cane is treated.
Spirit of Wine, or Alcohol, is not a natural product. It is found by the decomposition of grape-sugar by fermentation. There is a series of alcohols which exhibit a regular gradation, founded, so to speak, upon one, two, or three molecules of water. They are called respectively alcohols, glycols, and glycerins. Thus we have—
- Alcohols.
- Methylic alcohol.
- Ethylic “
- Prophylic “
- Amylic “
- Glycols.
- Enthelein glycol.
- Prophylene “
- Butylene “
- Amylene “
- Glycerins.
- (Ordinary Glycerine is the only one known.)
The cetyl and melissylic alcohols are contained in spermaceti and bees-wax respectively. The usual alcohol is the Vinic, a transparent, colourless liquid, which is the spirituous principle of wine, spirits, and beer, and when sugar is fermented the alcohol and carbonic acid remain.
Spirits of wine has a very powerful affinity for water, and thus the use of stimulants in great quantity is to be deprecated, for alcohol absorbs the water from the mucous membranes of the stomach and the mouth, making them dry and hard. The state of “intoxication,” unfortunately so familiar, is the effect produced by alcohol upon the nerves. We append a list of the beverages which are most in use, and the percentage of alcohol in each according to Professor Hart:—
| Port | 15 | per cent. |
| Madeira | 14·5 | ” |
| Sherry | 14 | ” |
| Claret | 8 | ” |
| Ale | 6 | ” |
| Porter | 5 | ” |
Fig. 432.—Hydrometer.
Spirit of wine is contained in many mixtures, and for the purpose of ascertaining how much alcohol may be in wine, or any other liquid, a hydrometer is used (fig. 432). This instrument consists of a glass tube with a bulb at the end. It is put into water, and the place the water “cuts” is marked by a line on the stem, and called zero 0°. Spirit of wine has less specific gravity than water, so in absolute alcohol the instrument will sink lower than in water, and will descend to a point which is marked 100. In any mixture of alcohol and water, of course the hydrometer will rise or sink between the extreme points accordingly as the mixture may contain less alcohol or more. So a scale can be furnished. The instrument, as described, was invented by MM. Gay-Lussac and Tralles, and called the “percentage” hydrometer. There are many other instruments marked in a more or less arbitrary manner. We append a comparative table of a few hydrometers. (See [page 420].)
Ether, or sulphuric ether, is a mixture of spirits of wine with sulphuric acid, and distilled. It loses water, and the product is ether, which is volatile, and transparent, with a peculiarly penetrating odour. It will not mix with water, and if inhaled will produce a similar effect to chloroform.
Comparative Table of Hydrometers.
| Specific Gravity. | Percentage Volume (Tralles). | Percentage Weight, at 60° F. | Degree, according to Cartier. | Degree, according to Beck. | Degree, according to Baumé. |
|---|---|---|---|---|---|
| 1·000 | 0 | 0 | 10 | 0 | 10 |
| 0·991 | 5 | 4·0 | ... | ... | ... |
| 0·985 | 10 | 8·0 | 12 | ... | ... |
| 0·980 | 15 | 12·1 | ... | 3 | 13 |
| 0·975 | 20 | 16·2 | ... | ... | ... |
| 0·970 | 25 | 20·4 | 14 | 5 | ... |
| 0·964 | 30 | 24·6 | 15 | 6 | 15 |
| 0·958 | 35 | 28·9 | ... | ... | 16 |
| 0·951 | 40 | 33·4 | ... | 9 | 17 |
| 0·942 | 45 | 37·9 | 18 | ... | ... |
| 0·933 | 50 | 42·5 | ... | 12 | 20 |
| 0·923 | 55 | 47·2 | 21 | 14 | ... |
| 0·912 | 60 | 52·2 | ... | 16 | 24 |
| 0·901 | 65 | 57·2 | 24 | 19 | ... |
| 0·889 | 70 | 62·5 | 27 | ... | 28 |
| 0·876 | 75 | 67·9 | ... | 24 | ... |
| 0·863 | 80 | 73·5 | 30 | 27 | 32 |
| 0·848 | 85 | 79·5 | 35 | 30 | 35 |
| 0·833 | 90 | 85·7 | ... | 34 | 38 |
| 0·815 | 95 | 92·4 | 40 | 38 | 42 |
| 0·793 | 100 | 100·0 | 44 | 44 | 48 |
Chloroform is transparent, and will sink in water. Diluted alcohol with hypo-chloride of lime, will produce it. When inhaled, chloroform produces a pleasing insensibility to pain, and is useful in surgery.
A certain compound of alcohol with mercury dissolved in nitric acid will cause decomposition, and white crystals will eventuate. These compound crystals are termed fulminating mercury.
We must now pass rapidly over the few remaining subjects we have to notice, such as fats and soaps, wax, oils, etc.
Fats are of the greatest use to man, particularly in cold climates, for upon them depends the heat of the body. Fatty acid, if liquid, is known as oleic acid; if solid, stearic acid. Soaps are compounds of fatty acids. Many “fats” are consumed as food, others as fuel or for lighting purposes, in the shape of oils. Such oils are not primarily useful for burning. Petroleum and other mineral oils are found in enormous quantities in America.
There are what we term fixed oils, and essential or volatile oils. A list is annexed as given by “Hadyn’s Dictionary of Science”:—
Fixed Oils.
- Drying.
- Linseed oil.
- Poppy oil.
- Sunflower oil.
- Walnut oil.
- Tobacco-seed oil.
- Cress-seed oil.
- Non-Drying.
- Almond oil.
- Castor oil.
- Colza oil.
- Oil of mustard.
- Rape-seed oil.
- Olive oil, etc.
Essential Oils.
- Oil of anise.
- Oil of bergamot.
- Oil of carraway.
- Oil of cassia.
- Oil of cedar.
- Oil of cloves.
- Oil of lavender.
- Oil of lemon.
- Oil of mint.
- Oil of myrrh.
- Oil of nutmeg.
- Oil of peppermint.
- Oil of rose.
- Oil of turpentine.
Vegetable oils are obtained by crushing seeds; animal oils come from the whale and seal tribe. Paraffin oil comes from coal. Linseed is a very drying oil, and on it depends the drying power of paint. We know olive oil will not dry on exposure to the air. Oiled silk is made with linseed oil. When oil is drying in the air considerable heat is evolved, and if oiled substances be left near others likely to catch fire, spontaneous combustion may ensue. Oil of turpentine is found in the pine and fir trees, and many of the oils above mentioned are used by perfumers, etc., the rose oil, or attar of roses, being an Eastern compound.
Fig. 433.—Crushing mill.
Allied to the volatile oils are the RESINS, which are non-conductors of electricity. They are vegetable products. They are soluble in alcohol, in the volatile oils, or in ether, and these solutions are called varnishes; the solvent evaporates and leaves the coating. Turpentine, copal, mastic, shellac, caoutchouc, and gutta-percha are all resinous bodies. Amber is a mineral resin, which was by the ancients supposed to be the “tears of birds” dropped upon the seashore. Moore refers to this in his poetic “Farewell to Araby’s Daughter”—
“Around thee shall glisten the loveliest amber
That ever the sorrowing sea-bird has wept.”
Amber is not soluble either in water or alcohol; it is, however, soluble in sulphuric acid. It takes a good polish, and when rubbed is very electrical. It is composed of water, an acid, some oil, and an inflammable gas, which goes off when the amber is distilled.
The well-known camphor is got from a tree called the “Laurus Camphora”; it is a white, waxy substance, and can be obtained by oxidizing certain volatile oils. It is generally produced from the Laurus Camphora in a “still.” The behaviour of a piece of camphor in pure water is curious, but its motions can be at once arrested by touching the water or dropping oil on the surface. This phenomenon is due to the surface tension of the liquid, which diminishes when it is in contact with the vapour of the substance.
Nitrogenous Substances.
There are certain albuminous compounds which we must mention here. These are albumen, fibrine, and caseine. Albumen is the white of egg; fibrine is, when solid, our flesh and muscular fibre, while caseine is the substance of cheese. These are very important compounds, and the albuminous bodies are of the very highest importance as food, for the solid portion of blood, brain, and flesh consist, in a great measure, of them. Albumen, fibrine, and caseine contain carbon, hydrogen, nitrogen, and oxygen, with sulphur and phosphorus.
Albumen. The most familiar and the almost pure form of albumen is in the white of eggs, which is albuminate of sodium. It also exists in the serum of the blood, and therefore it is largely found in the animal kingdom. It can also be extracted from seed or other vegetable substances, but it is essentially the same. Albumen is very useful as an antidote to metallic poisons. It forms about 7 per cent. of human blood. It is soluble up to about 140° Fah.; it then solidifies, and is precipitated in a white mass. Albumen is used in the purification of sugar, etc.
Fibrine is found in a liquid condition in blood. The vegetable fibrine (gluten) is prepared by kneading wheat flour in a bag till the washings are no longer whitened. Like albumen it is found both in a solid and liquid state.
Caseine is seen in the skin which forms upon milk when heated, and forms about 3 per cent. of milk, where it exists in a soluble state, owing to the presence of alkali; but caseine, like albumen, is only soluble in alkaline solutions. As we have said, it is the principal constituent of cheeses. Caseine is precipitated by the lactic acid of milk, which is produced by keeping the milk too warm. Caseine, or curds, as they are called, are thus precipitated. The milk is said to be “sour,” or turned.
Milk, the food of the young of all mammalia, is composed chiefly of water, a peculiar kind of sugar, butter, and caseine. It is this sugar in milk which causes the lactic acid mentioned above. The actual constituents of milk are as follows:—
| Water | 873·00 |
| Butter | 30·00 |
| Sugar | 43·90 |
| Caseine | 48·20 |
| Calcium (phosphate) | 2·31 |
| Magnesia | 0·42 |
| Iron | 0·07 |
| Potassium (chloride) | 1·44 |
| Sodium | 0·24 |
| Soda (with caseine) | 0·42 |
| 1000·00 |
The sugar of milk is non-fermenting, and can be procured from whey by evaporation.
Decomposition.
We have seen that animals and plants are composed of many different substances, and so it will be at once understood that these substances can be separated from each other, and then the decomposition of the body will be completed. When the sap sinks or dries up in plants they are dead. When our heart ceases to beat and our blood to flow we die, and then, gradually but surely, decay sets in. There is no fuel left to keep the body warm; cold results, and the action of oxygen of the air and light or water decays the body, according to the great and unalterable laws of Nature. “Dust thou art, and unto dust shalt thou return,” is an awful truth. The constituents of our bodies must be resolved again, and the unfailing law of chemical attraction is carried out, whereby the beautiful organism, deprived of the animating principle, seeks to render itself into less complicated groups and their primary elements.
This resolution of the organic bodies is decomposition, or “spontaneous decomposition,” and is called decay, fermentation, or putrefaction, according to circumstances. The Egyptians, by first drying the bodies of the dead (and then embalming them), removed one great source of decay—viz., water, and afterwards, by the addition of spices, managed to arrest putrefaction.
Fermentation is familiar in its results, which may be distilled for spirituous liquors, or merely remain fermented, as beer and wine. Fusel oil is prepared from potatoes, rum from cane sugar, arrack from rice. The power of fermentation exists in nature everywhere, and putrefaction is considered to be owing to the presence of minute germs in the atmosphere, upon which Professors Tyndall and Huxley have discoursed eloquently.
Plants are subjected to a process of decomposition, which has been termed “slow carbonization,” under certain circumstances which exclude the air. The gases are given off, and the carbon remains and increases. Thus we have a kind of moss becoming peat, brown coal, and coal. The immense period during which some beds of coal must have lain in the ground can only be approximately ascertained, but the remains found in the coal-measures have guided geologists in their calculations.
Having already mentioned some products of distillation, we may now close this portion of the subject and pass on to a brief consideration of minerals and crystals. We have left many things unnoticed, which in the limited space at our disposal we could not conveniently include in our sketch of chemistry and chemical phenomena.
CHAPTER XXXI.
MINERALOGY AND CRYSTALLOGRAPHY.
THE MINERALS—CHARACTERISTICS—CRYSTALS AND THEIR FORMS—DESCRIPTIONS OF MINERALS.
Minerals are constituent parts of the earth. All parts of minerals are alike. There are simple minerals and mixed. The former are the true minerals, and are generally considered under the heading Mineralogy. The others constitute a branch of Geology, as they form aggregate masses, and as such compose a large portion of the earth. We must learn to distinguish minerals and crystals as inorganic forms of nature. In the animal and vegetable kingdoms we have forms which are possessed of organs of sight, smell, taste, and certain structures indispensable to their existence and development. But in minerals we have no such attributes. They are INORGANIC, and have a similar structure; a fragment will tell us the story as well as a block of the same mineral. These inorganic substances are possessed of certain attributes or characteristics. We find they have FORM. They have chemical properties, and they behave differently when exposed to light and electricity. They are generally solid. All the elements are found in the mineral kingdom, and a mineral may be an element itself, or a chemical combination of elements. These compounds are classed according as the combination is more or less simple. An alliance of two elements is termed a binary compound, of three a ternary compound, forming a base and an acid.
We have learnt from our chemistry paper that there are between sixty and seventy elementary bodies in nature. When we speak of “elements,” we do not mean to apply the popular and erroneous definition of the word. Earth, air, fire, and water are not elements; they are compounds, as we have seen. The list of elements has been given; we will now give the names of the more important minerals. We have no space for a detailed description, but in the British Museum the cases contain some hundreds, and the student will find them classified and described with the greatest care, and according to the arrangement of Berzelius.
Principal Minerals as arranged by Professor Ansted.
The above is the arrangement best suited for beginners.
Professor Nichol prefers the following arrangement:—
These are only a portion of the minerals, but it would be scarcely interesting to give the list at greater length. In the foregoing we recognize the metals and various combustible and non-combustible substances familiar to us, existing, as people say sometimes, in “lumps.” But if any one will take the trouble to examine a “lump,” he will find the shape is definite and even. These regular forms of the minerals are called CRYSTALS, from the Greek word krustallos, ice. The term was originally applied to quartz, for in olden times it was thought that quartz was really congealed water. We can define a crystal as “an inorganic solid bounded by plane surfaces arranged round imaginary lines known as axes.” It must not be imagined that crystals are small bodies; they may be of any size. There are crystals of many hundredweight; and although the usual crystal is comparatively small, it may be any size.
Crystallization has occurred by cooling, or by other natural means; and we can form crystals by evaporation from certain salts deposited in water. So we may conclude also that the evaporation of water in the early periods deposited many forms of crystals. We have crystals in the air, such as snowflakes, which are vapours crystallized. Carbon, when crystallized, is the diamond. Boron is very like it. Oxygen cannot be crystallized. Alumina makes sapphires and ruby with silica. Alumina and earth give us spars, tourmaline, and garnets. Limestone also has beautiful forms, as in Iceland spar. Crystals, therefore, are certain forms of nature, corresponding in the inorganic kingdom to the animals and plants of the organic.
Let us look a little more at these. Here we have a group of crystals of different forms. Earths are metals combined with oxygen, and the principal earths are alumina, lime, and silica. To these three we are chiefly indebted for the ground we live on, and from which we dig so many useful metals and other minerals. Earths are coloured by the substances mixed with them. We can thus find copper, silver, gold, lead, etc., by noting the appearance of the soil. True earths are white. Strontia and baryta are also earths, and the latter is used in firework manufactories. Our chief assistants are Alumina, which furnishes us with bricks and slate; Lime, which gives us marble or stones for building in a carbonate form. Quicklime, by which is meant lime freed from the carbonic acid, is well known; and plaster of Paris is only lime and sulphuric acid in combination. The Silicates, such as sand and flint, are in daily demand. Agate, cornelian, Scotch pebbles, rock-crystal, etc., belong to the same family. Even our gems are crystallized earths, and, as already stated, diamonds are merely carbon.
Stone, as we know, is quarried; that is, it is dug out of the earth. But perhaps many readers do not know why a stone-mine is called a “quarry.” Most kinds of stone (granite and marble are the exceptions) are found in layers, or strata, rendering them easy of removal. The blocks of stone are cut with reference to these layers in a more or less square manner, and “squared up” before they are carried away. Thus the term “quarry,” from an old French word, quarré, or carré, as now written, signifying a square. In granite quarries the stone being very hard is bored, and loosened by means of gunpowder or dynamite blasting. Slate, on the contrary, is easily divided into slabs. We will now resume the subject of Crystals.
Fig. 434.
1.—Emerald. 3.—Garnet. 5.—Diamond.
2.—Agate. 4.—Ruby. 6.—Rock crystal.
We have said that crystals vary in size, and this variety may be traced, in the cases of crystallization from fluids, to the slowness or the rapidity of the cooling process. If the work be done slowly, then the crystals obtain a size commensurate with the time of cooling, as they are deposited one upon the other. The form of minerals is the first important point, and to ascertain their forms and structure we must study Crystallography. We shall find faces, or planes,—the lines of contact of any two planes,—called edges, and the angles formed where these planes meet. We may add that crystals have, at least, four planes, making six edges and four angles. Nearly all crystals have more than this, for the forms are, if not infinite, very numerous, and are divided into six (by some writers into seven) different systems or fundamental forms from which the varieties are derived. The axis of a crystal is an imaginary line drawn from an angle to the opposite one.
The first form, the monometric, or cubic system, with three equal axes at right angles, is represented by fig. 436. This crystal is limited by eight equilateral triangles. It has twelve edges and six angles. If we describe a line from any one angle to an opposite one, that line is called an axis, and in the case before us there are three such axes, which intersect each other at right angles.[23] Such crystals are regular octohedra. There are irregular forms also, whose axes do not come at right angles, or they may be of unequal length. The substances which we find crystallized in this form or system are the diamond, nearly all metals, chloride of sodium (salt), fluor-spar, alum, etc.
Fig. 435.—Stone quarry.
When we say in this form we do not mean that all the minerals are shaped like the illustration (fig. 436). We shall at once see that the system admits of other shapes. For instance, a regular crystal may have been cut or rubbed (and the experiment can be made with a raw turnip). Suppose we cut off the angles in fig. 436; we then shall have a totally different appearance, and yet the crystal is the same, and by cutting that down we can obtain a cube (fig. 437). Take off its angles again we obtain a regular octohedron once more, as shown in the diagram opposite.
Fig. 436.—Regular octahedron—first system.
We will exhibit the gradations. Suppose we cut fig. 437; we will obtain (fig. 438) the cube. The next is merely the cube with angles and edges cut off; and if we proceed regularly we shall arrive at fig. 442, the rhombic dodecahedron, or twelve-sided figure, whose equal planes are rhombs.
We can, by taking away alternate angles or edges situated opposite, arrive at other secondary crystals. From the original octohedron we can thus obtain figs. 443 and 444. These are known as tetrahedron. The pentagonal dodecahedron is another secondary form (fig. 445).
Fig. 437.—Octohedron angles removed.
Fig. 438.—The cube.
Fig. 439.—Cube with angles removed.
The cube, or hexahedron, the octohedron, and the rhombohedron are all simple forms, being each bounded by equal and similar faces, or surfaces. We can thus understand how certain primary or original natural forms of crystals can be changed in appearance by connection. Of the various substances crystallizing in this system we find salt, iron pyrites, gold, silver, copper, and platinum, and the sulphide of lead called galena, in the cube or hexahedron form. The diamond and fluor-spar, alum, etc., appear in the first form (I), fig. 436 (octohedron). The cube, we see, has six equal faces, eight equal angles, and twelve equal edges. Galena, as will be observed from the illustration herewith, shows this peculiarity in a very marked manner (fig. 446).
Fig. 440.—Another intermediate form of octohedron between figs. 436 and 438.
Fig. 441.—Cube deprived of edges and angles.
Fig. 442.—Rhombic dodecahedron (garnet crystal).
Figs. 443 and 444.—Secondary forms of first system.
Fig. 445.—Pentagonal dodecahedron.
The second crystalline form is the Hexagonal, and in this system three of the four axes are equal and in the same plane, inclined at an angle of 60°, with a principal axis at right angles to the others. In crystals of this system are found quartz and calc-spar.
The third system is termed the Quadratic or the diametric. This form has three axes, all at right angles, two being equal and the other longer or shorter than the former two. In this system crystallize sulphate of nickel, zircon, oxide of tin, etc.
Fig. 446.—Galena, or sulphide of lead.
Fig. 447.—Oxide of tin.
The fourth, or Rhombic system, or the trimetric. Here we have three rectangular axes, all unequal and intersecting at right angles. The sulphate and nitrate of potassium crystallize in this system.
Fig. 448.—Rock crystal—second system.
Figs. 449 and 450.—Quadratic, or third system.
Fig. 451.—Prism of quadratic system.
The fifth is the oblique, or Monoclinic system, which displays three unequal axes, two of which are at right angles; the third, or principal axis, is at right angles to one and oblique to the other of the preceding. Ferrous sulphate, tartaric acid, and gypsum crystallize in this system.
Fig. 452.—Rhombic, or fifth system of crystals.
Fig. 453.—Crystals of the fifth system.
The sixth, or Triclinic system, or the doubly oblique. In this system we have three axes differing in length, and all forms which can be arranged about these unequal and oblique axes. Sulphate of copper will be found in this group. The system has been called anorthic, or triclinic, because the axes are unequal and inclined, as in the oblique prism based upon an obliqued angled parallelogram. Axinite crystal, as annexed, will show one form in this system.
Fig. 454.—Sixth system.
As may be gathered from the foregoing, it is not easy to determine a crystalline form with certainty,—a great part of the crystal may be invisible. A crystalline mass is a mineral, which consists of an arrangement of crystals heaped together. If it does not possess these the mineral is amorphous, or shapeless. We will now endeavour to describe some of the physical characteristics of minerals.
Fig. 455.—Wollaston’s Goniometer, an instrument for measuring the angles of crystals.
The Goniometer (see fig. 455) is the instrument used for measuring the angles of crystals. Wollaston’s reflecting instrument is most generally used. It consists of a divided circle, graduated to degrees, and subdivided with the vernier. The manner of working is easy, though apparently complicated. The vernier is brought to zero, when an object is reflected in one face of the crystal. The crystal is turned till the same object is viewed from another face. The angle of reflection is then measured, and can be read off on the circle.
We have already referred to the physical characteristics of the minerals, and one of these attributes is cohesion. When we find a substance is difficult to break, we say it is “hard.” This means that the cohesion of the different particles is very great. Minerals vary in hardness; some are extremely difficult to act upon by force, and a file appears useless. At the other side we find some which can be pricked or scratched with a pin; and these degrees of hardness being put as extremes, we can in a manner relatively estimate the hardness of all other minerals. We can test this by scratching one against another; whichever scratches the other is the harder of the two, and thus by taking up and discarding alternately, we can at length arrive at a comparative estimate of the hardness of all. Such a scale was arrived at by Mohs, and arranged in the following order. The softest mineral comes first:—
- Talc.
- Gypsum (rock-salt).
- Calcareous spar.
- Fluor-spar.
- Apatite-spar.
- Felspar.
- Quartz.
- Topaz.
- Corundum.
- Diamond.
Talc, we see, is the softest, and diamond the hardest. Thus “diamond cut diamond” has passed into a proverb expressive of the difficulties one “sharp” person has to circumvent or “cut out” another. Diamond is used by glass-cutters. When geologists wish to express the degree of hardness of any substance, they mention it with reference to the foregoing list; and if the substance be harder than fluor-spar, but not so hard as felspar, they determine its hardness five, or perhaps between five and six, or between four and five, according as it is harder or less hard than apatite. Thus hardness, or power of cohesion, resistance to exterior force and pressure, is a prime characteristic of the mineral kingdom. The file is the best test.
We now come to another phase of the physical character of our minerals—cleavage. This is the term employed to express the facility of cutting in a certain direction which in the mineral is its direction of cleavage. Take mica, for instance. There is no difficulty in separating mica into thin layers; we can do so with our fingers. The layers, or flakes, or laminæ are so arranged that they exhibit less cohesion in one direction than when tried in other ways. We cut with the grain, as it were in the direction of the fibre when wood is concerned. Here we have another popular saying expressive of this,—“against the grain,”—which signifies an act performed unwillingly and unpleasantly. Cleavability, therefore, means cutting with the grain, as it were, and various minerals are possessed of different degrees of cleavage. It sometimes happens that electric excitement is observed when cleavage takes place. One place will become positive, and the other negative. Mica, arragonite, and calcareous spar will exhibit this action after cleavage or pressure. When a crystal of tourmaline is heated, it will develop positive electricity at one end of its principal axis, and negative at the other. Even if it be broken, the extremities of the fragments will exhibit similar phenomena, and so far like a magnet, which, as we have seen, possesses this attribute of “polarity.” But a curious fact in connection with this is that, if the heating cease the polarity ceases for a second or two, and yet as cooling goes on the polarity is restored, with the difference that the positive end has become negative, and the end previously negative has come over to the opposite pole. Electricity, therefore, must be hidden away in every portion of our globe, and will some day be proved to be the mainspring of all life.
Fracture in minerals is also to be noticed. Those substances which we cannot laminate we are obliged to break, and we may require to break a mineral in a direction different from or opposed to its direction of cleavage. Under such circumstances we must break it, disintegrate it, and observe the fracture. Sometimes we shall find the surfaces very even, or uneven, or what is termed conchoidal. This is observable in the breaking of flint. There are various ways in which minerals display fracture, and the particular manner and appearance denotes the class to which the mineral belongs.
We may pass over the question of the specific gravity of minerals, as we have in a former part explained this. It is important, however, to ascertain the specific gravity. As a general rule, minerals containing heavy metals are of high specific gravity.
But the relation of minerals (crystals) with regard to light is of great interest and importance. When we were writing of polarization, we mentioned the faculty a crystal has for double refraction, by which it divides a ray of light into two prolonged rays taking different directions, the plane of vibration of one being at right angles to that of the other. This property is not possessed by all crystals. Some act as ordinary transparent media. Some crystals transmit only one polarized ray, and tourmaline is called a polarizer; and if light be passed through it to another polarizer, it will be transmitted if the latter be similarly held; but if the second be held at right angles to it the ray will be stopped. We can easily understand this if we suppose a grating through which a strip of tin is passed; but the strip will be stopped by bars at right angles to it. The coloured rings in crystals can be observed when a slice of a double refracting crystal is examined. The rings are seen surrounding a black cross in some instances, and a white cross in another. The effect when examined in the polariscope is very beautiful. Selenite is probably the best crystal for exhibiting colours.
Minerals sometimes reflect, sometimes refract light; they are said to possess lustre and phosphorescence. All these properties may be considered as belonging to the crystals which are transparent, semi-transparent, translucent, or opaque, according to the degrees in which they permit light to pass through them. All minerals are electric or non-electric, and the variety can be ascertained by rubbing and placing the mineral near the electrometer. But all do not exhibit magnetic properties. Taste and smell are strongly marked in some minerals—salts, for instance, and sulphur; some are soapy to the touch, some appear cold to the fingers. Chemistry is very useful to us in determining the nature of the mineral, and the amount of it enclosed in the substance under examination. These delicate operations are termed qualitative and quantitative analysis. The application of heat is increased by means of the blowpipe, which is in effect a small bellows. We can thus, and particularly by means of the oxy-hydrogen blowpipe, obtain a very intense heat with little trouble. When the fragments of a mineral are held in the flame by platinum “tweezers,” or tongs, then the fusibility of the substance, and the colour of the blow-pipe flame will be of great assistance in determining the nature of the mineral. It is also curious to observe the different forms into which the various substances expand or contract under the influence of the blowpipe. We may have a rugged slag, an enamel, or a glass, or a bead, or “drop” of metal. The varied substances produce various colours—yellow, green, orange, or red, according to circumstances. Strontia is a vivid red, copper is green, lime orange, and so on.
Fig. 456.—The blowpipe.
It is very little use to attempt a study of mineralogy without some acquaintance with chemistry. In dealing with minerals, and in studying geology, we must try to keep our knowledge of chemical science in our minds, and thus fortified we can more easily understand the steps leading to the classification of minerals. It is impossible to teach mineralogy or geology from books. Nature must be studied, the specimens must be seen, the earth must be examined. The advance in mineralogy may be—probably will be—slow, but crystals will teach something; and when we can pass a viva voce examination in chemistry and crystallography, expressing, by the symbols, the various substances under discussion, we shall have made a considerable advance in the science. We shall have an idea of the component parts of various substances, and be able to class the various minerals according to their chemical constitution. Beginning with the metalloids, we shall pass to the metals and various compounds, salts, resinous substances, etc., such as amber.
It is impossible in the space at our command to describe all the minerals, and yet it is necessary to enumerate the most important. We may, therefore, take them in the following order. It should be added that most of the simple minerals occur in comparatively small quantities, but sometimes we find them in aggregate masses (rocks). We append a table.
SYNOPTICAL TABLE OF THE MINERALS.
- First Class.—Metalloids.
- Sulphur.
- Boron.
- Carbon.
- Silicium (Silicon).
- Second Class.—Light Metals.
- Potassium.
- Sodium.
- Ammonium.
- Calcium.
- Barium.
- Strontium.
- Magnesium.
- Aluminum.
- Heavy Metals.
- Iron.
- Manganese.
- Cobalt.
- Copper.
- Bismuth.
- Lead.
- Tin.
- Zinc.
- Chromium.
- Antimony.
- Arsenic.
- Mercury.
- Silver.
- Gold.
- Platinum.
- Third Class.
- Salts.
- Earthy resins.
Sulphur is found in Sicily and Italy and other parts of Europe, in a native state, but as such has to be purified. The crystals take the form as shown in the margin. Cleavage imperfect; it is brittle. Sulphuric acid is a very important combination, and a very dangerous one in inexperienced hands. Sulphur combines with a number of elements, which combinations are “Sulphides.” (See [Chemistry section].)
Fig. 457.—Crystals of sulphur.
Selenium is a metalloid resembling sulphur, but less common. It is inodorous.
Boron is usually found near volcanic springs, and in combination with oxygen. It is soluble. Taste, acid bitter, and white in colour; friable. It is known as Sassoline, or boracic acid. (See [Biborate of Soda] for one of the borates.)
Carbon is one of the most important of our minerals. In the form of coal we have it in daily use, and in the form of diamond it is our most valuable gem. In the latter form it is the hardest of all minerals, a powerful refractor of light, lustrous, and transparent. It is found in the East Indies and Brazil; more lately Cape diamonds have been brought to Europe, but they do not equal the Eastern gem. Almost fabulous prices have been given for diamonds, which, after all, are only carbon in a pure state. Another form of carbon is graphite (plumbago, or blacklead). It is much used for pencils and in households. It is found in Cumberland, and in many other localities in Europe and Canada.
Carbon appears in one or other of the above forms in regular octahedrons or their allied shapes. Anthracite, another form of carbon, is used as fuel for strong furnaces. It leaves little “ash,” and is smokeless when burned. Coal, in all its forms, is evidently derived from wood. Thousands of years ago vegetable matter must have been embedded in the ground and subjected to carbonization. There are different kinds of coal, all of which come under one or other of the following heads: cubical coal, slate coal, cannel coal, glance-lignite,—the last being, as its name implies, an imperfect development of wood; it is a brown coal. We are not here concerned with coal as a fuel. Charcoal is also a form of carbon prepared from wood and finds a counterpart in coke, which is prepared from coal. Carbon, as we have already seen, plays an important part in electric lighting and in the Voltaic Battery. Peat, or as it is called in Ireland, “turf,” is one of the most recent of the carboniferous formations. It is much used as fuel. It is cut from moors (“bogs,” as they are sometimes called), and the various deposits can be traced. Bog-oak is no doubt the first step towards peat, as peat is the step towards coal. The brown turf is newer than the black, and both kinds may be seen stacked in small square “bricks” along the Irish canals and in the yards of retailers of fuel.
Fig. 458.—Crystals of carbon.
Silicon. Silica occurs generally in combination with alumina, and never in a free state. In combination with oxygen it is called silicic acid. Silica, when crystallized, is usually called quartz.
Quartz has several varieties. We need only enumerate them, they will all be immediately recognized. We give illustrations of the crystals of quartz (fig. 459):—
1. Rock crystal appears in beautiful six-sided prisms.
2. Amethyst is coloured by protoxide of manganese, supposed by the ancients to be a charm against drunkenness.
3. Common quartz, or quartz rock, forms granite in combination, and is also known as “cat’s-eye,” “rose” quartz, etc.
4. Chalcedony, sometimes termed cornelian: used for seals, etc.
5. Flint: much used in potteries. “Flint and steel” have been superseded by phosphorus.
6. Hornstone: something like flint, resembling horn.
7. Jasper: of various colours; opaque and dull in appearance.
8. Silicious slate: a combination; used as a whetstone.
9. Agate: a mixture of quartz, amethyst, jasper, and cornelian; very ornamental.
10. Opal: a peculiar variety, containing water. It is not found in the form of crystal, but in vitreous masses. Its changeableness of hue is proverbial. The “noble” opal is much prized.
11. Smoky quartz, or cairngorm.
12. Onyx and Sardonyx.
Fig. 459.—Quartz crystals in various forms.
We now arrive at some minerals which contain metals.
Potassium. This metal is so frequently combined in minerals with alumina that we may refer to it with the latter in sequel. There are two natural potassa salts—nitre, and sulphate of potassa. Nitre is known as saltpetre, and is of great use in medicine. It is the chief ingredient in the composition of gunpowder.
Sodium. We have a number of minerals in this group—viz., nitrate of soda (nitratine), which occurs in large quantities in Peru; rock salt, chloride of sodium, known as salt. It crystallizes in the cubic system. Colour usually white, but it occurs in secondary rocks in company with gypsum, etc. It is sometimes of a reddish colour, or even green and yellow. Biborate of soda is borax, and is found in and on the borders of a Thibetian lake. There are several other combinations with soda: the sulphates of soda—viz., thenardite and glauberite, anhydrous and hydrated respectively, carbonate of soda, and so on.
Ammonia combinations occur in lava fissures, and are not often met with in consequence of their volatile nature.
Fig. 460.—Spar crystal.
Calcium. This forms an important group of the minerals, which are very white in colour, and not very hard in substance. Calcium is the metallic basis of lime. Fluoride of calcium, known as fluor-spar, most frequently crystallizes in cubes in the first system. Anhydrite is the anhydrous sulphate of calcium. The hydrated sulphate is called gypsum. One variety of the hydrated sulphate is selenite, another is known as alabaster. Apatite, or asparagus stone, and pharmacolite are in this group.
Rhombohedron (r).
Primary rhombohedron (r).
Six-sided prism (g) regular.
Primitive rhombohedron (r), with acute form (r½).
Obtuse rhomtrahedron (r½), ending in prism (g).
Equal six-sided prism (a), ending in regular (r).
Obtuse rhombohedron.
Fig. 461.—Crystals of Carbonate of Lime.
Carbonate of lime, not content with one system of crystals, makes its appearance in two. It is therefore divided into two minerals—namely, calcareous spar and arragonite. In the former it possesses various forms, as will be observed in the accompanying diagrams. It is a very important mineral, as will be readily acknowledged; it enters largely into the composition of all shells and bones. The minute shells, deposited by millions at the bottom of the sea, have combined to raise our chalk cliffs. Carbonate of lime is a constituent of water, as the deposits at the bottoms of kettles, upon the sides and bottoms of water-bottles, and the stalactites all testify. A little good vinegar will quickly dissolve this deposit. Calc-spar is crystallized, and the Iceland spar is celebrated. Marble, which is another form of carbonate of lime, is white, hard, and granular. It is sometimes varied, but the pure white is the most valuable. Chalk, we know well, is soft, and is useful for writing. We have also aphrite, schiefer spar,—compact limestone in various forms,—and finally, arragonite, called from the place of its nativity, Arragon,—a colourless and somewhat transparent vitreous crystal.
Barytes. The sulphate of baryta is known as heavy spar; the crystals are of tabular forms, but numerous modifications exist. One of the forms is represented in the margin.
Fig. 462.—Tabular form of heavy spar.
Strontium is the metallic basis of strontia. Sulphate of strontium is celestine, the mineral which colours the blow-pipe flame a fine crimson. There are certain varieties. Strontia salts are chemical preparations. A beautiful pyrotechnic “red fire” is produced by mixing nitrate of strontia with sulphur, antimony, charcoal, and chlorate of potassia.[24] There is a carbonate of strontia in the same crystalline system.
Magnesium. With this metal we have a large group of minerals. Magnesite is carbonate of magnesia, and occurs as talc-spar. The magnesium limestone crystallizes as bitter spar. This dolomite is like marble or common limestone, according to colour. Talc is a combination of magnesia with silicic acid. The hydrated carbonate is termed “white magnesia.” The sulphate of magnesia is found in Siberia, and we have boracite, and native magnesia called periclase. The sulphate is generally present in mineral waters, such as the Seidlitz and Epsom Springs. Large masses have been found in the extensive caverns of Kentucky and Tennessee, etc.
Fig. 463.—Crystal of augite.
Meerschaum is a hydrated silicate of magnesia. It is found in Anatolia and Negropont, also in France and Australasia. Serpentine is another similar composition. It is found in Cornwall, where it is carved into various ornaments. It is sometimes called snakestone. There are many other hydrated silicates of magnesia—viz., gymnite, picrosmine, pycrophyll, etc.
Fig. 464.—Alum crystals.
There is another family allied to magnesia, called Augites. These minerals are black or dark-green, and are contained in lava and basalt: Augite and Hornblende are the chief representatives of this family. The former crystallizes in the fourth system (see fig. 463), and there are several varieties—diallage, bronzite, diopside, etc. Hornblende belongs to the same system, and is a large factor in the composition of gneiss, syenite, and porphyry. Tremolite is a hornblende, and asbestos (amianthos), and mountain-cork are also varieties. The attribute of asbestos for sustaining heat is well known, and may be usefully employed for fire-proof purposes. The well-known jade-stone of China and calamite are other varieties.
Aluminum, or Aluminium, gives us a large class of minerals. It is the metallic basis of alumina, which, combined with silica, is the chief component of our clay. Silicic acid and this base combine to form many minerals, and contains nearly all the precious stones. Corundums consist of pure alumina, and crystallize in the hexagonal system. The following stones are varieties of this mineral:—Sapphire, a beautiful blue; ruby, a red oriental; topaz, yellow oriental; amethyst, violet; all being sapphires more or less. The finest crystals are found in the East Indies in the sands of rivers and diluvial soils. The common corundum is very hard, and is used for polishing. Emery is well known, and is found in mica-slate. It is of a bluish-grey colour, and is also a polisher.
Alum forms another family, of which we may first mention aluminite, a “basic sulphate” of alumina and found in small quantities. Alum-stone is found in Italy. Alum occurs in large crystallized masses. (See illustration, fig. 464.) There are different minerals with a composition very similar to alum, in which the potassa base of alum is supplied by others. Thus we have the potassa alum, soda alum, manganese alum, ammonia alum—all being very nearly of the same constituents, and having similar crystals in the regular system, and are thus termed isomorphous, or similarly-formed. The potassa, or potash alum, is the commonest form, and is found abundantly in England, on the Continent of Europe, and the United States. Soda alum is called salfatarite, and magnesia alum pickeringite; manganese alum is apjohnite; phosphate of alumina is wavellite.
There are compounds of alumina and magnesia called Spinels. They are hard minerals, and the same isomorphous changes take place with them as are observable with the bases of alum. There are therefore varieties such as the spinel ruby found in the East Indies, very red in colour; the balas ruby not so red, and the orange-red, termed rubicelle. Ceylon is remarkable for some fine specimens of spinels. Chromite is like the spinel, but is known as chrome iron.
Zeolites are principally compositions of silica and alumina. They contain water, and are white, vitreous, and transparent. There are several varieties of them—natrolite, stilbite, etc. We will now pass on to the Clays, which are a very important family of the aluminum group.
There are a number of hard minerals which, when disintegrated, form certain earthy masses. These we term clay, or clays, which possess various colours and receive certain names, according to the proportion of metallic oxides they contain. All clays have an affinity for water, and retain it to a very great extent. The earth has also a peculiar smell. Clay is used in various ways; pottery, for instance, we read in the Bible as having been an employment from very ancient times. One attribute of clays, the retention of water, is of the greatest use to the world in providing moisture for plants and seeds. We may mention other characteristics of clay. It absorbs oil very quickly, and therefore is useful for removing grease-spots. It cannot be burned, so we have fire-bricks and fire-clay in our stoves and furnaces. There are various clays—pipe-clay, for instance, which is white; potters’ clay is coarser. There is porcelain clay as well as porcelain earth, of which more below. Yellow ochre and sienna are clays used by artists. Bole is a reddish clay; and tripoli is employed for polishing. There are, besides, andalusite, or chiastolite and disthene, crystalline forms of clay.
Porcelain has been known to the Chinese for centuries. In 1701 it was discovered in Germany by Böttcher, a chemist, who while endeavouring to make gold by Royal command, found the porcelain, and was thereby enriched. Porcelain earth is frequently found; is known in many places as kaolin, and usually comes from the decomposition of felspar. But in Cornwall we find it as decomposed granite, and the filtering process can be viewed from the railway, while both gneiss and granulite have been known to yield kaolin. It is also found in China, Saxony, and France. It is free from iron, and when ground and mixed with silicic acid, it is handed to the potter or moulder. After the vessels have been dried in the air they are put into the kiln, and then become white and hard. After that they are glazed in a mixture of porcelain earth and gypsum, or ground flints and oxide of lead, made fluid with water in the glazing of earthenware. The vessel is then put into the furnace again, or “fired,” as the process is called, and then comes out white, hard, and partly transparent.
Fig. 465.—Porcelain furnace.
Fig. 466.—Stampers.
Fig. 467.—Flint mill.
Earthenware utensils are made of a coarser material,—clay and powdered flints,—from which all the gross matter has been eliminated. Flint is not difficult to break, if made hot and thrown into cold water. A stamper is then used to break the flints. They are first ground in a mill and purified like the clay, then they are mixed and beaten, while moist, into “putty,” and turned, or forced, into moulds. The handles are fixed on afterwards. The ware is baked for two days and glazed. The various colours are obtained by mixing different clays and oxides—iron or manganese. Biscuit porcelain is made by pouring a creamy mixture of porcelain earth into plaster-of-Paris moulds, and when a thin case has formed within, the liquid is poured out again. It is then dried in the mould and shrinks. The mould is taken to pieces, and the thin biscuit porcelain is left.
Fig. 468.—Felspar.
Felspars are very like the zeolites, except that the former contain no water. Felspar crystallizes in a number of different forms. We annex illustrations of specimens. This spar is found in rocks, granite, gneiss, etc. One variety is the moonstone, of a peculiar lustre. Felsite is amorphous felspar. Albite contains soda instead of potash. Labradorite is nearly a pure lime felspar, and is remarkable for its colours, like a pigeon’s breast. Spodumene is like albite, and leucite, soda-lite, etc., belong to this family.
Fig. 469.—Felspar crystal.
Lapis-Lazuli is a felspar distinguished by its blue tint. It was used for ultramarine colouring at one time, which colour can also be made chemically. Lapis-lazuli is found in Siberia and China. It is a mixture of mineral species. Hauyne is something like it. Obsidian is a sort of black glass, and occurs in various colours in vitreous masses. It is derived from the fusion of rocks, and is employed in the manufacture of boxes, etc. Pumice stone bears a resemblance in composition to the foregoing, but is porous, and so called spongy. It contains both potash and soda in some quantities. Pearlstone and pitchstone also attach themselves to this family group.
The Garnets possess many curious forms of crystals, which are coloured and used as gems. Tourmaline is a very particularly useful crystal, and is used in the investigations concerning the polarization of light. It is found of nearly all colours. The garnet and staurolite crystals are shown (figs. 470, 471).
The former is silicate of alumina with the silicate of some other oxide, which is not always the same. This change, of course, gives us a series, as in the case of alum above mentioned.
Fig. 470.—Garnet crystal.
The red varieties, called almandine, or precious garnets, are distinguished from the duller, “common” species by their clear colour. Bohemia is the most productive soil for the garnets.
Mica includes, as we have already noticed, a group of minerals which have a peculiarly laminated structure. These layers are by no means all alike, but they present a smoothness to the fingers which is highly characteristic. The chief constituents are alumina and silica, occasionally with magnesia. Mica slate is very common, and is often used instead of glass in window-frames. Muscovite, lepidolite, and phlogopite are all micas of the “potash,” “lithia,” and “magnesia” varieties.
In the list of minerals associated with the lighter metals, we need only now mention the Gems, so well known. These stones are very hard in many instances, infusible, and exhibiting beautiful colours. Amongst them are diamonds, sapphires, and rubies, of which we have spoken; the topaz, noticed under corundum. The chrysoberyl (of a pale green, or occasionally reddish hue), of which the alexandrite of Siberia is a variety, is a compound of glucina with alumina; the beryl, a silicate of the same, and the emerald of beautiful green. Zircon is another gem, and “hyacinth” is its most valued form. The latter is found in basaltic rocks. The emerald crystallizes in the hexagonal system.
Fig. 471.—Staurolite crystal.
We may now consider the minerals formed by the heavier metals, such as Iron, Copper, Nickel, etc.
Iron. This well-known metal fills a very important place in our mineral arrangements, for the substances formed with iron ores occur in great variety of structure, and occasionally in very large masses. They are highly magnetic, and very hard. Were we here treating of iron as a metal, we could give some information respecting its extraction and manifold uses. All we need mention here is the fact that iron occurs in nature in various ores which are essentially composed of iron and oxygen. The iron is extracted in the blast furnace, in which the process is continued for years. The “slag,” or glassy scum, protects the molten iron from the air; its presence is necessary in all blast furnaces. The most important of the iron group of minerals are Magnetic Iron (magnetite), or loadstone. This mineral occurs in Sweden and North America, and is found in primary rocks, and in Scandinavia forms mountains. It crystallizes in the regular (octahedron) system, and often in the form in illustration in the margin. It is highly magnetic, as its name implies.
Fig. 472.—Magnetic iron.
Native iron very rarely occurs, and then only in thin layers. The most extraordinary specimens are those termed meteoric iron, which fall from the atmosphere in great masses; and the meteoric stones, which contain ninety per cent of iron, together with other constituents in small quantities—viz., nickel, cobalt, copper, manganese, carbon, sulphur, arsenic, etc.
Red hematite crystallizes in the hexagonal system. It possesses much the same (chemical) constitution as corundum (q. v.). It is brightly metallic, and shows a red streak. It occurs in various forms, as iron glance or specular iron, which is found in Sweden and Russia; micaceous iron, bloodstone, clay, ironstone, etc.
Brown hematite has not been found in crystals, but brown ironstone (fibrous) is crystalline. The earthy brown, containing clay, gives us yellow ochre and umber. Pea-iron ore and “morass” or “bog” ore also belong to this class. Limonite is the name given to these more recent formations, of which yellow ochre is a pure specimen.
Fig. 473.—Native oxide of iron.
The combinations of iron with sulphur (pyrites) are also important. Iron pyrites and magnetic iron pyrites are two which may be mentioned. The latter first.
Magnetic iron pyrites (or pyrrhotin) crystallizes in six-sided prisms, and is attracted by the magnet. The composition of this mineral has not been exactly ascertained, and no chemical formula has been found for it.
Iron Pyrites (bisulphide of iron) is known as cubic pyrites, yellow pyrites, and mundic. It is generally found in the regular system of crystals, either as a cube or as a pentagonal dodecahedron. (See first system of crystals, ante.) Its colour is yellowish. It is known also as green vitriol when oxidised, and forms beautiful green crystals (copperas). This salt is used in the preparation of Prussian blue and violet dyes. With gallic acid it makes ink.
There are many other “ferruginous” minerals, such as vivianite, green ironstone, white iron pyrites, arsenical pyrites, or mispickel, etc.
A carbonate of iron, called chalybite, or spathic ironstone, is very abundant in nature, and forms obtuse rhombohedrons. It is very useful for the production of steel, as it forms the clay iron ore found in coal districts in combination. In a fibrous form it is known as sphærosiderite. It is a most useful mineral.
Chrome iron (chromite) is useful for the preparation of chromium compounds. It crystallizes in the cubic system. It is magnetic, especially when treated. Chromic acid forms scarlet “needle” crystals, and by its assistance chromate of lead, or chrome yellow, is prepared. (Chromate of lead is found in a native state as crocoisite). See Chromium.
Manganese is contained in several minerals. It usually occurs as an oxide. It colours minerals variously. In a pure state manganese is white and brittle. The chief minerals are—
Pyrolusite (the binoxide of manganese of commerce) occurs in crystals. It is black. It is used in the preparation of chlorine and oxygen. The other minerals are known as manganite, which is also found associated with pyrolusite, as are hausmannite and braunite, the other oxides.
Nickel and Cobalt are generally found together, both being similar, and the minerals are compounds of arsenic or sulphur, and occur under similar circumstances. The principal are of Nickel and of Cobalt—
- Sulphide of nickel (ullmanite).
- Arsenical nickel (nickeline).
- Nickel glance (gersdorffite).
- Nickel pyrites (siegenite).
- Arsenical cobalt (smaltine).
- Cobalt glance (cobaltine).
- Cobalt bloom (erythrine).
- Cobalt pyrites (linnæite).
Nickel ores are used for extraction of the metal, which is used as a substitute for silver. The name is derived from the German, kupfernickel, or false copper. It was discovered in 1751.
Copper, again, forms a number of minerals, and the chief is the red oxide of the metal, called cuprite. It crystallizes in the cubic system. Its colour is red, and tinges a flame green. Cuprite yields excellent copper, and is found in Cornwall, and in many places on the continent. The black oxide is rarely found. It is known as melaconite.
Malachite (carbonate of copper) is remarkable for its beautiful green colour. In Australia it is worked for copper. It is chiefly ornamental. Siberia yields the finest specimens, but the mineral is found in Cornwall and Cumberland, as well as on the continent. Chessylite (from Chessy, in France) is frequently found with malachite. It has been called blue malachite, or the azure copper ore. It is used as a paint.
Besides the above, copper unites with sulphur to form minerals, such as the needle ore (bismuthic sulphide of copper), antimonial sulphide, bournonite; purple copper, and copper pyrites, which is very abundant, and furnishes us with most of our copper. There is also the “grey” copper ore, which contains various metals; even silver is obtained from it at times.
Bismuth gives us only a few minerals, of secondary importance. Native bismuth resembles antimony, but is reddish in hue. Bismuth ochre, bismuth blende, and bismuthine are the chief combinations.
Lead is more important, and is obtained from galena, the sulphide of lead, which is very abundant, and the principal lead ore. It can be at once distinguished by its high specific gravity and metallic lustre; the “cubic cleavage” also is very easy. It frequently is found containing silver, and even gold, antimony, iron, etc. There are several suphantimonites of lead, such as zinkenite, geocronite, etc., and the salts, such as sulphate of lead and white lead ore, or carbonate of lead (cerasite). The chromate of lead is found in the Ural Mountains.
Tin is not found in a native state, but as tinstone, or binoxide of tin, named cassiterite. It is found largely in Cornwall, and the mines there have yielded great quantities for generations. Tin pyrites, a union of sulphides of tin, iron, and copper, is also found in Cornwall.
Zinc is produced from the ore called (zinc) blende, or sulphide of zinc (black Jack). Its colour is very variable, sometimes red, but when pure is greenish-yellow. It is also found black and brown. The red oxide of zinc (or spartalite) is also worked for zinc. The carbonate, or zinc spar, is common, and used to make brass, as is calamine, which is possessed of a remarkable lustre, and is even luminous when rubbed. It is a silicious oxide of zinc, and is found in the sedimentary rocks. When heated, it displays strong electric properties.
Chromium occurs in very few mineral combinations; chromate of lead, chrome iron, and chrome ochre, or sesqui-oxide of chromium are the only important ones.
Antimony minerals are very hard; the tersulphide is the most common, and from this the metallic antimony is produced. Red antimony, the oxide, is a rarer ore.
Arsenic resembles antimony, and occurs in combination with many metals. White arsenic, or arsenious acid, is found in Bohemia, Alsace, Transylvania, etc. Orpiment and realgar are sulphides of arsenic, and are employed as colouring matters in paint and fireworks. Arsenic is very poisonous.
Mercury is occasionally found native, but more generally as cinnabar. Chloride of mercury (or calomel) is found associated with the cinnabar, or hepatic ore. Cinnabar is easily volatilized, and possesses high specific gravity. The Californian mines are very rich. Spain also produces a large quantity. It is opaque, and carmine in colour.
Silver occurs native, or in ores. The latter are as follows:—The sulphide, or the vitreous silver (argentite); antimonial silver; and the combined sulphides, of antimony and silver. There are many silver minerals, such as the chloride (horn silver, or kerargyrite), bromide, and carbonate of silver, bismuthic silver, etc. The bromide and iodide are bromargyrite and iodargyrite. Silver occurs most frequently associated with gold; natural alloys of these two metals are found, containing from 0·16 to 38·7 per cent. of silver, which causes considerable variations both of colour and density. In addition to this alloy, we may mention sylvanite (graphic tellurium), which contains, besides gold and silver, one of the rarer metals—viz., tellurium.
Fig. 474.—Gold crystals.
Gold is our most precious mineral, and is generally found native. It exists in sand and in certain rocks. It crystallizes in various forms, and in Mexico it is found in companionship with silver and copper sulphides. Australia and California render the most valuable supplies of the metal.
Platinum is also found native, and rarely is crystals. It is often alloyed with other metals, chiefly with iron or gold; also with diamonds. We have already considered it as a metal. Little remains to be said about salts and resins, for with the exception of those we have referred to under Chemistry, they are of little value. The bitumens, rock oil, etc., which exude from the earth, are very useful, and as asphalt and petroleum play an important part in the civilized world, but scarcely come under the strict rule of minerals as we consider them, and with this reference we close our sketch of Mineralogy.
CHAPTER XXXII.
NEW LOCOMOTIVE APPLIANCES.
THE KITE—THE AEROPHANE—ICE YACHTS—SAILING TRUCKS—WATER VELOCIPEDES.
The kite, known from the earliest times, and constructed by a number of people, is a very familiar object, which we shall not describe; for we will now speak of some similar appliances of a more interesting and uncommon description.
Fig. 475.—Mr. Penaud’s “High-flier.”
M. Penaud has invented some appliances in which twisted india-rubber is the principal agent. Fig. 475 represents a sort of kite, which rises in the air if one twists and then looses the india-rubber round the central bow. Fig. 476 represents another kind of invention; it is an “aerophane,” with a screw at the back, so fixed that it receives no shock from striking against any obstacle. After having twisted the india-rubber, and loosened our hold of the apparatus in a horizontal position, it will first descend for an instant, then, acquiring increased speed, it rises seven or eight feet from the ground, and describes a regular movement in the air for a distance of about fifty yards; the motion lasts for several seconds.
Some models have also been constructed capable of traversing a distance of over seventy yards, remaining for thirteen seconds in the air, as lightly poised as a bird, and without any connection with the ground. During the whole time the rudder restrains with perfect exactitude the ascending and descending movements as they occur; and we can plainly observe the various oscillations like those of sparrows, or more especially woodpeckers. At last, when the movements are coming to an end, the apparatus falls gently to the ground in a slanting line.
Fig 476.—M. Penaud’s “Aerophane.”
M. Penaud has also succeeded in constructing a mechanical bird, that we have seen set in motion, which will continue flying for several seconds; we give an illustration of it in fig. 477.
Another scientist, M. Tatin, has also produced some remarkable results. His efforts have been unceasingly directed towards the reproduction of the flight of a bird by means of more or less complicated arrangements. He has endeavoured to discover in the small appliances made with indiarubber, and used by MM. Penaud and Hureau de Villeneuve, what were the best shapes in which to reproduce the wings, in order to adapt them to a large apparatus acting by compressed air. After several attempts, he decided on the employment of long, narrow wings. Wenham had previously proved that a wing may be equally effectual whether it be narrow or wide, and M. Marcy has also declared that birds with a quick, narrow wing-stroke have always very long wings. By means of these long, narrow wings (fig. 478) M. Tatin has reduced the time during which the wing reaches a suitable position for acting on the air when it first descends. Granted the fact, so long established, that a bird flies more easily if it rests its wing against a great volume of air, it will be understood that the maximum speed of movement will also be the most advantageous as regards the reduction of expended force. The inventor, finding that he could not prevent his mechanical birds from losing force in proportion as they attained considerable speed, remedied this defect by placing the centre of gravity in front. In consequence of this, the bird in full flight preserves the same equilibrium as the bird hovering in the air, and its speed is, to a certain extent, passive, the mass of air pressing of its own accord against the wings, all expenditure of force therefore being utilized for suspension. Thus has M. Tatin been enabled to increase the weight of his appliances, without increasing the motive power, and yet obtains a double course.
Fig. 477.—Mechanical bird.
The movement made by the wing round a longitudinal axis, by means of which it always exposes its lower surface in front on rising, is obtained by the mechanism illustrated in fig. 478 a.
M. Tatin’s Bird.
This apparatus, looked at sideways or from behind, is composed of a light wooden frame, on which are two small supports crossed by an axletree so as to form two cranks. This axle receives a circular movement from an india-rubber spring. The crank on the foremost plane causes the rising and falling of the wings, which move round a common axis, and pass the dead points as the cranks of a locomotive do—so the action is maintained.
Fig. 478.—M. Tatin’s bird.
Fig. 478 a.—Detail of fig. 478.
Fig. 479.—Back view of apparatus.
But the wing does not only move as a whole; every part of it, particularly as it rises, shows a tendency to inclination, which is most marked towards the extremity; the part near the body alone preserves an invariable obliquity. M. Tatin was of opinion that it is with the screw that it is necessary to direct the twisting movement; and to obtain it with all its transitions, he has substituted for silk wings, which fold up, some wings composed entirely of strong feathers, arranged in such a manner that they slipped one over the other when in motion. The arrangement was perfect, but still not suitable for adaptation to the large bird. The inventor therefore again returned to the use of the silk wings, which he appears to have definitely adopted. By means of certain modifications which he has recently introduced in his larger apparatus—viz., a change in the shape of the wings, variation of the amplitude in the flapping, etc., M. Tatin has been enabled to make great progress. The bird, acting by means of compressed air, at first could only raise three-quarters of its own weight, but finally lifted itself entirely. And we must take into consideration that the apparatus has to struggle against the weight of the steering apparatus, which nullifying the vertical and horizontal reactions of the bird during flight, constantly fulfils the office of regulator.
We will now pass to the consideration of two ingenious appliances of a very clever inventor, M. Salleron.
Small Atmospheric Boat.
The little boat shown in fig. 480, which is about the size of an ordinary plaything, is a very ingenious, if not a practical, application of the specific lightness of air acting as a propelling force. In this instance steam plays but a secondary part, which consists in carrying off the air that causes the moving of the boat.
Fig. 480.—Atmospheric boat.
The apparatus, as represented in fig. 481, is of extreme simplicity, as will be seen at a glance. A small cylindrical boiler, B, connected with a capillary tube, is placed on two supports over a spirit-lamp, in such a manner that the opening from which the steam issues is directly opposite the mouth of the tube, T. This tube, after forming a sudden inclination, terminates at the back of the boat in an inclined drain, R. The steam driven through the tube, T, carries along with it a certain quantity of air, which, forced under the water, propels the boat along. The little vessel soon reaches considerable speed, leaving a long track behind it. It will be seen that this is not a mechanical apparatus, capable of absorbing force or diminishing the action of steam by causing its condensation.
Fig. 481.—Section of “atmospheric” boat.
Let us now calculate the force engendered by this apparatus. We know that a litre of water at boiling point gives 1,700 times its volume. The steam, as it quickly issues from the opening of the boiler, carries along at least ten times its volume, or 17,000 litres of air, which, driven under the water, assumes an ascending force equal to the difference of the densities of water and air, or about the weight of the displaced water. Therefore in a litre of water transformed into 1,700 litres of the steam, which carries off into the water 1,700 × 10 = 17,000 litres of air, a force is developed represented by 34,000 kilograms. In fact, by reason of the inclined position of the drain on which the pressure of air acts, and its restricted dimensions, the quantity of force employed in the propulsion of the boat is but a fraction of the total force produced. Moreover, the resistance of traction increases with the size of the boat, and as the dimensions of the inclined pipe cannot be indefinitely enlarged, the result is that the propulsive action is soon insufficient, so that the invention is not, in its present condition, applicable to navigation on a large scale. Its superiority to the steam-engine cannot, therefore, be demonstrated; and we are only now discussing the contrivance in order to show that it is possible, with only moderately powerful generators and extremely simple mechanical appliances, to obtain considerable dynamic effects, susceptible of more serviceable application than is commonly believed.
Circulating Fountain.
Fig. 482.—Circulating fountain.
The apparatus given in fig. 482 is the subject of a very charming experiment, showing the influence of capillarity on the movements of liquids. Two glass balls, B B´, are connected by two tubes; one straight and of rather large diameter, the other extremely slender, and winding in and out in a more or less complicated manner. The large tube passes into ball B´, and forms a slender point, J, at the orifice of the narrow tube. At the lower end of the ball is a bulb, which is closed with a cork, and contains a coloured liquid. The apparatus is fixed to a board with a ring at each end, by which it can be hung on the wall. When commencing the experiment, it should be hung so that the ball B´ is uppermost. The liquid then flows through into the ball B, without presenting any particular phenomenon. The apparatus is then turned, and the liquid descends again with great speed, shoots through the opening, J, and rises into the twisted tube. The air displaced from ball B´ also rises, however, and mingles with the liquid, and it can be seen circulating through the winding tube in a number of air-bubbles, mingled with drops of liquid, gradually transmitting the pressure of the column contained in the upper ball and straight tube; so that by means of a similar phenomenon to that of the fountain of Nero, the liquid rises higher than the level of the reservoir, a part falling into ball B, which causes the experiment to be a little prolonged. This circulation of air-bubbles and coloured drops through the twisted tube of the apparatus has a very pretty effect.
The Pneumatic Pencil.
This ingenious invention is productive of results similar to Edison’s electric pen. It is the invention of an American gentleman, Mr. J. W. Brickenridge, of Lafayette, Indiana. The illustration (fig. 483) explains the mechanism of the pneumatic pencil. The whole apparatus is figured on the left side of the picture, while the longitudinal section of the pencil is shown on the right, the small cut at the top being a vertical section of a portion of the motive power. Compressed air furnishes the power of pressure, which is accomplished by means of a perforating needle.
If the treadle is put in motion, a backward and forward movement is imparted to a flexible diaphragm, as in the upper section in the centre of the illustration. By this movement the air is permitted to enter, and is compressed by the diaphragm into the flexible tube with which the diaphragm is connected. The air is thus brought into contact with another diaphragm at the end of the tube and presses on it. The pencil is fixed to the latter. When it is desired to use the pencil the apparatus is set in motion, and by a series of sharp, quick perforations, any writing can be traced, as by the electric pen. This indentation can be copied over and over again in a press, the writing acting as the negative; and if ink be first run over it, as in a stencil plate, by a proper “roller,” the latter will come out as plainly as possible.
Fig. 483.—Pneumatic pencil.
Tube Wells.
The principle upon which the tube well depends is very simple. It is well known that in certain localities water lies a short distance beneath the surface of the ground, and a very little trouble would satisfy us upon the point, and render us quite independent of the water companies’ supply. On the supposition that the water exists underneath our garden at, say, twenty-five feet beneath the surface of the ground, we have only to drive into the soil a tube for that distance, and by the assistance of a common pump we shall obtain a pure supply of water.
We will now proceed to describe the manner in which these wells are sunk. The first step is to fix a platform firmly upon the ground and bore a hole, by which the tube is to enter the ground. This tube should be very thick, with an aperture of two inches or rather less, and three or four yards in length. The lower portion should be pierced with holes, as in the illustration, and terminating in a point of extremely fine-tempered steel. This tube can be driven into the ground by mallets, or by the suspended hammer, worked as shown in the illustration (fig. 484). This work will be easily accomplished, and when the first length of tube has been driven in, another can be fixed to it and hammered down in the same way.
Fig. 484.—Tube Well.
When the tubes have been driven to the depth indicated it will be as well to let down a sounding line, a simple cord sustaining a pebble. If the stone be pulled up dry, another length of tube can be added, or the tubes can be pulled up, and another trial made. If, on the contrary, the pebble come up wet, the object is accomplished, and a small pump can be fixed to the upper end of the tube, as in fig. 485. At first the water will be found a little thick and muddy, in consequence of the disturbance of the soil and the particles adhering to the end of the first tube; but after an hour or so it will be found that the water has become quite clear. It need scarcely be said that if the water possesses sufficient ascensional force to rise to the level of the ground a pump need not be employed. An Artesian Well will, in that case, be the result.
Fig. 485.—Abyssinian Pump.
The operation described on page 456 can usually be performed without any difficulty. Sometimes, however, the tube may come in contact with a large stone, and in that case the experiment must be tried elsewhere; but, as a rule, the pointed tube, in consequence of its small size and penetrative power, pushes any moderately-sized obstacle aside, readily turns aside itself, or passes between pieces of stone to the desired depth. Nine times out of ten the operation will be successful, and the experiment will not occupy more than an hour, under ordinary circumstances, and the tubing (and pump) may be obtained at a moderate price, which can even be diminished by arrangement. Ordinary wells are relatively very difficult to sink, and the soil thrown out from the pit is in the way, while a parapet is necessary to protect the opening. Besides, should water not be found after much work, the expense and trouble of digging will be uselessly incurred. Thanks to the tube system, we can search or probe for water anywhere with ease, and if we do not find it in one spot we can easily move on to another without incurring any serious trouble or expense.
We believe the idea of these “instantaneous wells” originated in the United States during the War of Secession, when some soldiers of the Northern army sunk rifle barrels into the ground, and obtained water in a barren land. To Mr. Norton the development of the idea is due, and in the Abyssinian Expedition the utility of the notion was fully demonstrated. Since that time M. Donnet of Lyons has modified and improved the tube-well, and arranged all the materials, including wider tubing and the hammers upon a carriage, thus giving greater facilities to the workmen and to those desirous of sinking such wells.
The general arrangement of M. Donnet, and the carriage with its equipments utilized, is depicted in fig. 484; the actual sinking of the well is carried out just as originally performed by Mr. Norton.
A New Swimming Apparatus.
Fig. 486.—Swimming apparatus.
We have to mention a novel means of swimming, which may prove useful to those who distrust the natural buoyancy of water and their own powers of keeping afloat or swimming. The simple apparatus, shown in fig. 486, is the invention of an American named Richardson, a citizen of Mobile, U.S.
Fig. 487.—Nautical Velocipede.
The machine consists, essentially, of a shaft, upon which a float is fixed, and at the end of the shaft is a small screw propeller. The shaft is put in motion by a wheel arrangement worked by the hands, and by a crank moved by the feet. The swimmer rests upon the float, with his head well above water. The float sustains him, while the propeller forces him through the water, without his feeling fatigued, at the rate of about five miles an hour. A certain amount of practice is necessary to obtain complete command of the machine, but when mastered the swimmer can proceed, without much exertion, at a rapid rate. The apparatus itself is not difficult to make, and persons who have tried it speak highly of its convenience and of the facilities it may afford. Captain Boyton’s swimming-dress is another useful invention, but the means of mechanical propulsion are wanting, while in this new apparatus the swimmer can drive himself through the sea with ease and expedition, and even a non-swimmer may thereby save life without danger to himself, or the person he wishes to rescue.
Fig. 488.—Trained seal drawing canoe.
The Nautical Velocipede, which also deserves some notice at our hands, is the invention of M. Croce-Spinelli, who tried it upon the great lake of Vincennes and also on the Seine, when it was the object of much curiosity; but when the Franco-German war broke out the experiments were discontinued, and the inventor did not live to perfect the apparatus. He fell a victim to his love for ballooning. But M. Joberts, a practical machinist, has lately taken up the idea broached by Croce-Spinelli, and has brought out a new water velocipede of very ingenious construction, with satisfactory results. The machine is described as follows. There are two hollow tin “floats” of cylindrical form, and tapered at the ends. These floats are joined together by a platform made of very light wood, on which the seat of the worker is raised, and underneath is the machinery for propelling the velocipede. The motive power is very simple, and corresponds to that employed to propel the bicycle on land, by the feet of the rider, the wheel being furnished with paddles in the water velocipede.
Fig. 489.—Double yachts.
A rudder, which can easily be worked by cords, gives the velocipedist complete control of the machine, the steering being performed by a handle similar to that which the bicyclist uses to turn the machine he rides. In fact, the “water” velocipede is an adaptation of the “terrestrial” machine so familiar to all readers. This velocipede is equally adapted for sea or lake progression, the waves of the former being, under ordinary circumstances, no obstruction, for very little motion is imparted to the sitter. For those desirous to bathe in deep water the machine offers many facilities; and in the case of attack of cramp or faintness, rescue would not be difficult, as the swimmer could support himself upon the pointed cylinders of the water velocipede till assistance arrived. On the other hand, it is very necessary to know how to swim before attempting to work the machine.
Fig. 490.—Ice boats.
Before describing the ice-yachts which are used in Canada when winter’s cold grasp lies on water and land, we will mention a very curious experiment in water locomotion made a year or two ago. The illustration explains itself. It is not an imaginary sketch, it is the record of fact.
This sagacious seal was exhibited in London, and was in the habit of performing certain tricks, one item of his performance being to draw the light canoe (as represented), and another accomplishment consisted in “striking the light guitar,” to the astonishment of the spectators, amongst whom was the writer. The instrument was placed between his fins, or “flappers,” and the seal twanged it more or less melodiously. He was very tame, and obedient to his master and trainer.
We all have heard of, even if we have not seen, the twin steamer Castalia, which, pending the opening of the tunnel beneath the Channel, was supposed to reduce sea-sickness to a minimum. The Castalia did not answer, however, but an American has planned certain double yachts, of which we give an illustration. The sailing-boats, as represented, have had much success upon the lake of Cayuga, and are quite seaworthy,—in fact, it is impossible to overturn them.
The weight of one of these yachts is about fifteen hundred pounds, and the draught six inches. Having two keels they answer the helm very readily. The boat, in the centre of the illustration, belongs to Mr. Prentiss, and is called the Pera Ladronia. It is a very fast “ship.”
From navigation in water, we now come to navigation on water. The ice-boats are much used in Canada, and their simple but effective construction will be readily perceived from the accompanying illustration. The Americans state that these ice-yachts can run before a good breeze as fast as an ordinary train. There are, or were, models of some such (Finland) yachts in the South Kensington Museum with two sails. The American yacht, as a rule, has only one sail, and the owners say—but we will not vouch for the truth of the allegation—that they frequently run far ahead of the wind that primarily propelled them!
Sailing on Land.
Fig. 491.—Sailing carriage of the 17th century, from a drawing of the period.
It is quite possible to sail upon land, although this statement may appear contradictory in terms. “The force of the wind upon sails,” says Bishop Wilkins in his work, “Mathematical Magic,” printed in London in 1648, “can be applied to vehicles on land as well as to ships at sea. Such conveyances,” he adds, “have long been in use in China and in Spain, as well as in flat countries, such as Holland, where they have been employed with great success. In the last-named country they are propelled with greater speed than are ships before a fair wind; so that in a few hours a boat containing several persons actually travelled nearly two hundred miles, with no trouble to any one on board except the steersman, who had little difficulty in guiding the boat.”
The astonishment expressed by the good bishop was quite justified, for, as a matter of fact, a carriage or boat on wheels, with sails, as shown in the illustration, achieved a distance of nearly thirty-eight miles in an hour. This pace was quite unknown at that time; such a rate of travelling had never entered the minds of people then. “Men running in front of the machine after a while appeared to be going backwards, so quickly were they overtaken and passed.” “Objects at a distance were approached in the twinkling of an eye, and were left far in the rear.” So it is evident that, had locomotion by steam not been adopted, the mode of sailing on land would have eventually become the most rapid mode of transit, and it is rather remarkable that it was never adopted as a mode of travel.
Fig. 492.—On the Kansas Pacific Railway.
But Bishop Wilkins had not to reproach himself on this account, for he adapted the principle of the windmill to carriages, “so that the sails would turn and move his car, no matter in what direction the wind was blowing.” He proposed to make these sails act upon the wheels of a carriage, and trusted to “make it move in any direction, either with the wind or against it!” This suggestion has been lately adopted in the United States, and it is curious that after two hundred and fifty years no better mode for utilizing wind-power on land has ever been found. Perhaps the ice-boats already mentioned may be the forerunners of some new system of “land transport,” for which enormous kites have been made available.
It is somewhat remarkable that if the introduction of railroads quite “took the wind out of the sails” of any other mode of locomotion on terra firma, it is that very iron track which has led to the reintroduction of sails as a mode of progression upon the rails. In the United States at the present time there are many vehicles propelled by sails across the immense prairies at a pace, with a strong wind, which equals that of the trains. We are indebted to Mr. Wood, of Hayes City, Kansas, for the photograph from which the picture of the sailing-waggon, invented by Mr. Bascom, of the Kansas Pacific Railway, is copied. This carriage travels usually at thirty miles an hour, and a speed of forty miles an hour has been obtained when the wind has been high and blowing directly “aft.” The distance of eighty-four miles has been accomplished in four hours when the wind was “on the beam,” or a little forward of it, and on some curves with an almost contrary breeze.
The newest machine has four wheels, each thirty inches in diameter; it is six feet in length, and weighs six hundred pounds. The sails are carried upon two masts, and they contain about eighty-one square feet of canvas. The main, or principal mast, is eleven feet high, four inches in diameter at the base, and two inches at the top. As in the case of the ice-boats, it is claimed for the sailing carriage that it frequently outstrips the wind that propels it along the track. On the other hand, there is a difference between the best sailing points of the two kinds of vehicle. The ice-boat goes quickest with the wind “dead aft,” the carriage makes best time with the wind “on the beam”—i.e., sideways. The greater friction and larger surface exposed to the influence of a side-wind no doubt will account for the difference between the speed of the railway sailing-carriage and the ice-boat.
Mr. Bascom informs us that the carriage we have described is in frequent use upon the Kansas Pacific Railway, where it is employed to transport materials for the necessary repairs of the line, telegraph, etc., etc. It is a very cheap contrivance, and a great economizer of labour. We all have noticed the cumbrous method of “trolly-kicking” by “navvies” along the line. A trolly fitted with a sail would, in many cases, and on many English lines, save a great deal of trouble, time, and exertion to the plate-layers.
CHAPTER XXXIII.
ASTRONOMY.
INTRODUCTORY—HISTORY OF ASTRONOMY—NOMENCLATURE.
Fig. 493.—Celestial globe.
Astronomy is the science which treats of the heavenly bodies and the laws which govern them. The term is derived from two Greek words, astron, a star, and nomos, a law. It may be included in the study of Physics, for the motion of the planetary bodies and equilibrium, gravity, etc., all have something to say to the arrangements and positions of the stars. The space in which they are set is infinite, and known as the “Firmament,” or “Heaven.” The number of the heavenly bodies must therefore be infinite also. We can see a few stars, comparatively speaking, and there must be numbers whose light has never yet reached the earth. When we calmly reason upon the immeasurable distances and the awful rapidity of motion, with the masses of matter thus in movement, we are constrained to acknowledge that all our boasted knowledge is as nothing in the wondrous dispensations of Him “who telleth the number of the stars, and calleth them all by their names.”
Astronomy, no more than any other of the physical sciences, cannot stand by itself. We have seen how heat, light, electricity, etc., are all, in a manner, inter-dependent. So astronomy is dependent upon mathematics, particularly geometry and trigonometry, for the wondrous problems to be solved. But in the following sketch we do not propose to plunge the reader in the slough of calculations. We only desire to put plainly before him the great phenomena of nature with regard to the heavens, and the glorious orbs which so thickly stud the space above us. We need not detail the laborious calculations by which philosophers have arrived at certain discoveries. We may refer to the results and explain general principles, thereby indicating the road by which the student may arrive at the more difficult bypaths in the fields of scientific discovery.
The history of astronomy is nearly as old as the world itself, or rather as old as the human race. From the earliest ages we can picture men gazing upon the “spangled heavens,” and the wandering tribes of the desert were always very careful observers of the paths of the stars. To the nomads of the East the planetary system served as compass and clock, calendar and barometer.
We shall find, therefore, that many observations of the heavenly bodies were made by the ancients, and have descended to more advanced generations, and this leads us to remark that the science of astronomy can be studied without any very special or costly apparatus. In other branches of science numerous instruments are indispensable before we can reveal to ourselves the desired results. In astronomy, a telescope—even a good field glass, such as possessed by any household, will reveal many interesting facts. We will, by means of more expensive instruments, and by the aid of large telescopes particularly, enjoy the sight of the moon and planets. But even with the naked eye a great variety of phenomena may be observed. With a celestial globe in our hands upon a fine starry night, we can easily find out the position of the constellations, and trace their forms in the firmament.
It is to the Chaldeans, Indians, Chinese, and Egyptians, that our knowledge of astronomy is primarily due. They did much to facilitate the observation of the stars; they named the planets, grouped the stars, and marked the sun’s track in the sky. Astrology was cultivated in very remote ages. The Jews practised it; and the astrologers of subsequent periods played very important parts in divining the future of individuals, and casting their horoscopes. Many of these so-called predictions came true, “because,” as was remarked by Pascal, “as misfortunes are common they” (the astrologers) “are often right,” as they foretold misfortune oftener than good fortune. Still the fact remains that occasionally a very startling prediction was made, and proved true; such, for instance, as the laying waste of Germany by Gustavus Adolphus, which was foretold by Tycho Brahé after his consideration of a certain comet, and the date of the king’s death was also correctly prophesied. Astrology, therefore, held a very considerable influence over the human race during the Middle Ages.
We can only give a very brief historical summary of the science. We know that the destinies of individuals and nations were at a very early period attributed to the influence of the stars. We read that “the stars in their courses fought against Sisera,” and many expressions surviving to the present time serve to remind us that the stars were at one time paramount in men’s minds. Thus we have the phrases—“unlucky star,” “born under a lucky star,” “mark my stars,” “moonstruck,” etc. Even the common term “consider”—to take counsel of the stars—is thus accounted for, and many men have a habit of looking up to the ceiling of a room or to the sky when thinking deeply—considering with the stars. “Contemplate” is another term signifying the same thing; for templum, a temple, was formerly a space marked upon the sky in imaginary lines, and traced on the ground in accordance with the supposed diagram. Thus temple became a place for heavenly “contemplation,” and by an easy transition to a place of worship. In our old poets’ writings we have many allusions to the influences of the stars.
“Now glowed the firmament
With living sapphires; Hesperus, that led
The starry host, rode brightest, till the moon,
Riding in clouded majesty, at length
Apparent queen, unveiled her peerless light,
And o’er the dark her silver mantle threw.”—Milton.
Although from Thales, who lived B.C. 610, the real science of astronomy may be allowed to date, there can be no doubt that the ancients were acquainted with many phenomena. The Chaldeans were, doubtless, the first to place on record the rising and setting of the celestial bodies and eclipses, and used the water-clock (clepsydra). A list of eclipses from 2234 B.C. is stated to have been found at Babylon by Alexander the Great. The Chaldeans also divided the ecliptic into twelve equal parts, and the day and night into twenty-four hours. The Chinese, again, have recorded astronomical phenomena as far back as 2857 B.C.; and the Egyptians also were well versed in the science, although no records of much importance remain to us, unless the zodiac signs were their invention.
Thales predicted the eclipse of the sun B.C. 610. Aristarchus and Eratosthenes also made important observations. Hipparchus (160-125 B.C.) discovered the precession of the equinoxes, calculated eclipses, determined the length of the year, etc., etc.
Ptolemy, of Alexandria, A.D. 130-150, was the founder of a theory called the Ptolemaic System, which recognized the earth as the centre of all—the sun, moon, stars, etc., all revolving in very complicated courses around it, as figured in the diagram herewith. Even though his theory turned out to be untenable, he paved the way for his successors in other ways, and left a valuable collection of observations on record. In this volume, called the “Almagest,” he reviewed the state of the science, and gave a catalogue of stars, as well as a description of the heavens. He discovered the lunar evection.
Fig. 494.—Ptolemaic System.
After his time astronomy, though it was not neglected, appeared to droop, and it is at a comparatively late period that we again open the records—viz., in 1543, the year in which Copernicus died. This philosopher, who was born in 1473, promulgated the true theory of the solar system. He placed the sun in the centre of the planets, and by this he explained their motion around the sun, though they appeared to be carried round the earth. The book in which he explained his theory, “De Revolutionibus Orbium Celestium,” was not finished till a day or two before he died.
The justly celebrated Tycho Brahé was the most important of the successors of Copernicus, but he opposed the Copernican theory, while other able philosophers agreed with it. Brahé was a Dane; he died in 1601. He adopted the theory that the sun and moon revolved around the earth, while the (other) planets moved around the sun. This theory did not gain much credence, but he, again, though he could not defeat Copernicus, and though he was wrong in his assumption, made many important investigations. After him came Kepler, whose observations upon the planet Mars cleared away many complications, and he laid down three laws, which are as follows:—
1. Every planet describes an elliptic orbit about the sun, which occupies one focus of each such ellipse.
Fig. 495.—Copernican System.
2. If a line be drawn from the sun, continually, to any planet, this line will sweep over equal areas in equal times.
3. The squares of the periodic times of the planets are proportional to the cubes of their mean distances from the sun.
Kepler also remarked that gravity was a power existing between all bodies, and reasoned upon the tides being caused by the attraction of the moon for the waters.
Fig. 496.—Ellipse.
Fig. 497.—Radii Vectores.
It was about this time—viz., the beginning of the seventeenth century—that the telescope was invented, and logarithms came into use. The actual discoverer of this now almost perfected instrument is uncertain. Borelli, who wrote in the seventeenth century, ascribes the discovery to Zachariah Jansen and Hans Lippersheim, spectacle makers of Middleburg. Baptista Porta, also a spectacle maker, has had the credit of discovering the magnifying power of the lens, and, so far, the originator of the telescope.
Fig. 498.—Ecliptic and Equator.
But whoever invented it, the telescope did not penetrate into southern Europe till 1608-9. Galileo then made inquiries concerning the new instrument, and Kepler made some propositions for their construction. But Harriot had used the instrument so far back as 1611 or 1612, and had observed spots upon the sun’s disc. Galileo, in 1610, had also made observations with the telescope, and discovered the satellites of Jupiter. He thereby confirmed the Copernican theory;[25] and when Newton promulgated his immortal discovery of gravitation, after Picard’s researches, the relations of the sun and planets became more evident. His researches were published in the Principia, and then one-half the scientific world began to question the principle of gravitation, which was supported by Newton and his adherents. Subsequently the researches of Lagrange and Laplace, Adams and Leverrier, Sir J. Herschel, etc., brought astronomy into prominence more and more; and the innumerable stars have been indicated as new planets have been discovered. The spectroscope, which gives us the analyses of the sun and other heavenly bodies, has, in the able hands of living astronomers, revealed to us elements existing in the vapours and composition of the sun, etc. Stars are now known to be suns, some bearing a great resemblance to our sun, others differing materially. The nebulæ have been analysed, and found to be stars, or gas, burning in space—hydrogen and nitrogen being the chief constituents of this glowing matter. Instruments for astronomical observation have now been brought to a pitch of perfection scarcely ever dreamed of, and month by month discoveries are made and recorded, while calculations as to certain combinations can be made with almost miraculous accuracy. The transit of Venus, the approaches of comets, eclipses, and the movements of stars, are now known accurately, and commented upon long before the event can take place.
We will close this chapter by giving a brief explanation of the various definitions most usually employed in astronomy.
1. The Axis of the earth is an imaginary line passing through the centre (north and south); the poles are the extremities of this line.
2. The Equator is an imaginary circle passing round the globe, dividing it into northern and southern hemispheres. The equinoctial is the plane of the former circle extended to the heavens, and when the sun appears in that line the days and nights are of equal duration—twelve hours each.
3. The Ecliptic is the sun’s path through the heavens—though, of course, the sun does not actually move, and therefore the track, or supposed circle, is really the earth’s motion observable from the sun. When the moon is near this circle eclipses happen. The ecliptic cuts the equinoctial at an angle of about 23°. One half is to the north and the other to the south of the equinoctial.
Fig. 499.—The Zodiac.
4. The Zodiac is a girdle extending 8° on each side of the ecliptic, in which space of 16° the planets move. The zodiac is divided into twelve parts of 30° each, called the “Signs.” These names are as under written:—
| Northern Signs. | |
|---|---|
| Spring. | Summer. |
| Aries, the Ram, March. Taurus, the Bull, April. Gemini, the Twins, May. | Cancer, the Crab, June. Leo, the Lion, July. Virgo, the Virgin, August. |
| Southern Signs. | |
| Autumn. | Winter. |
| Libra, the Balance, September. Scorpio, the Scorpion, October. Sagittarius, the Archer, November. | Capricornus, the Goat, December. Aquarius, the Waterbearer, January. Pisces, the Fishes, February. |
5. Colures are two circles dividing the ecliptic into four equal parts, and making the seasons.
6. The Horizon is the boundary line of our vision, and is called the sensible (apparent) horizon. The true horizon is the circle—as on a globe—dividing the heavens into two hemispheres. The sensible horizon is enlarged according as the eye is elevated above the ground. A man six feet high can see a distance of three miles when standing on a plain. We can always find the distance visible when we know the height at which we stand, or, inversely, we can tell the height of an object if we know the distance. We have only to increase the height one half in feet, and extract the square root for the distance in miles. On giving the distance in miles reverse the operation.
Fig. 500.—Right ascension.
For instance, for the man six feet high, as supposed, add three feet, being half his height; that makes nine feet. The square root (or number multiplied by itself to give nine) is three, which is the number of miles the man can see on a plain. Or, again, suppose we can see a tower on the level, and we know we are twelve miles away from it. The square of twelve is one hundred and forty-four feet, one-third of that is forty-eight feet, which represents the half of the original height added to the whole tower in feet; so the whole tower is ninety-six feet high. Reversing, as in the former case, we can prove this by taking the tower at ninety-six feet high and trying to find the distance we can see from its summit = 96 + 48 = 144; the square root of 144 = 12, the distance required.
7. The Nadir and the Zenith are the poles of the horizon. The zenith is exactly overhead, the nadir exactly under foot. Circles drawn through these points are azimuth circles.
8. Meridians are circles passing through the poles at right angles to the equinoctial. Every place is supposed to have a meridian, but only twenty-four are upon the globe, and they represent the sun’s, or the planets’, “movements” every hour—15° being one hour, 360° being twenty-four hours (see fig. 500). One quarter of a degree equals one minute of time. Parallels of latitude are familiar circles parallel to the equator. Latitude in astronomy is the distance from the ecliptic at a right angle north or south. This will be explained as we proceed.
Fig. 501.—Orbit of planet.
9. Declination is the distance of the heavenly bodies from the equinoctial measured as a meridian.
The Tropics indicate the limits of the sun’s declination.
10. Disc is the term applied to the apparently flat surface of a planet, such as the moon, for instance.
11. The Orbit is the path described by a planet revolving round the sun. The plane of the orbit is an imaginary surface cutting through the centre of the sun and the planet, and extending to the stars. The diagram shows the plane of the earth’s orbit. The circle, A B C D (fig. 501), is the ecliptic. The inclination of an orbit is the plane of the orbit with reference to the plane of the earth; and, supposing the shaded part of the illustration to be water, a hoop held inclined towards the earth, with one half in and the other half out of the water, will describe the planetary orbit.
Fig. 502.—Conjunction of Venus and Saturn.
12. Nodes are the opposite points of a planet where its orbit cuts the ecliptic or the earth’s orbit.
13. Apogee is the point of a planet’s orbit farthest from the earth. Perigee is the nearest point.
14. The terms Culmination, Conjunction, and Opposition require no special explanation. But planets are in conjunction with each other when in the same sign and degree. A planet with the sun between it and the earth is in conjunction with the sun. With the earth between it and the sun it is in opposition.
15. Latitude and longitude upon a celestial globe are known respectively as “Declination” and “Right Ascension.”
16. The Radius Vector is a line drawn from a planet to the sun, wherever the planet may be (see fig. 497).
CHAPTER XXXIV.
ANGLES AND MEASUREMENT OF ANGLES.
THE QUADRANT—TRANSIT INSTRUMENT—CLOCKS—STELLAR TIME—SOLAR TIME—“MEAN” TIME.
We must say a few words respecting the various instruments and aids to astronomical observation before proceeding, for astronomy requires very accurate calculations; and though we do not propose to be very scientific in our descriptions, some little idea of the manner in which observations may be made is necessary. The first thing to see about is the Angle.
Suppose we draw four lines on a piece of paper, ab and cd. These intersect at a point, m. We have then four spaces marked out, and called angles. The four angles are in the diagram all the same size, and are termed right angles, and the lines containing them are perpendicular to each other.
Fig. 503.—Right angles.
But by altering the position of the lines (see fig. 504), we have two pairs of angles quite different from right angles; one angle, a´ m´ c´, is smaller, while a´ m´ d´ is much larger than the right angle. The former kind are called acute, the latter obtuse angles. We can therefore obtain a great number of acute angles, but only three obtuse, and four right angles around a given point, m.
Fig. 504.—Obtuse and acute angles.
The length of the sides of an angle have no effect on its magnitude, which is determined by the inclination of the lines towards each other. We now may consider the magnitude of angles, and the way to determine them. For this purpose we must describe a circle, which is figured in the diagram. But what is a circle?—A circle is a curved line which always is at the same distance from a certain fixed point, and the ends of this line meet at the point from which the line started.
Fig. 505.—The circle, etc.
If we fasten a nail or hold a pencil on the table, and tie a thread to it, and to the other end of the thread another pencil, we can describe a line around the first pencil by keeping the thread tightly stretched. This line is at all points at equal distance from the centre point. Any line from the centre to the circumference is called a radius, and a line through the centre to each side of the circumference is the diameter, or double the radius. The circumference is three (3·14) times the diameter. Any portion, say k i l, is an arc, and the line, k l, is the chord of that arc. A line like m n is a secant, and o p is a tangent, or a line touching at one point only.
Fig. 506.—Circle and angles.
We may now resume our consideration of the angles by means of the circle. Let us recur to our previous figure of the right angles, around which we will describe a circle. We see that the portion of the circumference contained between the sides of the right angle is exactly one-fourth of the whole. This is termed a quadrant, and is divided into 90°—the fourth of 360 equal parts or degrees into which the whole circumference is divided. The angle of 45° so often quoted as an angle of inclination is half a right angle. To measure angles an instrument called a Protractor is used.
Fig. 507.—The Protractor.
The Protractor, as will be seen from the accompanying illustration (fig. 507), is a semi-circle containing 180°. The lower portion is a diagonal scale, the use of which will be explained presently. The Protractor measures any actual angle with accuracy. If we put the vertical point of the angle and the centre point of the circle together, we can arrive at the dimensions of the angle by producing the lines containing it to the circumference. An angle instrument, figured herewith, may be assumed as the basis of most apparatus for measuring angles. An index hand, R R, moves round a dial like the hand of a clock, and the instrument is used by gazing first at one of the two objects, between which the angle we wish to determine is made—like the church steeples (fig. 508) for instance. The centre of the instrument is placed upon the spot where lines, if drawn from the eye to each of the objects, would intersect. The index hand is then put at 0°, and in a line between the observer and the object, A. Then the index is moved into a similar position towards B, and when in line with it the numbers of degrees passed over (in this imaginary case 20), shows the magnitude of the angle.
Fig. 508.—Determination of distance.
Fig. 509.—Measuring angles.
The simple quadrant is shown in the cut (fig. 510). This was so arranged that when any object in the horizon was being looked at through the telescope attached, a plummet line is at 0°. But if the telescope be raised to C S, the quadrant will move, and the line will mark a certain number of degrees of the angle which a line if drawn from the star makes with the line of the horizon. The “Astronomical Quadrants” are as shown in fig. 516, and consist of a quadrant of wood strengthened and fitted with a telescope. The circle is graduated on the outer edge, and a “vernier” is attached. The time is determined by the observation of the altitude of a star, and then by calculation finding out at what time the star would have the observed altitude. The quadrant is now superseded by circular instruments.
Fig. 510.—The quadrant.
Fig. 511.—Ellipse.
An ellipse is a flattened circle, or oval, and will be understood from the diagrams. Let us fix two pegs upon a sheet of paper, and take a thread longer than the distance between the pegs; draw with the pencil controlled by the thread a figure, keeping the thread tight. We shall thus describe an oval, or ellipse. The orbit of nearly all the heavenly bodies is an ellipse. The parabola is another curved line, but its ends never meet; they become more and more distant as they are continued. The comets move in parabolic curves, and consequently do not again come within our vision unless their direction be altered.
Fig. 512.—Ellipse.
Fig. 513.—Diagonal scale.
This figure has a long axis, ab (fig. 512); and perpendicular to this a short axis, de, passing through the centre, c. The two points, S S′, are called the foci of the ellipse; also, as is evident from the construction of the figure, any two lines drawn from the two foci, to any point of the circumference, for instance, S and S′m, or Sm′ and S′m′, etc., which represent the thread when the pencil is at m or m′, are together equal to the larger axis of the ellipse. These lines, and we may imagine an infinite number of such, are called radii vectores. The distance of the foci, S or S′, from the centre, c, is called the eccentricity of the ellipse. It is evident that the smaller the eccentricity is, the nearer the figure approaches to that of the circle. The superficies of the ellipse is found by multiplying the two half axes, ac and dc, by each other, and this product by the number 3·14.
The Diagonal Scale is shown in the margin. It is used to make diagrams so as to bring the relative distances before the eye. The larger divisions represent, it may be, miles, or any given distance; the figures on the left side tenths, and the upper range hundredths of a mile. So a measurement from Z to Z´ will represent two miles, we may say, with so many tenths and hundredths.
Fig. 514.—Transit instrument.
The Transit instrument is due to Roemer, a Danish astronomer. It consists of a telescope so constructed as always to point to the meridian, and rotates upon a hollow axis, directed east and west. At one end is a graduated circle. The optical axis of the telescope must be at exactly right angles to the axis of the instrument; it will then move on the meridian. There is an eye-piece filled with two horizontal and five vertical wires, very fine, the latter at equal distances apart. The star appears, and the time it takes to cross is noted as it passes between each wire, and the mean of all the transits will be the transit on the meridian. For if we add the times of all the transits across the wires, and divide by five the number of them we shall get at a true result.
Fig. 515.—The eyepiece of transit instrument.
A good clock is also a necessary adjunct for astronomical observations, and the astronomical clocks and chronometers now in use record the time with almost perfect accuracy. The improvement in telescopes, the use of micrometers, etc., have greatly facilitated observations. In the transit clock we have a most useful timekeeper, for the ordinary clocks are not sufficiently accurate for very close observations. The sidereal time differs from solar time, and the twenty-four hours’ period is calculated from the moment a star passes the meridian until it passes it again. The sidereal day is nearly four minutes shorter than the solar day, and the sidereal clock marks twenty-four hours instead of twelve, like the old dial at Hampton Court Palace over the inner gate. The Chronograph has also been useful to astronomers, for by “pricking off” the seconds on a roller by itself, the observer can mark on the same cylinder the actual moment of transit across each wire of the instrument, and on inspection the exact moment of transit may be noted.
The Equitorial is another useful instrument, and by its means the whole progress of a star can be traced. The Equitorial consists of a telescope fixed so that when it has been pointed at a certain star a clock-work movement can be set in motion, which exactly corresponds with the motion of the star across the heavens, and so while the star moves from its rising to setting it is under observation. Thus continuous observations maybe made of that particular star or comet without any jerking or irregular movement.
We can thus see the uniform motion of the stars which go on in greater or lesser circles as they are nearer to or farther from the pole; and with the exception of the polar star, which, so far as we are concerned, may be considered stationary, every star moves round from east to west—that is, from the east of the polar star to the west of it, in an oblique direction. Therefore, as Professor Airy remarks, “Either the heavens are solid, and go all of a piece, or the heavens may be assumed to be fixed or immovable, and that we and the earth are turning instead of them.”
Fig. 516.—Astronomical quadrant.
The Mural Circle is another very useful instrument, and is used by calling to aid the powers of reflection of quicksilver, in which a bright star will appear below the horizon at the same angle as the real star above the horizon, and thus the angular distance from the pole or the horizon of any star can be calculated when we know the inclination of the telescope. The Transit Circle is also used for this purpose, and is a combination of the transit instrument with the circle. In all calculations allowance must be made for refraction, for which a “Table of Refractions” has been compiled. From the zenith to the horizon refraction increases. The effect of refraction can be imagined, for when we see the sun apparently touching the horizon the orb is really below it, for the refraction of the rays by the air apparently raises the disc.
The clock and chronometer are both very useful as well as very common objects, but a brief description of the pendulum and the clock may fitly close our remarks upon astronomical apparatus and instruments. The telescope has been already described in a previous portion of this work, so no more than a passing reference to it has been considered necessary. We therefore pass on to a consideration of the measurement of time, so important to all astronomers and to the public generally.
Time was measured by the ancients by dividing the day and night into twelve hours each, then by sun-dials and water-clocks, or clepsydra, and sand-clocks. The stars were the timekeepers for night before any mechanical means of measurement were invented.
“What is the star now passing?—
The Pleiades show themselves in the east,
The eagle is soaring in the summit of heaven.”—Euripides.
Sun-dials were in use in Elijah’s time, and the reference to the miracle of the sun’s shadow going back on the dial as a guarantee to Hezekiah, will be recalled at once by our readers. These dials were universal, and till sunset answered the purpose. But the hours must have been very varying, and on cloudy days the sun-dials were practically useless.
The water-clocks measured time by the dripping or flow of water, and they were used to determine the duration of speeches, for orators were each allotted a certain time if a number of debaters were present. This method might perhaps be adapted to the House of Commons, and speaking by the clock might supersede clôture. We find allusions to these practices in the orations of Demosthenes. Even this system was open to objection, for the vases were frequently tampered with, and an illiberal or objectionable person was mulcted of a portion of the water, while a generous or popular adversary had his clepsydra brimming full. Some of these water-clocks were of elegant design, and a Cupid marked the time with arrows on the column of the clock of Ctesibius, while another weeping kept up the supply of water. The motive power was the water, which filled a wheel-trough in a certain time, and when full this trough turned over, and another was filled. The wheel revolved once in six days; and by a series of pinions and wheels the movements were communicated more slowly to the pillar on which the time was marked for 360 days, or with other arrangements for twenty-four hours.
The repeating of psalms by monks also marked the time, for by practice a monk could tell pretty accurately how many paternosters or other prayer he could repeat in sixty seconds. At the appointed hour he then awoke the monastery to matins.
Nature also marks time for us—as, for example, the age of trees by means of rings—one for each year; and horses’ teeth will guide the initiated to a guess at the ages of the animals, while the horns of deer or cattle serve a like purpose. But man required accuracy and minute divisions of time. He had recourse therefore, to machinery and toothed wheels. Till the mechanical measurement of time was adopted, the sunrise and sunset only marked the day, and the Italians as well as Jews counted twenty-four hours from sunset to sunset. This was a manifestly irregular method. To this day we have marked differences of time in various places, and at Geneva we have Swiss and French clocks keeping different hours according to Paris or Berne “time.” This, of course, is easily accounted for, and will be referred to subsequently.
Fig. 517.—Clock movement.
We have read that the first clock in England was put up in Old Palace yard in 1288, and the first application of the toothed-wheel clock to astronomical purposes was in 1484, by Waltherus, of Nuremberg. Tycho Brahé had a clock which marked the minutes and seconds. If we had had any force independent of gravitation which would act with perfect uniformity, so that it would measure an equal distance in equal spaces of time, all the various appliances for chronometers would have been rendered useless. In the supposed case the simple mechanism, as shown in the margin (fig. 517), would have sufficed. The same effect would be produced by the spring, were it possible that the spring by itself would always uncoil with the same force. But it will not do so: we therefore have to check the unwinding of the cord and weight, for left to itself it would rapidly increase in velocity; and if we likewise make an arrangement of wheels whereby the spring shall uncoil with even pressure all the time, we shall have the principle of the watch.
It is to Huygens that the employment of the pendulum in clocks is due, and the escapement action subsequently rendered the pendulum available in simple clocks, while the manner of making pendulums self-regulating by using different metals, has rendered timepieces very exact. Of course the length of a pendulum determines the movement, fast or slow; a long pendulum will cause the hands of the clock to go slower, for the swing will be a fraction longer. A common pendulum with the escapement is shown (fig. 518). Each movement liberates a tooth of the escapement. The arrangement of wheels sets the clock going. The forms of pendulum are now very varied.
Fig. 518.—Pendulum and escapement.
But in watches the pendulum cannot be used. The watch was invented by Peter Hele, and his watches were called “Nuremburg eggs” from their shape. The weight cannot be introduced into a watch, and so the spring and fusee are used. The latter is so arranged that immediately the watch is wound and the spring at its greatest tension, the chain is upon the smallest diameter of the fusee, and the most difficult to move. But as the spring is relaxed the lever arm becomes longer, and the necessary compensating power is retained. Watches without a “fusee” have a toothed arrangement beneath the spring.
Fig. 519.—Balance.
The Pendulum. A “simple” pendulum is impossible to make, for we cannot put the connecting line between the “bob” and the clock-work out of consideration, so “simple pendulums” are looked upon as “mathematical abstractions.” The most modern clocks have what is called a “deadbeat” escapement, and a compensating pendulum. Clocks are liable to alter by reason of the state of the air and varying temperature, and until all our clocks are placed in vacuo we must be content to have them lose or gain a little. There is a magnet arrangement by which the Greenwich Observatory clocks keep time by compensation, corresponding with the fall or rise of the syphon barometer attached to it. The description need not be added. We may here state that detailed descriptions of all the instruments used in the Observatory, together with full information as to their use, will be found in a very interesting work by Mr. Lockyer, entitled “Stargazing,” to which we are indebted for some corrections in our summary.
Fig. 520—Regulator.
We have spoken of solar time and sidereal time, and no doubt someone will inquire what is meant by mean time—an expression so constantly applied to the Greenwich clock time. Stellar time, we have seen, corresponds to the daily revolution of a star or stars. Solar time is regulated by the sun, and this is the astronomical time generally observed, except for sidereal investigations. But the sun is not always regular; the orbit of the earth causes this irregularity partly. The earth moves faster in winter than in summer, so the sun is sometimes a little fast and sometimes a little slow. Astronomers therefore strike an average, and calculate upon a Mean Sun, or uniform timekeeper. Mean time and true (apparent) time are at some periods the same—viz., on the 15th of April, on the 14th of June, on the 31st of August, and on the 24th of December. Twice it is after, and twice before it. The time occupied by this “mean” sun passing from the meridian and its return to it, is a mean solar day, and clocks and chronometers are adjusted by it.
Fig. 521.—Fusee and spring.
Twenty-four hours represents a complete revolution of the heavenly bodies. The mean solar time is 23h 56´ 4·091″, while twenty-four hours of mean time are equal to 24h 3´ 56·55″ of sidereal time. The difference between the times is given by Dr. Newcomb as follows, and is called the Equation of Time:—
| Differences between Mean and Apparent Time. | ||
|---|---|---|
| February 10th | True Sun | 15 minutes slow. |
| April 15th | ” | Correct. |
| May 14th | ” | 4 minutes fast. |
| June 14th | ” | Correct. |
| July 25th | ” | 6 minutes slow. |
| August 31st | ” | Correct. |
| November 2nd | ” | 16 minutes fast. |
| December 24th | ” | Correct. |
Measurement of Distances.
Before passing to consider the planetary system we must say a few words respecting the manner of ascertaining the distances of inaccessible objects, and by so doing, we shall arrive at an idea how the immense distances between the sun (and the planets) and the earth have been so accurately arrived at. To do this we must speak of parallax, a very unmeaning word to the general reader.
Fig. 522.—Works of a clock.
Parallax is simply the difference between the directions of an object when seen from two different positions. Now we can illustrate this by a very simple method, which we have often tried as a “trick,” but which has been very happily used by Professor Airy to illustrate the doctrine of parallax. We give the extract in his own words:—
“If you place your head in a corner of a room, or on a high-backed chair, and if you close one eye and allow another person to put a lighted candle upon a table, and if you then try to snuff the candle with one eye shut, you will find you cannot do it.... You will hold the snuffers too near or too distant—you cannot form any idea of the distance. But if you open the other eye, or if you move your head sensibly you are enabled to judge of the distance.” The difference of direction between the eyes, which is so well known to all, is ready a parallax. It can also be illustrated by the diagram herewith.
Fig. 523.—Parallax.
If two persons, A and C (fig. 523), from different stations, observe the same point, M, the visual lines naturally meet in the point, M, and form an angle, which is called the angle of parallax. If the eye were at M, this angle would be the angle of vision, or the angle under which the base line, A C, of the two observers appears to the eye. The angle at M also expresses the apparent magnitude of A C when viewed from M, and this apparent magnitude is called the parallax of M.
Let M represent the moon, C the centre of the earth represented by the circle, then A C is the parallax of the moon; that is, the apparent magnitude the semi-diameter of the earth would have if seen from the moon. If the moon be observed at the same time from A, being then on the horizon, and from the point B, being then in the zenith, and the visual line of which when extended passes through the centre of the earth, we obtain, by uniting the points, A C M, by lines, the triangle, A C M.
Therefore, as A M, the tangent of the circle stands at right angles to the radius, A C, the angle at A is a right angle, and the magnitude of the angle at C is found by means of the arc, A B, the distance of the two observers from each other. As soon, however, as we are acquainted with the magnitude of two angles of a triangle, we arrive at that of the third, because we know that all the angles of a triangle together equal two right angles (180°). The angle at M, generally called the moon’s parallax, is thus found to be fifty-six minutes and fifty-eight seconds. We know that in the right-angled triangle M C A, the measure of the angle, M = 56´ 58″, and also that A C, the semi-diameter of the earth = 3,964 miles. This is sufficient, in order by trigonometry, to obtain the length of the side, M C; that is, to find the moon’s distance from the earth. A C is the sine of the angle, M, and by the table the sine of an angle of 56´ 58″ is equal to 1652/100000; or, in other words, if we divide the constant, M C, the distance of the moon, into 100,000 equal parts, the sine, A C, the earth’s semi-diameter = 1,652 of these parts. And this last quantity being contained 60 times in 100,000, the distance of the moon from the earth is equal to 60 semi-diameters of the earth, or 60 × 3964 = 237,840 miles.
Fig. 524.—Parallax explained.
In a similar way the parallax of the sun has been found = 8″·6, and the distance of the sun from the earth to be 91,000,000 miles.
Let us first see how we can obtain the distance of any inaccessible or distant object. We have already mentioned an experiment, but this method is by a calculation of angles. The three angles of a triangle, we know, are equal to two right angles; that is an axiom which cannot be explained away. We first establish a base line; that is, we plant a pole at one point, A, and take up our position at another point, B, at some distance in a straight line, and measure that distance very carefully. By means of the theodolite we can calculate the angles which our eye, or a supposed line drawn from our eye to the top of the object (C) we wish to find the distance of, makes with that object. We now have an imaginary triangle with the length of one side, A B, known, and all the angles known; for if all three angles are equal to 180°, and we have calculated the angles at the base, we can easily find the other. We can then complete our triangle on paper to scale, and find out the length of the side of the triangle by measurement; that is the distance between our first position, A, and the object, C. It is of course necessary that all measurements should be exact, and the line we adopt for a base should bear some relative proportion to the distance at which we may guess the object to be.
In celestial measurements two observers go to different points of the earth, and their distance in a straight line is known, and the difference of the latitudes. By calling the line between the observers a base line, a figure may be constructed and angles measured; then by some abstruse calculation the distance between the centre of the earth and the centre of the moon may be ascertained. The mean distance is sixty times the radius of the earth. The measurement of the sun’s distance is calculated by the observations of the transit of Venus across his disc, a phenomenon which will again occur on 6th December, 1882, and on 8th June, 2004, the next transit will take place; there will be no others for a long time after 2004.
All astronomical observations are referred to the centre of the earth, but of course can only be viewed from the surface, and correction is made. In the cut above, let E be the earth and B a point on the surface. From B the stars, a b c d, will be seen in the direction of the dotted lines, and be projected to e i k l respectively. But from the centre of the earth they would appear at e f g h correctly. The angles formed by the lines at b c d are the parallactic angles, f i g h and h l show the parallax. An object on the zenith thus has no parallax. (See fig. 524.)
Fig. 525.—Halo Nebulæ.
CHAPTER XXXV.
THE SOLAR SYSTEM.
GRAVITATION—THE PLANETS—SIZE AND MEASUREMENT OF THE PLANETS—SATELLITES—FALLING STARS—COMETS—AEROLITES.
Fig. 526.—Planets compared with a quarter of the sun.
Gravitation is the force which keeps the planets in their orbits, and this theory, perfected by Newton, was partially known to Kepler. Newton brought this idea into practical shape, and applied it mathematically. We know that every object in the world tends to attract every other object in proportion to the quantity of matter of which each consists. So the sun attracts the planets, and they influence him in a minor degree. Likewise the moon and our earth reciprocally attract each other. But as the sun’s mass is far greater than the masses of the planets he influences them more, and could absorb them all without inconvenience or disturbance from his centre of gravity. We have, in a former portion of this work, spoken of the law of universal gravitation, which is the mutual attraction of any two bodies to each other, is directly proportioned to their masses (not size), and inversely to the square of their distances apart.
This law operates amongst the heavenly bodies, and it is to the never-changing action of gravitation that the planets are kept in their places. Let us see how this is effected. We have read of force, and motion, and rest. Every body will remain at rest unless force compels it to change its position, and it will then go on for ever in a straight path unless something stops it. But if this body be acted on simultaneously by two forces in different directions, it will go in the direction of the greater force. Two equal forces will tend to give it an intermediate direction, and an equal opposing force will stop it. The last axiom but one—viz., the two equal forces in different, not opposing directions, gives us the key to the curving line of the planetary motions. Were it not for the attraction of the sun the planets would fly off at a tangent; while, on the other hand, were not the impelling force as great as it is, they would fall into the sun. Thus they take an intermediate line, and circle round the centre of the solar system—the Sun.
The solar system consists of the sun and the planets which revolve in space around him. These stars are called planets because they move in the heavens. We shall see that they have certain motions—going from east to west, from west to east, and sometimes they appear to be quite motionless. This change of place, appearing now at one side of the sun and now at another, has given them their title of “wanderers” (planets). Besides the planets there are comets and meteors, asteroids and satellites, all circling round the sun in more or less regular orbits. And there must be families of comets, and whole systems of meteors that have not yet appeared to us, and which make up the comets, as has been lately suggested.
Five planets were known to the ancients, and were named after gods and a goddess: Mercury, Venus, Mars, Jupiter, Saturn. In later years a great number were discovered. We must, however, confine ourselves to the consideration of the principal ones, eight in number, including our own Earth, Uranus and Neptune completing the list. Of these Venus and Mercury are the inferior, or interior planets moving between us and the sun. Mars, Jupiter, Saturn, Uranus, and Neptune are superior, or exterior, and pass quite round the heavens. All the planets are spheroids, and vary greatly in their magnitude, as will be seen by the illustration (fig. 528), the largest body being the sun. Mercury, Mars, and Venus, are not so large as the Earth. The other principal planets are considerably larger than our globe.
Fig. 527.—The Moon.
Mercury is the smallest of the planets, Venus being nearly as large as the earth. Then comes Mars, which, though smaller than Venus, is larger than Mercury. Jupiter is the largest of all—the giant amongst planets, as Jove was the ruler of the gods of mythology. Saturn comes next, though much smaller than Jupiter, but bigger than all the rest together. Next Uranus, then Neptune, larger than Uranus, but farther away from us. We shall speak more in detail about these in their order separately.
Fig. 528.—Comparative size of the sun seen from the planets.
Fig. 529.—Sizes of the planets.
Taking the earth as 1, the comparative VOLUMES of the planets are as follows:—
Mercury 1/25, Venus 4/5, Mars 1/5, Jupiter 1300, Saturn 900, Uranus 80, Neptune 230. Sir John Herschel gives the following illustration of magnitudes and distances:—
Fig 530.—Sizes of planets.
“Choose any well-levelled field or bowling green; on it place a globe two feet in diameter; this will represent the sun. Mercury will be represented by a grain of mustard seed on the circumference of a circle 164 feet in diameter for its orbit; Venus a pea, on a circle 284 feet in diameter; the Earth also a pea on a circle 430 feet; Mars a rather large pin’s head on a circle of 654 feet; Juno, Ceres, Vesta, and Pallas grains of sand in orbits of 1,000 to 1,200 feet; Jupiter a moderate-sized orange on a circle nearly half a mile across; Saturn a small orange on a circle four-fifths of a mile; and Uranus a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half in diameter”
Fig. 531.—Orbits of planets.
From an inspection of the following table the relative distances of the principal planets from the sun, their diameters, and other information respecting them may approximately be obtained. The dates of the discovery of the more modern pair are added:—
| Names. | Diameters in English miles (about). | Distance from the sun (about). | Sidereal period of revolution. | Time of rotation on their axis. | Date of discovery. |
|---|---|---|---|---|---|
| d. h. m. | d. h. m. | ||||
| Mercury | 3,000 | 35,000,000 | 87 23 16 | 24 5 — | Antiquity |
| Venus | 7,500 | 66,000,000 | 224 16 50 | 23 21 21 | do. |
| Earth | 8,000 | 91,000,000 | 365 6 9 | 23 56 4 | do. |
| Mars | 4,500 | 139,000,000 | 686 23 31 | 24 37 20 | do. |
| Jupiter | 85,000 | 476,000,000 | 4,332 14 2 | 9 55 21 | do. |
| Saturn | 70,000 | 872,000,000 | 10,759 5 16 | 10 16 — | do. |
| Uranus | 33,000 | 1,753,000,000 | 30,686 17 21 | 9 30 — | Herschel, 1781. |
| Neptune | 37,000 | 2,746,000,000 | 60,126 17 5 | . . . | Leverrier and Galle, 1846. |
| The Sun | 850,000 | . . . . | . . . . | 25 7 48 |
Altogether there are a great number of planets and asteroids, which latter are minor planets circulating outside the orbit of Mars. They have nearly all classical names, such as Juno, Ceres, Vesta, Flora, Ariadne, Pallas, Pomona, Thalia, etc., and are all at distances from the sun ranging between 200,000,000 and 300,000,000 of miles, the periods of sidereal revolution ranging from 1,100 to 3,000 days, so their years must be from four times to nine times as long as ours. Altogether about two hundred of the minor planets have been discovered, and they are all very much smaller than the earth; some, indeed, being very tiny—only a few miles in diameter, but very massive. They do not appear to possess any satellites—at least, none have been discovered, for such very small bodies as they must be, supposing they exist, would be quite invisible even with our perfected instruments.
Fig. 532.—Mars.
Fig. 533.—Jupiter.
Fig. 534.—Saturn.
Satellites, however, or “planetary moons,” as they are sometimes designated, are plainly perceived attending upon the great planets. There are twenty of these at present under observation. One we are all familiar with, and the moon, par excellence, lends a beauty to our nights which no other light that we can enjoy or command can ever do. It is remarkable that only this moon is specially mentioned in the Bible in connection with the sun. The stars are usually grouped, although, of course, the sun and moon are equally “stars” in the firmament. Mars possesses two moons and Jupiter four; Uranus also rejoices in the latter number; Neptune, like the Earth, has only one. It is reserved for Saturn to outstrip all the rest in his attendants, for no less than eight satellites wait upon that enormous planet. No doubt there are many more of these moons to be found, and every year will doubtless bring us further knowledge respecting them. Mars’ moons were only discovered very lately (in 1877), although they were known to exist; but being very small, unlike the others, they were missed. So we may conclude that the remaining satellites will remain for some time undiscovered, even if they actually are in existence. Jupiter’s moons are supposed to be as large as our own moon; Neptune and Uranus can boast of equally-sized attendants. But it is impossible to estimate the riches of astronomical lore which are beyond our ken. Millions of tiny planets are believed to exist, but their immense distance from us precludes all investigation. We are but mites in the scale.
Fig. 535.—Meteor shower.
Meteors, to which we have already referred, are small erratic bodies rushing through the planetary system, and getting hot in the process, appear in the atmosphere surrounding our earth as “shooting stars.” Some of these falling bodies have reached the earth, and several can be seen in the British Museum. Numbers, of course, are burnt up before they reach us, and who can tell what destruction such a catastrophe may represent, or whether it be or be not an inhabited world which has thus plunged to destruction by fire? They are of a metallic or stony nature. On certain nights in August and November it has been calculated that these meteors will appear. They fall from certain constellations apparently on these occasions, and are called after their names—as Leonides, from Leo, in the November displays.
Fig. 536.—Star shower.
The star-showers at times attain the dimensions of a very beautiful display of rockets. Millions of them rush round the sun; and when, as occasionally happens, our earth comes near them, we have (as in 1866) a grand display of celestial “fireworks.” But the individuals composing the mass of falling stars are very small. These meteors are very much like the comets we last year had an example of, and it has been lately suggested that there is a great degree of affinity between the comets and the meteors;—in fact, that a comet is merely an aggregation of meteors. They have been supposed to be bodies of burning gas. Their mass is very great, and their brilliant tails are many millions of miles in extent.
Comets are thus distinguished by their tails, and differ very much in their orbits from the planets. The latter are direct in their wanderings, but comets are most irregular and eccentric. The name bestowed upon comets is from the Greek Kome, hair; for when the comet recedes from the sun the “tail” may be said to come out of the head, and appear as a hair in front, so to speak. But though all comets have tails, there are many luminous bodies (classed with comets) which have no tails.
Fig. 537.—Halley’s Comet.
The comet which created the most excitement was Halley’s in 1456, of which we append an illustration (fig. 537). A comet had been observed in 1607, and Halley made a calculation that it would reappear in 1757. The calculation for its actual appearance was made by Claivant, and the expected visitor passed the perihelion in March 1759. This comet, on its appearance at Constantinople, is said to have caused much consternation, and Christians regarded it as a “sign,” for the Turks had just then captured Constantinople, and were threatening Europe. Pious people included it in their supplications for deliverance from their most dreaded enemies, and “Lord, save us from the Turks, the devil, and the comet,” was a common prayer.
Fig. 538.—Great comet of 1811.
There have been several very beautiful comets. Encke’s, Coggia’s, etc., and the comet of 1858 (Donatis) must be in the recollection of middle-aged readers. Others came in 1861 and 1874. In 1881 two comets appeared. Some comets of antiquity were very remarkable, and are reputed to have equalled the sun in magnitude. One tail is usually supposed to be the distinguishing mark of a comet, but in 1774 one appeared with six tails, arranged something like a fan. Sometimes the tail is separated from the head. Of the actual consistency of comets we cannot give any lengthened details. They apparently consist of elements similar to the meteors—namely, of solid masses, and have been supposed to be aggregations of meteors. Some appear at regular intervals, and their approach can be determined with accuracy. Of course we only see those which are attracted by the sun, or those which revolve in the solar system. There must be thousands of other comets which we never see at all.
The diagram (fig. 540 in the next page) represents a portion of the path of the comet of 1680. This visitor pursued its course for two months at a velocity of 800,000 miles an hour. The tail was estimated to extend 123,000,000 of miles, and a length of 60,000,000 of miles was emitted in two days. When this great comet was approaching the sun, or its perihelion, as such approach is termed, three minutes more would have seen it rush into the orb had its enormous pace been slackened, but as it was proceeding so rapidly, and being just then 144,000 miles away, it escaped. We can scarcely estimate the results of such a collision. This comet appeared B.C. 34, and again at intervals of about 575 years. It may be expected in 2255. It is to Halley that the discovery of the elliptical orbits of comets is due.
Fig. 539.—Path of Biela’s Comet.
M. Biela’s comet was the cause of much anxiety in 1832, for a collision with the earth was apprehended. Fortunately a month intervened between the period at which the comet was expected at a certain place in the system and the earth’s arrival at that spot, so, as it happened, about 60,000,000 of miles intervened. We cannot say what the exact effect of such a collision would be, but some wonderful atmospheric phenomena and increased temperature would certainly result from the contact. Now the comet is supposed to have an effect upon the vintage, as “comet” wines are regarded with much favour. If comets, as is believed, do consist partly of solid particles, a collision might be unpleasant; but the weight is, as a rule, a mere nothing compared to their vapoury volume, which is enormous. That the tails must be of a very attenuated medium is evident, as we can see the stars through them, and we know that a very thin cloud will obscure a star. The “menacing” comet, mentioned in the Spectator February 1881, will not do much damage, so the scare was needless, as Mr. Proctor has explained.
Fig. 540.—Path of comet, 1680.
Aerolites, or “Meteorites,” are falling bodies (meteors), which reach the earth in solid form. The greater mass of falling stars are burnt up ere they reach us, or are dissipated in space. But many instances of aerolites descending might be adduced. They usually consist of metals, such as iron and nickel mixed with sulphur, magnesia, and silica. The theory concerning falling stars has been already mentioned.
Fig. 541.—The heavens as seen from Saturn.
We have thus far taken a brief general view of the solar system, with a few of the phenomena of the heavens. Our next step will be to consider the sun, the planets, and the asteroids, according to the order of magnitude. The asteroids we cannot consider separately, but the sun, moon, earth, and the principal planets will yield us much interesting information as we examine them more closely. We shall then, as far as possible, look into the domain of the fixed stars, constellations, and the nebulæ, commenting, as we proceed, upon the varied celestial and terrestrial phenomena connected with the movements of the heavenly bodies. As is due to the great centre of our system, we will commence with the Sun. But before proceeding to do so, we must say a few words about the motion of the heavenly bodies—that is, the apparent motion of the rising and setting of the sun and stars.
The attentive observation of the starry heavens, even during a single night, will convince us that all the visible stars describe circles which are the smaller, the nearer the stars are to a certain point of the heavens, P (fig. 542). In close proximity to this point there is a tolerably bright star, called the Pole Star, which has scarcely any motion, but appears to the eye as always occupying the same position. Hence a line, PP´, drawn from this star, through the centre of the earth, c, represents the axis around which all the heavenly bodies perform their apparent motions. The part of the celestial axis, PP´, passing through the earth, is the earth’s axis; the north pole, of which p is on the same side as the pole star, and the south pole, p´, is on the opposite side.
Fig. 542.—Celestial axis.
We have, therefore, by the aid of the stars, determined the position of the earth’s axis, and by this latter we can assign to the equator its proper place. For if pp´ be the earth’s axis, aq´ is the greatest circle drawn round the earth, equally distant from both poles, and the plane of which cuts the earth’s axis at right angles.
Furthermore, let us suppose the plane of the equator to be extended till it reach the celestial concave; we thus find the place of the celestial equator, A Q, or equinoctial, as it is generally termed in opposition to the equator, which always means the terrestrial equator. The equinoctial divides the heavens into the northern and southern hemispheres. We cannot actually describe the equinoctial and make it visible, but we can imagine its line of direction by observing those stars through which it passes.
We are now in a condition to assign to an observer different stations in relation to the earth’s axis on the earth’s surface, which will essentially modify the aspects under which celestial phenomena are represented. One of these stations may be supposed to be at one of the two poles, for example, at p, or at any one point of the equator, as at a, or, finally, on any portion of the surface of the earth which lies between the pole and the equator, as, for example, o.
Fig 543.—Great Nebulæ in Orion.
CHAPTER XXXVI
THE SUN.
MOTION OF THE SUN—THE SEASONS—CHARACTER OF THE SUN—SUNSPOTS—ZODIACAL LIGHT.
Suppose that we rise early in the morning we shall, as the reader will say see the sun rise—that is, he appears to us to rise as the earth rotates. By the accompanying diagram (fig. 544) we can understand how Sol makes his appearance, and how he comes up again; not, it will be observed, after the manner stated by the Irishman, who declared that the sun “went down, and ran round during the night when nobody was looking.” The earth rotates from west to east, and so the sun appears to move from east to west. If we look at the diagram we shall see that after rising at O, the sun advances towards the meridian in an oblique arc to A, the highest or culminating point—midday. He then returns, descending to W; this path is the diurnal arc. At Q similarly, during his passage in the nocturnal arc, he reaches the lowest or inferior culmination. HH´ is the meridian.
Fig. 544.—Sun’s motion.
On the 21st of March, this path brings the sun on the “ equinoctial” line mentioned at the close of the last chapter. Day and night are then of equal duration as the arcs are equal. So this is the Vernal (or spring) Equinox. Some weeks after the sun is at midday higher up at S´, so the diurnal arc being longer, the day is longer, (Z is the zenith, Z´ is the nadir, P P´ is the celestial axis). From that time he descends again towards the equinoctial to the autumnal equinox, and so on, the diurnal arc becoming smaller and smaller until the winter solstice is reached (S).
Fig. 545.—The ecliptic.
From what has been previously said, it is evident that the sun has a twofold apparent motion—viz., a circular motion obliquely ascending from the horizon, which is explained by the rotation of the earth, and by our position, o, to the earth’s axis, p p´, and also by a rising and setting motion between the solstitial points, S and S´, which causes the inequality of the days and nights. Independently of the daily motion of the sun, we observe that at the summer solstice, on the 21st of June, at midday, the sun is at S´, and one half year later—viz., on the 21st of December, at midnight, the sun is at s, from which he arrives again in the space of half a year at S´; so we are able to represent this annual motion of the sun by a circle, the diameter of which is the line, S´ s. This circle is called the Ecliptic.
The plane of the ecliptic, S´ s, cuts the plane of the equinoctial, A Q, at an angle of 23½°, and the axis of the ecliptic, S″´ s″, makes the same angle with the axis of the heavens, P P. The two parallel circles, S´ s´ and S s, include a zone extending to both sides of the equinoctial, and beyond which the sun never passes. These circles are called the Tropics, from τρέπω, I turn, because the sun turns back at these points, and again approaches the equinoctial. The parallel circles, S″ s″, and S″´ s″´, described by the poles of the ecliptic, S″´ s″, about the celestial poles, P P, are called the arctic and antarctic circles.
Whenever the sun crosses the equinoctial, there is the equinox; but the points of intersection are not invariably the same every year. There is a gradual westerly movement, so it is a little behind its former crossing place every year. (See diagram, fig. 547.)
Fig. 546.—The Seasons.
[This is the “Precession of the Equinoxes,” because the time of the equinoxes is hastened, but it is really a retrograde movement. Hipparchus discovered this motion, which amounts to about fifty seconds in a year. So the whole revolution will be completed in about 28,000 years.]
Fig. 547.—Precession of Equinoxes.
It is obvious, then, that the sun is the most important star in the universe; and when we come to speak about the earth we shall consider the seasons, etc., more fully. Now we must endeavour to explain what the sun is like, and this can only be done with specially darkened glasses, for a look at the sun through an ordinary telescope may result in great, if not permanent, injury to the eye.
The sun is not solid so far as we can tell. It is a mass of “white-hot” vapour, and is enabled to shine by reason of its own light, which the planets and stars cannot do; they shine only by the sun’s reflected light. So we may conclude the sun to be entirely gaseous, but, thanks to the recent researches in spectrum analysis (already explained), by which the light of the sun has been examined by means of the spectroscope, and split up into its component colours, Mr. Lockyer and other scientists have discovered that a number of elements (metals) exist in the sun in a fused, or rather vapourous state, in consequence of the intense heat. Hydrogen exists in the sun, with other gases unknown to us here, and many metals, discovered by their spectra, which are the same under similar circumstances.
The sun is supposed to be spherical in shape,—not like the earth, flattened at the poles,—and to be composed of materials similar to what the earth is composed of, and what it would be if it were as hot as the sun is. Thus we can argue by analogy from the spectra of earthly elements, that, as the sun and star light gives us similar spectra, the heavenly bodies are composed of the same elements as our globe. We can thus form our opinion of the sun’s constitution. Mr. Neisen says:—
“With the aid, therefore, of the additional information given us by the spectroscope, it is not very difficult to form a true idea of the probable condition of the surface of the sun, which is all that we can see. It is the upper-lying strata of a very dense atmosphere of very high temperature—an atmosphere agitated by storms, whirlwinds, and cyclones of all kinds, traversed by innumerable currents, and now and then broken by violent explosions. Above the brilliant surface which we see is a less dense and somewhat cooler upper stratum, which, though hot enough to shine quite brightly, is quite invisible in the presence of the brighter strata beneath it.”
Fig. 548.—Sun spots.
Sun Spots, as they are generally called, are hollows in the sun’s vapoury substance, and are of enormous extent; and there are brilliant places near those spots, which are termed faculæ. These spots have been observed to be changing continuously, and passing from east to west across the sun, and then to come again at the east to go over the same space again. Now this fact has proved that the sun turns round upon his axis, and although he does not move as we imagine, from east to west, round the earth, the orb does move—in fact, the sun has three motions: one on his axis; secondly, a motion about the centre of gravity of the solar system, and a progressive movement towards the planet Hercules.
If we examine the surface of the sun through a proper telescope, we shall find that the even surface we can plainly distinguish at sunset is marked, and the brightness is greater towards the centre of the orb. We can perceive various irregularities; we shall find spots, faculæ (little torches), etc. These spots were discovered by Galileo and other astronomers, and were, as we have stated, found to be surface markings, and not a series of bodies passing between the earth and the sun. The rotation of the sun was measured, and it was found that the orb revolved in about 25-1/3 days, and in such a manner as to be slightly inclined to the plane of the ecliptic.
Herschel observed a spot at least 50,000 miles in diameter, which is more than six times the diameter of the earth. The sun spots are observed to be constantly changing, and are naturally observed differently as the revolution proceeds. The dark pole, or “nucleus” (umbra), as it is called, is surrounded by a less dark surface called the penumbra, but the umbra is not really dark; it is extremely bright when viewed alone, as has been proved by Professor Langley, while the heat is even greater in proportion. But the umbra of a sun spot must be below the level of the penumbra, for the shape changed as the sun revolved on its axis. The penumbra was wider on the side nearest the edge of the solar disc, and the umbra may be due to the uprushing or downpouring of gas or vapour like “whirlpools in the solar atmosphere.”
Near the sun spots the long streaks, or faculæ, are often observed by the borders of the disc, and a transition of the luminous part of the photosphere[26] into darkness has been observed, and bright bridges crossing the spots, and then gradually getting dark, were seen by M. Chacomac. The sun spots vary in direction, but the same general course is continued. Sometimes they describe curves, sometimes lines.
Fig. 549.—Direction of sun spots.
During solar eclipses the sun exhibits what are termed “red prominences,” which are the luminous vapours existing around the sun. When the orb is eclipsed, we can see the bright-coloured vapours shooting out from underneath the dark shadow, and this light is termed the “coronal atmosphere”; the vapours are called the sun’s chromosphere. In the coronal atmosphere are certain curious shapes of vapour thrown up, and frequently changing,—projecting, in fact, from the gaseous envelope. These red prominences were first observed in 1842, and in 1851 it was proved that they appertained to the sun, for the moon hid them as the eclipse began. Before the prominences were discovered, the red light surrounding the solar disc was known, and called the “sierra” (now chromosphere), or chromatosphere. “The luminosity of these prominences is intense,” says Secchi, “and they rise often to a height of 80,000 miles, and occasionally to more than twice that; then bending back, they fall again upon the sun like the jets of our fountains. Then they spread into figures resembling gigantic trees, more or less rich in branches.” We give some illustrations of the appearances of these prominences.
Fig. 550.—Solar prominences.
The zodiacal light is often observed. It is a glow, and frequently of a rosy tint. It may be seen in England in March or April before sunset, or in the autumn before sunrise; and it is doubtful whether it be a terrestrial or an extra-terrestrial light—a lens-shaped object surrounding the sun. Some philosophers maintain that the light is caused by multitudes of minute bodies travelling round the sun; but Herr Gronemann has lately fully discussed the observations, and the drift of his contention is under stated:—
Fig. 551.—Solar prominences.
There are valid observations against two items in the support of the old theory—viz., the affirmed connection of the evening and morning cones seen on the same night (if the corresponding sides be prolonged), and the participation of the cones in the daily motion of the heavens. The zodiacal light is sometimes seen when daylight has not yet disappeared; and, on the other hand, it sometimes fails to appear, though there is complete darkness. There would seem to be a real lengthening and shortening. It has been observed by Schiaparelli, that the light is much more difficult to make out when it passes through the meridian, than when it is only 30° above the horizon, and is less easily seen when the air is clearest than when a sort of mist is present. Indeed, the bright parts of the Milky Way may be seen to be weakened by mist, while the zodiacal light at the same height is unaffected. The zodiacal light has temporary variations of light intensity, and it shows from time to time remarkable changes of form and position, so sudden and short as to be hard to explain on the planetary hypothesis. The elongations of the cones show a half-yearly period, which is independent of the transparency of the air. The cone follows the observer northwards or southwards, so that there is no parallactic action; and this peculiarity (so adverse to the extra-terrestrial hypothesis) cannot be explained by reflection or absorption of light. As to spectroscopic observations, the author finds (1) that the zodiacal light consists partly of proper light; (2) that its connection with polar light is but secondary, temporary, and accidental; (3) that the cause of the second phenomenon is such that it may strengthen the zodiacal light and modify its spectrum; and (4) that the results of spectrum analysis rank with other arguments tending to find the source of the zodiacal light in the neighbourhood of the earth (like the polar light). Herr Gronemann, then, thinks the zodiacal light a terrestrial phenomenon, though he will not say that it cannot be influenced by cosmic action. He throws out the suggestion that the cone may be a kind of optical illusion, arising from some fine matter—gas or dust—being more accumulated near the observer in one direction than another. The apparent length of the cone might be conditioned by the conical shadow of the earth, and the changes of length be due to cosmic and electric influences. In any case, there is need of a more scientific theory than the old one.
Fig. 552.—On the sun’s disc.
We may conclude our brief notice of the great luminary to which we are indebted for everything, by a resumé of his distance from us, his diameter, and a few other plain facts and figures. In the first place the actual distance of the sun from the earth has never accurately been determined, but perhaps the next transit of Venus will assist the observers to a nearer estimate. It is quite sufficient for our purpose, however, to state that the sun’s distance from the earth is 92,000,000 of miles. The distance varies in winter and summer. In the former period the sun is nearer than in summer, and yet as the rays strike over us, and pass us, as it were, we feel less heat. When, as in summer, we are more in the focus of the rays, we feel the greater heat.
We have already spoken of parallax, and it is by finding the solar parallax that the distance of the sun from us is found. This parallax has not been exactly ascertained, or rather authorities differ, and as difference of 0·01″ in the solar parallax means something over 100,000 miles of distance, it is evident that exactness is almost impossible. If 8″·80 be settled as the solar parallax, 92,880,000 miles is the distance of the sun from the earth. If 8″·88 be taken we have nearly 92,000,000 exactly.
The volume of the sun is 1,253,000 times that of the earth, and yet the density of the former is only about one-fourth of the latter, so the attraction of gravitation at the sun must be more than that of the earth’s surface twenty-seven times. A body dropped near the surface of the sun would fall 436 feet in the first second, and have attained a velocity of ten miles a minute at the end of the first second. The diameter of the sun depends in our calculations upon its distance from the earth. If we suppose that to be 92,880,000 miles, the diameter is 866,000 miles. If we take 91,000,000 of miles as the distance from the earth the diameter is 850,467 miles. The sun makes (apparently) the circuit of the heavens in 365 days, 6 hours, 9 minutes, and 9·6 seconds; the transit from one vernal equinox to the next being only 365 days, 5 hours, 48 minutes, 48·6 seconds, owing to the precession of the equinoxes already mentioned.
When we consider the power and grandeur of the sun we may well feel lost in the contemplation. The sun balances the planets and keeps them in their orbits. He gives us the light and heat we enjoy, and coal-gas is merely “bottled-up sunlight.” In darkness nothing will come to maturity. We obtain rain and dew owing to the sun’s evaporative power; and no action could go on upon earth without the sun; and yet we receive only about 1/2070650000 part of its heat and light.
As to the colour of the sun, Professor Langley states that it is really blue, and not the white disc we see. The whiteness is due to the effect of absorption exerted by the vapourous metallic atmosphere surrounding our luminary; and if that atmosphere were removed, his colour would change.
CHAPTER XXXVII.
THE EARTH.
FORM OF THE EARTH—MOTION OF THE GLOBE—RATE AND MANNER OF PROGRESSION—LATITUDE AND LONGITUDE—THE SEASONS.
We have learnt from our books on Geography that the earth is shaped like an orange,—that is, our globe is round and flattened slightly at the “poles,” and we can easily see that the earth curves away, if we only try the experiment mentioned in a foregoing chapter—viz., how far a person standing (or lying) on the ground can see on a level. Our power of eyesight is not limited to three or four miles, but a man of ordinary height standing on the plain cannot see more than three miles, because the earth is curving away from him.
We know that at the seaside we can see ships gradually appear and disappear. When approaching us the masts and top-sails appear first, then the main-sails, and then the ship itself. A sailor climbing up the mast can see farther than the captain on deck, because he can see over the curve, as it were. When the vessel is at a considerable distance we see her “hull down” as it is termed,—that is, only her sails are visible to us, and at last they disappear also. If we want any other proof that the earth is round we can see when an eclipse takes place that the shadow on the moon is circular. So we may be certain of one fact; the earth is round, it is a globe. So much for the rotundity of the earth.
But the earth appears to us, except in very mountainous districts, as being almost a plane. This is because of its extent; and even from very high mountains we can only see a very small portion of the earth, and so, on a globe sixteen inches in diameter, the highest hills would be only about 1/100 of an inch, like a grain of sand.
The motion of the earth is known to most people, though as everything upon the globe passes with it, and a relative fixity is apparent, this is, of course, not real rest. The earth is moving from west to east at a tremendous rate,—viz., nearly nineteen miles a second! We think a train at sixty miles an hour a fast train; but what should we think of an express going more than 68,000 miles an hour! Yet this is about the rate at which our globe whirls around the sun. Her fastest pace is really 18·5 miles a second; the least about one mile per second less.
That is one motion of the earth; the other is its motion on its axis. If we send a skittle ball rolling we perceive it turns round as it proceeds. So the earth rotates on its axis, N S, in the accompanying diagram; the extremities of the axis are called the poles. The line in the middle is the equator, which is divided into 360 equal parts, each being 69-1/10 miles in length; so there are 180 lines, or rings, drawn upon the globe from N to S, and these are meridians. In England the degrees are calculated from the Greenwich meridian. We can thus obtain the distance of localities east or west, as we may briefly show (fig. 554).
Fig. 553.—Evidence of the spherical form of the earth.
Fig. 554.—Latitude and Longitude.
The distance of any meridian from the first meridian is termed the longitude, and it is employed in describing the situation of a place on the earth’s surface. Suppose L (fig. 554) a city, its longitude will be 30°, since it lies on a meridian which is 30° from the first. So, for example, the longitude of Oporto is 8° 37´ west, Paris 2° 22´ east, Vienna 16° 16´ east, Bagdad 44° 45´ east, reckoned from the meridian of Greenwich, and so on. At the 180th degree we have proceeded half round the globe, and reached the farthest distance from the first meridian, and are now on the opposite side of the earth, and proceeding in a similar manner in the opposite direction we get west longitude.
It will readily be perceived that a knowledge of the longitude alone is not sufficient to determine the situation of a place on the earth’s surface. When we say, for example, that the longitude of a place is 30°, it may lie on any point whatever of the line, N L S, on the whole hemisphere (fig. 554). This point must therefore be determined more accurately, and hence the first meridian is divided into 90 equal parts north and south of the equator towards the poles. These are called degrees of latitude, and the lines drawn through these round the globe, parallel to the equator, are called circles or parallels of latitude, and diminish as they approach the poles.
Hence, by the latitude of a place we mean its distance from the equator towards the poles, and we speak of north and south latitude according as the place is situated in the northern or southern hemisphere.
So, for example, the point L (fig. 554), which has 30° longitude and 60° N. latitude is in Sweden.
The latitude is also observable by ascertaining the altitude of the polar star above the horizon when in the northern hemisphere. The longitude is found by the chronometer; for if we know the time at Greenwich we can calculate how far we are east or west of it by seeing whether the local time be an hour (say) earlier or later, and that difference shows we are 15° to the east or the west as the case may be.
The earth’s rotation, according to sidereal time, is less than solar time, as we have seen, so we have 365 solar days and 366 sidereal days; so a person going round the world gains or loses a day as he travels east or west according to his reckoning, as compared with the reckoning of his friends at home. We can easily ascertain the earth’s motion by watching the stars rise and set. Now the path in which the earth moves is called an ellipse,—very nearly a circle,—but it does not always move at the same rate exactly. We will now look at the relations of the sun and the earth.
Let us take an example. Suppose we have a rod, at each end of which we fix a ball (see diagram), and let one ball be three times as large as the other, the common centre of gravity will be at c, at one quarter of the distance between the centres, and there the bodies will be in equilibrium. If these masses be set spinning into space they will revolve at that distance from each other, the attraction of gravitation and the force in opposition to it equalizing each other.
Fig. 555.—Earth and Sun.
The earth, as we know, proceeds with a tremendous force around the sun, not in a circle, remember, but in an ellipse or oval track, from which it never moves year by year in any appreciable degree. Now what prevents this earth of ours from rushing off by itself into space? Why should not the earth fly away in a direct line? The reason is because the sun holds it back. The force of the sun’s gravitation is just sufficient, or we may say so enormously great, that it suffices to retain our globe and all the other planets in their various orbits at the very same distance, and to counteract the force which launches them through space. Therefore, as we have already noticed when considering the sun, it is to that ruler of the day that we are indebted for everything.
What would happen, then, if the earth were suddenly to increase her velocity, or the sun to contract his mass?—We should be flung into infinite space, and in a short time would be frozen up completely. Our present diurnal course would probably proceed, but all life and existence would cease as we whirled with distant planets through infinity.
Suppose, on the contrary, we were to stop suddenly. We have some of us read in a foregoing part of this volume that heat is the motion of molecules in ether, and that when a body strikes another heat is developed by contact and friction. If the earth were to be stopped suddenly, “an amount of heat would be developed sufficient to raise the temperature of a globe of lead of the same size as the earth 384,000° of the Centigrade thermometer. The greater part, if not the whole of our planet, would be reduced to vapour”, as Professor Tyndall says.
Fig. 556.—Transit of earth across Sun, seen from Mars.
In the diagram (fig. 546, on page 497) we shall at once find the explanation of the constantly-recurring seasons, and the amount of our globe which is illuminated by the sun at various times. It will be easily understood that the poles have six months day and six months night. When the earth is at an equinox, one half of the surface is illuminated and the other half in shade, therefore the days and nights are equal. But when the north pole turns more and more towards the sun, the south pole is turning away from it in the same ratio,—the days and nights respectively are getting longer and longer, and at the north and south poles day and night are continuous, for the small spaces round the poles are, during a certain period, wholly in sunshine and shade respectively.
In March (in the diagram, fig. 546) we see that exactly one half of the earth is illuminated, and the other is darkened. So in September, when we have the opposite view. In June the earth is more inclined apparently to the sun, and more of the surface is exposed to it, so the days are longer in some parts. The opposite effect is visible in December.
The summer heat and winter cold are accounted for by the more or less direct force of the sun’s rays, for the more the angle of incidence is inclined the fewer rays reach the object; and if the rays fall at an angle of 60°, the heat is only half what it would be if they came vertically. When the days are shortest the sun is lowest, and therefore gives less heat to the earth at certain periods.
The wonderful precision which has adapted the position of the earth on its axis, will be apparent from the illustration (fig. 557). Here we have a table and some bottles, a candle to represent the sun, and a ball of worsted and a knitting-needle to represent the earth and its axis. Suppose we place the ball in the position at a, with its axis perpendicular to the plane of the orbit. As the earth would turn and go round the sun in this supposed case, we should find the days and nights equal, and the sun would quickly scorch up the tropics, and the other portions would have a never-changing spring or winter all the year for ever. This would not be so pleasant, for variety is the charm of nature, and the salt of life. So we may put a aside, as the earth would be scarcely habitable under the supposed conditions, and try b. Here we find the poles directed to the sun. The whole northern hemisphere would thus be illuminated one half year, and the southern similarly; such rapid changes from heat to cold and back again would not suit us. So we fall back upon c, the actual appearance of the position of the earth, and here we find all the most favourable circumstances existing for us. This inclination gives rise to all the varied phenomena of the pleasant gradations of heat and cold, summer and winter, the charming changes of season, and the wonderful results of the ever-recurring days and nights, months and years, as the earth spins round. So we see that the sun does not really rise and set upon the earth; the globe rotates, and brings us into view of the sun, and as we turn we lose his light.
Fig. 557.—Inclination of Axis.
In the foregoing brief description we have learnt some few facts concerning our earth. We have ascertained that the planet we inhabit is round; we have also seen that the earth moves around the sun and around its own axis, and also that it moves at a tremendous rate; we know that that rate is just counterbalanced by the attraction of gravitation, and the course round the sun gives us varying seasons, day and night. There are many subjects relating to the earth which will be more properly included under Physical Geography. We may here just add the diameter of the earth, and proceed to inquire concerning the moon. The polar diameter of our globe is 7,899 miles; the equatorial diameter 7,925 miles. It is distant from the moon 238,500 miles. We will close this chapter with the letters and characters of the Greek alphabet used in astronomical works to designate the stars.
- Α α = Alpha
- Β β = Beta
- Γ γ = Gamma
- Δ δ = Delta
- Ε ε = Eps[=ilon
- Ζ ζ = Zeta
- Ê ê = Eta
- Θ θ = Theta
- Ι ι = Iota
- Κ κ = Kappa
- Λ λ = Lambda
- Μ μ = Mu
- Ν ν = Nu
- Ξ ξ = Xi
- Ο ο = Omicron
- Π π = Pi
- Ρ ρ = Rho
- Σ ς = Sigma
- Τ τ = Tau
- Υ υ = Upsīlon
- Φ φ = Phi
- Χ χ = Chi
- Ψ ψ = Psi
- Ω ω = Omega
Fig. 558.—A ship disappearing below the horizon.
CHAPTER XXXVIII.
THE MOON.
WHAT IS IT LIKE?—MOON SUPERSTITIONS—DESCRIPTION OF THE MOON—PHASES—TIDES—ECLIPSES.
From the early days of childhood every man and woman has been familiar with the moon. This satellite of earth has been domesticated, so to speak, amongst us; and while the sun and other stars have been glorified in poetic and prose effusions, the moon has been always more tenderly addressed. The soft (reflected) light of our attendant moon is much more attractive than the brilliancy of the greater light “that rules the day.” The moon is regarded as our particular property, and has awakened an interest in our minds since the time that we could, as we fancied, see the “Man in the Moon.”
Fig. 559.—The earth as seen from the moon.
In ancient times the moon was supposed to possess some light of her own, and to be inhabited by immense creatures; and various theories continued to be promulgated respecting her, until the telescope came into use, and then astronomers began to find out many new things concerning Luna. Now, what has the telescope told us regarding our moon?—It shows us that there are mountains and craters, and numerous traces of volcanic action. At one time it was supposed that the dark masses apparent in the surface of the moon, and which can easily be distinguished by the naked eye, were seas, and maps of the moon were made, marking continents and craters.
If it were possible to reach the moon, as M. Jules Verne’s travellers did, we should find a very irregular and corrugated surface—plains and mountains without water. We should be able to see the stars in the daytime, because there is no atmosphere around the moon, and there is a silence that “might be felt.” The appearance of our earth from the moon, and the beauty of the stars in the unclouded and waterless space around the satellite, must be very grand, and has been, in a measure, depicted in the illustration (fig. 559) on the opposite page.
Fig. 560.—The Moon: the ring plain Copernicus.
In this illustration (fig. 560) we have some idea what the moon is like. We see the rugged and cratered appearance of the disc; it is a desert waste, so far as we can ascertain, without inhabitants, and, in all probability, without vegetation. For there being no moisture amongst the plains and craters and mountains of our satellite, we must conclude that the moon is dead. It is a very interesting,—nay, a fascinating study. When we take up our telescope and look from the window at the heavens the most beautiful object within our small telescopic vision is the moon shining like a silver plate, and we wonder what is up there. With a small telescope even we can discern many interesting features in the moon at the full, which will assist us in verifying the diagrams in books and their explanations.
As the moon is only a few miles away, comparatively speaking, and as the large telescopes now in use bring us within a measurable distance of the surface, we are enabled to speak more positively about our changeable satellite than of any of the planets. When we look steadily at the full moon we perceive upon its surface dark and light tracts called “seas,” though they are dried up now. Thus we hear of the “Sea of Serenity,” the “Sea of Storms,” and the “Sea of Tranquillity”; and in the map upon a subsequent page you may see the names of the seas, mountains, and the general formations of the surface of the moon. Maps of the moon are now to be procured, though no personal visits can be made to the satellite. It is very interesting to observe or to read about the structure of the moon, for we may thus learn how similar the earth and her attendant are in formation; but one important agency—that of water—has made a considerable difference in the appearance of the formations. In the moon we have mountains, plains, and rugged craters; the surface is not level, because the sunlight is visible sooner at some points than others. The chief mountain chain is the Apennines, and has a great elevation; many traces of volcanic agency are discoverable amid the great desolation, and awful silence reigns throughout.
Fig. 561.—Telescopic appearance of the Moon.
As is well known, water has a great erosive power, and its action disintegrates the surface of the earth with rapid persistency. So the physical appearance of the globe has become much changed in the course of ages: ravines exist where plains used to extend, and rivers cut their way through deep gorges to the sea. The sands and other deposits are overlaid, and thus the whole outward appearance has been altered. Not so the moon. With a very attenuated atmosphere without clouds or rain, there is no moisture, no lake, no water in the moon now. What may have been we can only conjecture. If there ever have been lakes or seas they have all been absorbed.
Fig. 562.—Formations near Mostig. Low power.
The heat upon one side of the moon must be very great at one period, and the cold on the opposite side intense, as one would think—yet upon this fact authorities differ somewhat. If the moon possess no atmosphere of any kind it would be fearfully cold and extremely hot at intervals, but a surrounding medium, even of very little density, would modify the extremes; and while we must accept the fact that the temperature varies very much we need not place it above 100° of heat, nor below 20° of cold. So from close observation and comparison we are enabled to form a very fair opinion of the “past” of the moon, and to ascertain that the same forces of nature which have moulded the planet we inhabit, have been at work in the moon also. When we study “Selenography,” therefore, we shall find a record of a history which may some day bear a parallel to the history of our physical world.
Fig. 563.—The ring-plain Copernicus, as seen with small magnifying power.
The moon, as all are aware, moves round the earth attendant upon us, but entirely under the control of the sun; our satellite, moreover, has been the subject of many superstitions. A great many rites and even domestic actions—such as the killing of fowls—were regulated by the moon; and in Scotland, Scandinavia, and other portions of Europe, she has always been regarded as effecting destiny. There are many interesting myths connected with the moon, and indeed with astronomy generally, and from a volume entitled “Notes on Unnatural History,” some very amusing extracts might be made. It will not be out of place to mention a few of these myths.
Fig. 564.—The walled-plain Plato.
The Chinese have an idea that a rabbit exists in the moon, and is the cause of the shadows we see. The Buddhists think a holy hare is up there. In the Pacific Islands there is a belief of a woman in the moon; she was sent there because she wished her child to have a bit of it to eat; and Mr. Buchanan has versified the old Scandinavian myth about the two children kidnapped by the moon as they returned from a well with a bucket of water slung upon a pole. The Jews placed Jacob in the moon, and the Italians say that Cain inhabits the luminary with a dog and a thorn bush. In the Inferno of Dante this is referred to, and we know that in A Midsummer Night’s Dream we have the moon coming out to shine upon the loves of Pyramus and Thisbe with the dog and the thorn-bush; and in the Tempest the same idea is mentioned by Caliban. Readers of Longfellow will recall the lines how “the good Nokomis answered” Hiawatha, who asked about the moon—
“Once a warrior, very angry,
Seized his grandmother, and threw her
Up into the sky at midnight.
Right against the moon he threw her,
‘Tis her body that you see there.”
But modern scientific research has exploded all these charming old myths, and laid bare the facts for us. We must now resume.
Fig. 565.—Map of Moon showing [principal formations].
The moon moves around us in 27d 7h 43m 11·461s. Its diameter is about 2,160 miles, and it is much less dense than our earth, and so the force of gravity is less there than here. Its mean distance from us is 238,833 miles. The moon goes through certain changes or phases every twenty-nine days or so; and while rotating on its own axis our satellite goes round the earth, so that we only see one side of the moon, inasmuch as the two motions occupy almost exactly the same space of time. So we generally see the same space of the moon, though there is a slight variation at times. This movement or swaying of the central point is called the moon’s “libration,” and is an optical effect, due to the inequalities in the motion of the moon in its orbit, and to the inclination of its equator and orbit to the ecliptic.
We append a map of the moon, on which the mountains, seas, and craters can be perceived, according to the list. The hill ranges extend for hundreds of miles, and the elevation reaches 30,000 feet, and even more in places. The so-called craters do not resemble volcanoes when viewed closely, but take the form of basins or valleys surrounded by lofty hills. One great plain called Copernicus is more than fifty miles across. Respecting the appearance of the moon let us quote Mr. Lockyer.
Fig. 566.—The Apennines and walled plain Archimedes.
“Fancy a world without water, and therefore without ice, cloud, rain, snow; without rivers or streams, and therefore without vegetation to support animal life;—a world without twilight or any gradations between the fiercest sunshine and the blackest night; a world also without sound, for as sound is carried by the air, the highest mountain on the airless moon might be riven by an earthquake inaudibly.”
Phases of the Moon.
We have said that the moon revolves around the earth in the same time as she turns upon her own axis, and always presents one side to us when she appears. Any one can ascertain this by putting a candle upon a round table, and walk round it facing the candle. The experimentalist will find that he will turn upon his own axis as well as turn around the table. Thus we shall see how the moon changes, for to be as changeable as the moon is proverbial. These different aspects or phases we shall now proceed to explain.
Fig. 567.—Phases of the Moon.
The time intervening between one “new” moon and another is 29d, 12h, 44m, 2s, and is termed a synodic revolution. This is longer than the sidereal revolution, because the earth is also moving in the same direction and the moon has to make up the time the earth has got on in front, as it were. So the moon travels nearly thirteen times round the earth while the latter is going round the sun.
The revolutions of the moon have been a measurement of time for ages, and her varying appearances during lunation are always observed with interest. The illustration (fig. 567) will assist us materially. The sun’s rays fall in a parallel direction upon the earth and moon, and let us suppose that S is the sun in the diagram and T the earth; C at the various points is the moon, the capital letters, A, B, C, etc., indicating the planet as she appears from the sun, and the small letters show how she appears to us from the earth.
Let us suppose that the sun, earth, and moon are in conjunction—or in a direct line. The phases, C and G, are the moon’s “quadratures.” At A we see the sun shining on the moon, but we only have the dark side. It is then “new” moon; but by degrees, as she goes round in her orbit, we perceive a small crescent-shaped portion, lighted up by the sun at B and b. At c´ we have the first quarter or half-moon. When she is in opposition she is at full moon, and so on to the last quarter and conjunction again.
Fig. 568.—Crescent Moon.
The moon’s phases may be easily shown by means of a medium-sized lamp to represent the earth, a smaller one to serve as moon, and a light to act as sun all at the same height. Colour the “lunar” globe white, and if we move it about the “earth” globe, we shall see the various phases of the moon in the sharp shadows.
The Tides.
The ebb and flow of our tidal waters depend upon the moon to a great extent. The phenomenon is so common, that we need only refer to it, but the cause of the tides may be stated. Twice every day we have the tides twelve hours apart, and the flow and ebb are merely examples of the attraction of gravitation, which is exercised upon all bodies, either liquid or solid. The tides are highest at the equator and lowest at the poles, because the tropics are more exposed to the influence of the lunar attraction.
Fig. 569.—Moon’s attraction.
By the small diagram (fig. 569) we shall be able to see in a moment how the moon acts. The moon being nearer to the earth at b, the water will be naturally attracted to the ball, m, and cause high water (a); and a similar effect will be produced opposite, because the earth is attracted, so the waves are higher than the ground which has been attracted away from the water, and the waters will flow in and cause a high tide at d, but not so high a tide as at the opposite point, a. It can then be understood that there will be low water at the other two sides, e and f, because the water has been taken away, so to speak, for the high tides at a and d. We shall learn more of this under Physical Geography.
Fig. 570.—New Moon.
The moon revolves round the earth in a changeable elliptical orbit, intersecting the ecliptic at certain points called Nodes. When the moon is nearest to the Earth she is said to be in perigee when farthest from us she is in apogee (the line uniting these points is the line of apsides), the difference in distance being about 4,000 miles. She passes the sun periodically, and so if the moon moved in the plane of the ecliptic there would be eclipses of the sun and moon twice a month; but as the orbit is inclined a little, she escapes by moving north or south. We will now endeavour to explain this theory.
Eclipses.
We have briefly considered the Sun and Earth and the Moon separately. We are now about to regard the effects produced by them when they come in each other’s way and cause Eclipses, which are observed with so much interest. There are eclipses of the sun and of the moon. The former occur at the time of new moon, and the latter at full moon; and this will be at once understood when we remember that the sun is eclipsed by the moon passing between us and the sun; and the moon is eclipsed because the shadow of the earth falls upon her when she is opposite the sun, and therefore “full.”
Fig. 571.—Solar eclipse with corona.
Readers of the voyages of Columbus will remember that he managed to obtain supplies from negatively hostile Jamaica savages by pretending to cause an eclipse of the moon, which he knew was about to take place, and to the ancients eclipses were of dire portent. Even in enlightened Rome, to ascribe an eclipse to the causes of nature was a crime. The Chinese have an idea that great dragons are devouring the moon when she is eclipsed.
There are total, partial, and annular eclipses. The former terms speak for themselves; the latter name is derived from “annulus,” a ring; for a ring of light is left around the dark portion eclipsed, and is only seen in solar eclipses. In one sense the eclipse of the sun is really an eclipse of the earth, because it is caused by the shadow of the moon falling upon the earth.
Fig. 572.—Umbra and penumbra.
If a bright body, A, be larger than the dark body, B, there will be two kinds of shadows—viz., the umbra and the penumbra. For instance, the umbra is the central dark part in the cut (fig. 572), and the penumbra is the lighter portion. As soon as the eye is placed on the umbra, it can perceive no part of the source of light, A, which appears to be eclipsed. On the other hand, the penumbra originates in that locality where only a portion of the light proceeding from a luminous object can fall; hence an eye in the penumbra would see a part, but not the whole of the illuminating body. This shadow also forms a cone, the apex of which, if extended, will fall before the opaque body. If we receive the shadows so projected at m n, for example, on a white sheet, we have in the centre a dark circle, which is the umbra, surrounded by the penumbra, which gradually decreases in intensity towards the exterior (see fig. 573). The farther we hold the sheet from the body producing the shadow, the umbra decreases, and the penumbra is enlarged. For where (in solar eclipses) the umbra falls there is totality; within the penumbra partial eclipse only.
Fig. 573.—Lunar eclipse.
Lunar Eclipse.—Let A (fig. 573) be the sun, and B the earth, the length of the umbra of the latter will exceed 108 diameters of the earth. Since the moon is only about thirty terrestrial diameters distant from the earth, and as the diameter of the earth’s shadow, at this distance, is nearly three times as large as the apparent diameter of the moon, it follows that when the latter enters this shadow, she must be totally eclipsed, for at those places where the moon’s shadow falls there is total eclipse. If the moon’s orbit were coincident with the ecliptic, or if both moon and earth moved round the sun in the same plane, there would be an eclipse at every conjunction, and at every opposition,—i.e., a solar eclipse would happen at every new moon, and a lunar eclipse at every full moon. But we have seen that the lunar orbit cuts the ecliptic only in two points; consequently an eclipse of the moon is possible only when, at the time of opposition, the moon is in one of her nodes, or in close proximity to it, which can only occur twenty-nine times in the space of eighteen years.
A lunar eclipse begins on the eastern margin of the moon, and is either total, when her whole disc enters the umbra, or partial, when only part of her disc is in the shadow. A total eclipse may last for two hours.
We shall understand this better, perhaps, with the diagrams.
Fig. 574.—Solar eclipse.
Solar Eclipses.—When the moon and the sun are in conjunction, the moon’s place may be represented by M (fig. 574) between the earth, T, and the sun, S. If this conjunction occur when the moon is in one of her nodes, or within 16° of it, the shadow of the moon will fall upon the earth, and the sun will be eclipsed. At other places the sun will not be entirely covered; and if the moon be moved farther off, so that its shadow will not reach the earth, and so not cover the sun up completely, we shall have an annular eclipse, because a rim of the sun will be visible.
The lunar umbra extends from the moon by a space about equal to her distance from the earth, and hence only a small portion, d, of the earth’s surface enters the lunar umbra. To the inhabitants of this part of the earth the sun will be totally eclipsed, and the eclipse will be annular if only the margin of the sun’s disc remain uneclipsed by the lunar shadow. This is only possible when the moon is in her apogee, or greatest distance from the earth, where her apparent diameter is less than that of the sun, which it cannot in general exceed more than 1´ 38″. Hence the duration of a total eclipse of the sun cannot be more than 3¼ minutes.
On the contrary, the penumbra of the moon is diffused over a much larger portion, n m, of the surface of the earth, since its section is five-ninths of the earth’s diameter. The inhabitants of this portion of the earth do not receive light from all parts of the sun, consequently a part of this luminary is invisible to them, and the eclipse is said to be partial.
Solar eclipses commence on the western margin of the sun, and advance to the eastern. On account of the proximity of the moon to us, an eclipse of the sun is, in all places above the horizon of which the sun appears, visible neither at the same time, nor is it of equal duration, nor of equal extent: in some parts it may not be visible at all. In favourable situations, the diameter of the umbra, where it reaches the earth, amounts to about 167 miles, and on this small strip of the earth’s surface only can the sun appear totally eclipsed.
Fig. 575.—Lord Rosse’s monster telescope.
CHAPTER XXXIX.
THE PLANETS AND ASTEROIDS.
Mercury.
Including our own globe there are eight principal planets—viz., Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. The two first-named being between us and the sun, are termed interior planets; the others are exterior. Mercury, Venus, and Mars are smaller than Earth. The other four are much larger.
Fig. 576.—An Orrery.
We have already described the planets as bodies wandering through the zodiac, and reflecting the sun’s light. Their orbits are very different from the moon’s; for instance, planets take a retrograde motion as well as a direct one. The sun and the planets revolving around him constitute the solar system.
We will commence our brief consideration of them with Mercury, the planet nearest to the sun.
The distance of Mercury from the sun is 35,000,000 of miles, less than half the distance our earth is from him, and so receives much more heat and light than we do. The sun to the Mercurians, if there be any inhabitants upon the planet, must appear about seven times larger than he does to us. Mercury’s year is about eighty-five days in length, so the seasons must be shorter if they follow the same rotation as ours. It passes through space with an exceedingly rapid motion, and so probably the ancients called the swift planet Mercury after the winged messenger of Jove.
Mercury is not an easy planet to observe, owing to its proximity to the sun, yet the ancients managed to descry it. But it can be seen just before sunrise and sunset in autumn, and in spring if the weather be clear. It possesses phases similar to our moon. Some authorities have stated that Mercury has an atmosphere, but this circumstance, as well as its formation, is still shrouded in mystery. Mercury’s day is a few minutes longer than ours.
Fig. 577.—Transit of Mercury.
A transit of Mercury is represented in the accompanying illustration (fig. 577). This phenomenon took place in 1845, but there have been many others noticed. The first recorded took place in November 1631, and these transits always occur in May or November.
Venus.
Venus is the planet next in order, and revolves about 66,000,000 of miles from the sun. It is the nearest planet to the earth, and is somewhat smaller than the latter. This planet is both a morning and evening star, and is very brilliant—so much so, that any close observation with the telescope is not possible; and when at her nearest point she is invisible as she passes between us and the sun, and of course when fully illuminated she is directly beyond the sun, and enclosed in his rays. But under other circumstances she is distinctly visible as a crescent in the evening, and nearly full as a morning star. Venus goes round the sun in 224 days, and her day is rather less than ours.
Fig. 578.—Orbit of Venus.
Venus has long been celebrated as the morning and evening star, as “Lucifer” and “Hesperus.” “Lucifer, son of the morning,” is mentioned by Isaiah. That Venus possesses an atmosphere denser than our own can scarcely be doubted. The observations made during the successive transits, particularly the last (1874), seem to have established the fact that aqueous vapour exists around, and water in, Venus. No satellite can be found, though the ancients reported such an attendant upon this planet.
The apparent diameter of Venus varies considerably in consequence of her varying distances at the inferior and superior conjunction. When nearest the earth, if she presented her fully illuminated disc to our gaze, we should see a miniature moon, and even under the circumstances Venus throws a shadow, so brilliant is her light.
Fig. 579.—Venus, at quadrature.
The transits of Venus have been referred to, and, like those of Mercury, are simply a passing, or “transit,” of the planet across the illuminated disc of the sun. The transits afford means to ascertain the volume and distance, etc., of the sun, and this year (1882) the next transit is expected. There will not be another for more than one hundred years.
Fig. 580.—Venus, near inferior conjunction.
Whether Venus has a constitution similar to our globe is of course doubtful. The matter is less dense than the earth, and there is an atmosphere half as dense again as ours. Spots have been noticed crossing the planet, which may have been vapours or clouds, and the rotation of Venus on its axis was calculated from these spots as being 23h 21m 22s. The seasons in Venus must be very different from ours, as her inclination is greater than our earth, and as the sun is so much nearer to her than to us her tropical and polar regions are close, and a vertical sun is scarcely enjoyed by two places for three successive days, and she may have two winters and summers, two springs and autumns!
Mars.
Having already considered the earth, we pass on from Venus to Mars. The orbit of the latter planet is exterior to the earth’s, as is proved by his never appearing “horned,” nor ever passing across the sun’s disc. Therefore no “transits” of Mars can take place as transits of Venus and Mercury.
Yet Mars is most favourably situated for astronomical observation by us, because it turns its full disc to us. Venus is nearer to us than Mars—but, as we have explained, when she comes nearest to us she is quite invisible. Astronomers have been enabled to ascertain a good deal concerning the planet of war—“the red planet Mars.”
Mars has been considered very like the earth. We perceive seas and continents, and the shape of Mars is like the earth. But our globe is larger than Mars, which is much less dense, so the force of gravitation is less also. Mars moves upon his axis in about twenty-four hours and a half, and takes rather more than 686 days to revolve round the sun. (See page 489.) Thus its days are a little longer than, and its years twice as long as our days and years. When in “opposition,” or on the opposite side of us from the sun, Mars is at his brightest. This happened in September 1877. He will come close again to us in 1892.
Fig. 581.—Mars seen from the earth.
All planets are wanderers, but of all the wanderers Mars has the most eccentric orbit. He curls about, so to speak, in loops and curves in a very irregular manner, and therefore his distance from the earth varies very considerably; and this eccentric behaviour of the warlike planet must have, as we believe it did, puzzled the ancients very much. But—and here reason came to human aid—this very fact, this great eccentricity of the planetary motions, caused Copernicus to investigate the subject with great attention, and he at length explained the true reason of these irregular orbits from the hypothesis that it was around the sun, and not around the earth that the planets moved in regular orbits.
It is quite ascertained that Mars is very like our earth in miniature. We annex a diagram of the planet, and when it is examined with a good telescope the seas and continents can be quite distinctly perceived. At the poles there appears to be a white or snowy region at varying periods, which would lead us to the conclusion that the atmospheric changes and the seasons are similar to our own; and as the inclination of the planet is nearly the same as the earth, this supposition may be accepted as a fact.
Thus we see that Mars is the most like earth of all the planets, and its inhabitants—if, indeed, it is now inhabited—must have a beautiful view of us when the weather is fine, for we are so much larger. Mars is also attended by two satellites, or moons, as Professor Hall reported from Washington in 1877. These moons have been named Deimos and Phobos, and are both very small, their diameter being only about six miles; but late astronomers have reasoned that they must be three times this diameter.
Fig. 582.—Earth seen from Mars.
There have been numerous theories concerning Mars being inhabited, and of course these suggestions made respecting life on one planet may, with varying circumstances, be applied to another. Each planet may have had, or may yet have, to pass through what has been termed a “life-bearing stage.” We on earth are at present in the enjoyment of that stage. So far as we can tell, therefore, Mars may be inhabited now, as he bears much the same appearance as our planet. Certain changes are going on in Mars, and all planets, just as they go on here in our earth, and as they did long, long ages before the earth was populated, and which will continue to go on after life on the earth has ceased to exist.
Mars is, as we know, much further away from the sun than earth is, and must receive less direct heat. When he was created, or formed, we can only conjecture, but in all probability he cooled before the earth did, as he is smaller. Here another theory concerning the state of Mars arises, and in support of it we may quote an American authority upon the planet.
“His mass is not much more than one-ninth of the earth’s, while his surface is about one-third of hers. Then, if originally formed of the same temperature, he had only one-ninth her amount of heat to distribute. If he had radiated it away at one-ninth of her rate, his supply would have lasted as long, but radiation takes place from the surface in proportion to the surface, hence he parted with it three times as fast as he should have done to cool at the same rate as the earth, and must have attained a condition which she will not attain until three times as long an interval has elapsed from the era of her first existence than has already elapsed. Geologists agree that the last-named period must be measured by many millions of years; hence it follows that twice as many millions of years must elapse before our earth will be in the same condition as Mars, and Mars must be three times as far on the way toward planetary decrepitude and death as our earth. Then assigning two hundred thousand years as the extreme duration of the period during which men capable of studying the problems of the universe have existed, and will exist on this earth, the theory holds that Mars would have entered on that stage of his existence many millions of years ago, and that the appearance of the planet itself implies a much later stage of planetary existence.”
Mars is a very interesting study, and the reddish hue which is so distinctive is perceived in certain spots when examined by the telescope’s aid. These red places were discovered by Cassini. Mr. Dawes made drawings of Mars, and Mr. Proctor has by their aid constructed a regular map of Mars, and a chart of the surface of the planet. There is much more land than water on Mars, as the bright surfaces which indicate land are much more extensive than the darker portions which betoken the existence of water. But these “markings” are not always visible, in consequence of something coming between us and the land on Mars, and this has been attributed to the production of vegetation, which a French savant declared was ruddy-coloured, and that this autumnal tint departed in the winter.
The seasons of Mars are not equal, in consequence of his wandering propensities, and winter is warmer up there than our winter, while summer is cooler than our summer. That there are clouds and an aqueous atmosphere surrounding Mars we learn from spectroscopic observation and analysis, and in fine we may look upon Mars as similar to our earth. Respecting the question of its habitation we take the liberty to quote Mr. Richard Proctor:—
“I fear my own conclusion about Mars is that his present condition is very desolate. I look on the ruddiness of tint to which I have referred as one of the signs that the planet of war has long since passed its prime. There are lands and seas in Mars, the vapour of water is present in his air, clouds form, rains and snows fall upon his surface, and doubtless brooks and rivers irrigate his soil, and carry down the moisture collected on his wide continents to the seas whence the clouds had originally been formed. But I do not think there is much vegetation on Mars, or that many living creatures of the higher types of Martian life as it once existed still remain. All that is known about the planet tends to show that the time when it attained that stage of planetary existence through which our earth is now passing must be set millions of years, perhaps hundreds of millions of years ago. He has not yet, indeed, reached that airless and waterless condition, that extremity of internal cold, or in fact that utter unfitness to support any kind of life, which would seem to prevail in the moon. The planet of war in some respects resembles a desolate battle-field, and I fancy that there is not a single region of the earth now inhabited by man which is not infinitely more comfortable as an abode of life than the most favoured regions of Mars at the present time would be for creatures like ourselves.”
Fig. 583.—CHART OF MARS (Names according to Proctor and Green).
A Peer Continent. B Herschel Continent. C Fontana Land. D Secchi Continent, East. E Secchi Continent, Central. F Secchi Continent, West. G Mädler Continent. H Leverrier Land. 1 Herschel Strait. 2 Dawes Ocean. 3 Maraldi Sea. 4 Oudemans Sea. 5 Trouvelot Bay. 6 Funchal Bay. 7 Campani Sea. 8 De la Rue Ocean.
The Moons of Mars.
We must devote a few lines to the satellites of Mars, which during the last four years have proved a very interesting study for the astronomers, and some very interesting facts have been ascertained concerning the ruddy planet, which is now proved not to be “moonless Mars,” as the poet declared.
There are two satellites, which, in consequence of their distance from him, being so different, vary in apparent size. The outer one is twelve thousand, the inner one about three thousand five hundred miles from the planet, so the former would revolve in about thirty hours in a direction from west to east, and the inner moon goes round in the same way in about seven hours and a half. Mars revolves in twenty-four and a half hours from west to east. So the outer moon rises for him in the east, and the inner one in the west. This is accounted for by the fact that one travelling slower than Mars rises in the east, the other outruns him, and comes up in the west.
But if we suppose ourselves upon Mars we shall find that, after all, we have only one moon properly so called. The outer satellite is very small and very far away, so it is useless to give light—at most, it is no bigger than Mars appears to us on earth. So the Martians do not see two moons passing each other in the sky—that is, unless their eyes are of greater range and power than ours. Thus they have one moon rising in the west, appearing in all its phases every night, while our moon takes twenty-eight days to pass through her phases; for we must remember that Mars’ moon takes only seven hours and forty minutes to pass through its orbit, and therefore each quarter will not occupy quite two hours.
The Minor Planets, or Asteroids.
Passing onward from Mars towards Jupiter we arrive at a number of smaller planets, which will not concern us very much, as they are very small and scarcely visible without a good telescope. But a very interesting chapter in the history of astronomy was commenced when the discovery of these bodies was begun. In old times astronomers noticed a very considerable gap between Mars and Jupiter, which was remarkable when the regular progression of the distances between the planets was remembered. So Kepler was of opinion that some planet would be discovered having its orbit in that space between Mars and Jupiter. It is, however, to Piazzi, the Italian, that the discovery of the zone of asteroids is due.
Dec. 8th. Dec. 9th.
Fig. 584.—Field of view showing motion of minor planets amongst the stars.
Piazzi was surveying the constellation Taurus, where he fancied he had discovered a change of place in a star which he had observed on the 1st of January in that year (1801). He was quite sure of this change next day (the 3rd of January), and he expressed his opinion to Bode and Oriani. But letters took a long time to pass in those days, and when the other astronomers had received the advices the new star had been lost in the sun’s glory. But after a year, on the 31st December, 1801, the planet was again seen and the discovery was proved. The new planet was named Ceres.
The discovery of Ceres led to other discoveries. For, while searching for her, Olbers found other minor planets, and so on to the present day. Now we have nearly two hundred asteroids, and more are probably to be found in the zone beyond Mars.
It would answer no purpose to give a list of the asteroids. We need only remark that the first four were discovered in quick succession, and then a lapse of thirty-eight years occurred before the fifth was found, thus—
| Ceres, | discovered by | Piazzi at Palermo, January 1st, 1801. |
| Pallas, | ” | Olbers at Bremen, March 28th, 1802. |
| Juno, | ” | Harding at Lillienthal, September 2nd, 1804. |
| Vesta, | ” | Olbers at Bremen, March 20th, 1807. |
| Astræa, | ” | Hencke at Driessen, December 8th, 1845. |
Since 1848 there have been numerous minor planets discovered every year.
The hypothesis that all these asteroids are fragments of one large planet which has been destroyed was started by Olbers; and in confirmation of this view it has been determined that the asteroids have essentially the same character. The orbits of these minor planets are different from the larger “wanderers,” and cross each other, as will be seen from the accompanying diagram, so that a collision may one day ensue.
Fig. 585.—Orbit of asteroids.
Planetoids and extra zodiacal planets are titles which have been bestowed upon these bodies, of which Vesta is the first in order in the system, and revolves in 1,325 days, at a mean distance of 225,000,000 of miles from the sun. Juno and Ceres take each about four of our years to revolve in their orbits, at greater distances still, averaging 260,000,000 of miles. Pallas and Ceres are most alike in their periods and distance from the sun; the principal asteroids are only about 300 miles in diameter, while the smaller are very tiny indeed, and one certainly has quite disappeared.
Jupiter, the Giant.
Jupiter has been well named the Giant planet, since his diameter is eleven times greater, and he is thirteen hundred times larger than our planet. His inclination is very small, and you now know that under such circumstances he enjoys very small changes of seasons. Jupiter has four moons, or satellites, and an illustration of the “Jovian System” is herewith given.
Fig. 586.—The Jovian System.
Jupiter himself was well known to the ancients, but Galileo was the discoverer of the “moons.” His telescope was, of course, a very imperfect instrument, and while he was gazing at the planet he noticed three stars close by the bright disc, two on one side, but next day Galileo perceived them all at the same side. Next time he looked there were only two, and after many anxious observations he found out, not only that Jupiter had three attendant stars, but four!
These moons were found to revolve round Jupiter in times varying from nearly two days to nearly sixteen days, according as they were at a less or greater distance from him. They were found to have their times of eclipses and transits, etc., also. These moons act with respect to Jupiter very much as the inner planets act with respect to the sun, for observation showed Galileo that the satellites sometimes appeared on one side of the planet, and at other times on the opposite side.
Fig. 587.—Satellite in Transit.
From the diagram of the Jovian System we shall understand the orbits of the moons, which are all of nearly equal size,—two thousand miles in diameter,—and cause eclipses of the sun to Jupiter. If the earth be in the same direction as the sun the moons are lost to view. The satellites disappear into the shadow, and are eclipsed at 1′″, 2′″, 3′″, 4′″, respectively, but they do not always come into view again immediately they have passed through the planet’s shadow, because the earth is a little at one side of the sun. So when the satellite gets behind the edge of Jupiter, his shadow being on the opposite side to the satellite’s, it is said that the “moon” is in “occultation”; when it disappears in the shadow it is “eclipsed.” Cassini discovered the “transit” of Jupiter’s moons. The annexed diagram illustrates the eclipses, etc., very clearly. At the four points, A B C D, we have the earth; J is Jupiter with his moons; 1 2 3 4 is their orbits. At a moon No. 1 enters his shadow, and emerges at b. From the earth at D a will be visible, but not b, because Jupiter is in the way. So at B, the coming out, or emersion, will be visible, but not the entrance into the shadow, or immersion. At A the satellite is in transit d, on the disc of the planet, J.
Fig. 588.—Eclipses of Jupiter’s Moons.
From the observation of the eclipses of Jupiter’s moons the rate of the transmission of light was discovered by Roëmer in 1675, and its progressive motion was calculated. The eclipses were noticed to take place later than the calculated time, when the planet was approaching conjunction. Roëmer suggested that the delay was owing to the greater distance the light had to travel—a distance equal to the diameter of the earth’s orbit, or about 190,000,000 of miles. The time was about sixteen minutes. Light was found to travel at the rate of nearly 12,000,000 of miles a minute.
Let us now endeavour to picture Jupiter himself. Here we have an illustration of the planet. He is the biggest, and the brightest, except Venus, of all the planets. He revolves at a distance of 476,000,000 of miles from the sun, and his year is equal to nearly twelve of ours, while his day is scarcely ten hours long, showing a rapidity more than twenty times the rate of our earth. Jupiter, therefore, must have a very much greater diameter than the earth.
Fig. 589.—Jupiter.
There is much less sunlight and heat found on Jupiter than upon Earth, because he is so much farther from the sun than we are, but at the same time the heat comes at less intervals than with us. And here the theory already noticed respecting the gradual cooling of the planets will be remembered. Jupiter, we can easily imagine, would take much longer to get cool than Mars or the earth, and, though his rapid rotation would assist him, he must be still in the midst of a glowing atmosphere without form and void—perhaps a furnace for cloud and vapour generation.
Now when Jupiter is examined with the telescope it will be seen that he is crossed by belts of vapour (see also page 489); and when we consider the results of the spectrum analysis of the planet, we may fairly assume that Jupiter is in a very heated state, and that we cannot really perceive the actual body of the planet at all yet. There is an immense quantity of water thus surrounding Jupiter, and he is still in the condition in which our earth was before geology grasps its state, and long ere vegetation or life appeared. The waters have yet to be “gathered together into one place,” and the dry land has yet to appear upon Jupiter, who is a very juvenile, if a very enormous planet. Under these conditions we can safely assume that there are no inhabitants of Jupiter.
The belts, or zones, of Jupiter vary in hue, and the continual changes which are taking place in this cloud region tend to show that disturbances of great magnitude and importance are occurring.
It is useless to speculate upon what will happen in Jupiter when the disc is eventually cooled. The planet, we know, has not nearly reached maturity; the earth is in the full prime of its life, and the moon is dead and deserted. What the millions of years which must elapse before Jupiter has cooled may bring forth we need not try to find out. The earth will then, in all probability, be as dreary as the moon is now, and we shall have returned to dust.
Saturn.
Fig. 590.—Saturn.
We now come to the most curious of all the planets—Saturn, which is an immense globe surrounded by a beautiful bright ring, or rather series of rings, and attended by eight moons. He appears to possess much the same constitution as Jupiter, but enveloped in an even denser atmosphere than the latter. Saturn’s diameter is about nine times greater than the earth; he revolves on an inclined axis in about ten hours, and has seasonal alternations of unequal length. His year is about thirty of ours (10,759 days). The most striking phenomena in connection with Saturn are his rings.
Saturn’s rings are supposed to be a close agglomeration of stars, or satellites, revolving around the planet and encircling him in a belt. The rings are apparently broad and flat and thin, resembling roughly the horizon of a globe.
The globe of the planet is not exactly in the centre of the rings, which have been measured, and are approximately as below:—
Fig. 591.—Saturn’s rings at Equinox.
| Diameter of exterior ring | 169,000 | miles. |
| Diameter of interior ring | 124,000 | ” |
| Diameter of innermost ring | 100,000 | ” |
| Interval between innermost ring and Saturn | 19,000 | ” |
| Intervals between the rings | 18,000 | ” |
| Thickness of the rings | 130 | ” |
| Breadth of the rings | 37,000 | ” |
The rings were first recognised as such by Huyghens in 1659, but Galileo had remarked the curious appearance the planet presented. Cassini confirmed Huyghens’ discovery, and found that the ring was duplicated, and Mr. Ball made the same discovery. The two outermost rings are very bright, the inner ring being darker and partially transparent, for the ball of Saturn can be perceived through it.
Fig. 592.—Enlarging ring.
Fig. 593.—Ring shadow.
But the rings are not always so plainly seen as in the foregoing diagram. Sometimes they appear as a mere line of light on each side of the planet, as shown in the margin. This occurs at the time of the equinox (fig. 591). By degrees, however, as they become inclined, they appear broader (fig. 592). The inner ring may be formed of vapour, but the outer ones are of something more solid, as the shadows they cast upon the planet, and it casts upon them, at certain times (figs. 593 and 594).
Saturn possesses eight moons, seven of them revolving in orbits on the plane of the rings, but one is more inclined. These eight satellites have been named as follows:—
| Name. | Distance from Saturn in radii of Saturn. Miles. | Time of Revolution. | Discoverer. |
|---|---|---|---|
| d. h. min. | |||
| 1 Mimas | 3·36 (about) 120,000 | 22 37 | Herschel. |
| 2 Enceladus | 4·30 (about) 150,000 | 1 8 53 | Herschel. |
| 3 Tethys | 5·34 (about) 190,000 | 1 21 18 | Cassini. |
| 4 Dione | 6·84 (about) 240,000 | 2 17 41 | Cassini. |
| 5 Rhea | 9·55 (about) 340,000 | 4 12 25 | Cassini. |
| 6 Titan | 22·15 (about) 790,000 | 15 22 41 | Huyghens. |
| 7 Hyperion | 26·78 (about) 945,000 | 21 7 7 | Lassel and Bond. |
| 8 Iapetus | 64·36 (about) 2,250,000 | 79 0 8 | Cassini. |
Fig. 594.—Ring shadow.
But these eight moons are not so interesting as those belonging to Jupiter, because the great distance they are away precludes much examination of them. They vary much in size, Titan being the largest, and perhaps equal to Mars, Iapetus being next in magnitude. The light of these satellites and the rings is no doubt very great in the aggregate, and must have a magnificent appearance in the heavens (compare page 493). Very likely there are other attendants upon Saturn, but owing to the brilliancy of the rings it is impossible to distinguish them.
Uranus.
Uranus was discovered by Herschel in 1781, and has been called after its discoverer, and sometimes the “Georgium Sidus.” It revolves at an enormous distance from the sun—viz., 1,753,000,000 of miles. It takes about eighty-four of our years (30,686 days) to go round the sun, and possesses four moons. It is very much larger than the earth—about four times the diameter, and forty times its volume. We can only speculate concerning its physical constitution, which is assumed to be similar to that of Jupiter, while the changes of temperature and seasons must vary immensely. The names of the moons are Ariel, Umbriel, Titania, and Oberon. The outer pair can be seen without much difficulty.
Neptune.
The existence of this planet was determined by calculation before it had been seen at all. Uranus was observed to be disturbed in his orbit, moving sometimes faster than at others; and even before Uranus had been discovered Saturn and Jupiter had been seen to be affected by some body in the system. M. Leverrier determined to ascertain the cause of this, and came to the conclusion that some other planet was influencing Uranus. The Newtonian theory here received a most convincing proof. While Leverrier was calculating, Mr Adams of Cambridge leaped to the same conclusion, and wrote the result of his calculations to Professor Airy, and the planet was seen, but not reported upon. Meantime Leverrier published his calculations, and the observers at Berlin detected the new planet in September 1846.
Fig. 595.—Neptune in field of view with stars of 6th, 7th, 8th, and 9th magnitudes.
Very little can be said concerning Neptune, as its distance is too great for observation. It is at 2,746,000,000 of miles from the sun, and takes 164 years to go round it (60,126 days). It is about the same size as Uranus. It has one moon, which moves round the planet in 5d 21h, and is of great size.
CHAPTER XL.
THE FIXED STARS.
FIXED STARS—MAGNITUDE OF THE STARS—CONSTELLATIONS—DESCRIPTIONS OF THE ZODIACAL CONSTELLATIONS—NORTHERN AND SOUTHERN STAR GROUPS—DISTANCE OF STARS.
We have been considering the planets so far as they are known to astronomers, but no doubt we shall find out others some day beyond Neptune in space, for it must be assumed that there are other planets wandering about in the infinite firmament. At present, however, we cannot spare time for such speculation; we have got to peep at the stars and their groupings.
“What little bits of things the stars are,” a child said once in our hearing; and there were others present who were inclined to believe that the tiny light spots we could see looked small—not because they were distant, but because they were of no great magnitude; and when those children were told that the tiny stars were “suns” like our sun, giving heat and light millions and millions of miles away,—and, so far as we can tell, some are much bigger and hotter than our own sun,—they were very much surprised indeed, and one little girl aptly quoted Dr. Watts:—
“Twinkle, twinkle, little star,
How I wonder what you are”!
Now let us endeavour to learn something about these apparently tiny specks, and why they “twinkle.”
At a very early period in the history of astronomy the observers of the heavens grouped stars together in fancied resemblances to men or animals; and these “constellations,” as they are termed, are combinations of fixed stars—that is, of stars which do not wander about as the planets do. But these so-called fixed stars have motions; they are only relatively fixed with reference to their positions to each other as they appear to revolve daily round the earth. But stars have a movement of their own, which is termed their “proper motion.”
It is to Halley that the discovery of these real star motions is due. He saw three very bright stars (Sirius, Aldebaran, and Arcturus) were not in the places they had been assigned. The sun also has been found to possess a “proper motion,” and, with the planets, is travelling as determined by Sir J. Herschel, to a particular place in the constellation called Hercules. There are now star catalogues and star maps, for the heavens have been as closely surveyed as the earth, and by accurate observations it has now become possible to find the position of every star usually visible. Some of the stars are used as “clock” stars, by which sidereal time can be calculated accurately, and the clocks thereby corrected. The stars, though termed “fixed,” are in perpetual movement—Arcturus at the rate of fifty miles a second, and others less. Only the rates of a few are known.
The number of the stars is beyond our calculation, and even the number of stars only visible in the telescope amount to millions, and these are called telescopic stars. The visible stars amount to about six thousand, and of course these are the brightest up to the sixth magnitude. There are more visible in the southern than in the northern hemisphere. The magnitudes of the stars range in classes according to the brightness of the stars observed, for this is really the test from the first magnitude to the sixth; after that the telescopic stars are seen up to the fifteenth or sixteenth. We can only see about three thousand stars at any one time from any place, although, as remarked above, many millions may be observed with a good telescope, and as many more, probably twenty millions, are invisible.
We will now proceed to detail the constellations, which are familiar by name to everybody. We have already given the names of the zodiacal groups, which consist of many stars, each designated by a letter of the Greek alphabet so far as possible, then the Roman letters and numerals are employed. Thus α (Alpha) is the most brilliant star; β (Beta) the next bright γ (Gamma) the next, and so on; so the relative brilliancy of the stars in the constellation is indicated, but not the very biggest star of the first magnitude is intended by α, for the star δ in one constellation may equal α in another. John Bayer originated this method in 1603.
The arrangement of the constellations is plunged in the obscurity of ages, but B.C. 370 there were forty-five thus grouped. There are northern and southern constellations which are visible above our old friends Aries, Taurus, Cancer, etc. We will, as in duty bound, consider our old acquaintances first, and then give a list of the northern and southern groups of stars; but we shall find that the forms are in the greater part due to the imagination of the ancients, and do not bear out our ideas of the animals they are supposed to represent, while at the same time they cross and recross with other constellations in the skies in a very puzzling way.
Fig. 596.—Aries.
The first constellation is Aries, the Ram, which is celebrated in mythology as the proud possessor of the Golden Fleece, which we may remember was seized and carried away by Jason and the Argonauts. The Hellespont is so called from Helle, who fell from the Ram’s back when being carried upon it over the Black Sea. The Ram is here represented with the equinoctial ring.
We perceive in Aries two very bright stars near the head. These are (α) Arietis and (γ) Sheretan. The signs and constellations do not now correspond as they used to do, because of the change in the position of the stars, which gives rise to the Precession of the Equinoxes (vide ante., p. 497), so that the stars which two thousand years ago were in conjunction with the sun, are much more to the eastward. In olden time (when astronomy was young), the sun entered Aries on the 21st March, and now a change has taken place. But in about another twenty thousand years, they will all come right again. This will be perceived by reference to the celestial globe. The Ram has sixty-six stars in his constitution.
Fig. 597.—Taurus.
Taurus, the Bull, is the next constellation. He received his name from the celebrated animal into which Jupiter transformed himself when he wished to carry away Europa. The star Aldebaran (α) is the end of a kind of V in the Bull’s face. The Pleiades are on the shoulder to the right. This cluster of twinkling stars is well known, and will guide the observer towards the imaginary Bull, which we must nowadays describe as rather a fanciful delineation. Europe is called after Europa, because Jupiter, as a Bull, carried her to this continent. There are 141 stars in Taurus, according to the number found in the list of Aratus, and probably more.
Fig. 598.—Gemini.
Gemini, the Twins, which are supposed to be Castor and Pollux, though it is believed that two goats were the original sign—which statement, taken in connection with the ram and the bull, that were also turned out in the spring-time, may have something to recommend it. But now Castor and Pollux are generally recognized as the constellation. During the expedition for the Golden Fleece, the electric appearance, now known as St. Elmo’s Fire, became visible upon them, and their effigies were placed in the forepart of ships as a good omen. This led to the adoption of the “figure-head.” They were made into stars when Pollux was immortalized by Jupiter, for he divided the boon with his brother. The planet Uranus was discovered near this constellation, which contains eighty-five stars.
Fig. 599.—Cancer.
Cancer, the Crab, is the next in order, and the only derivation we can find for this is that Juno sent a crab to attack Hercules when he was busily engaged with the many-headed Hydra. The crab was directed to pinch the hero’s foot, but it appears rather a lame device for the Queen to adopt. The crab, however, was killed by Hercules, and placed amongst the stars by Juno as a reward; so he gained immortality cheaply. He, Cancer, contains more than eighty stars, but none of them of any particular note. Some writers explain the sign as reminding the ancients of “the retrograde movement of the sun to the north”; but as a crab does not move “backwards,” we will still adhere to mythology as equally satisfactory at any rate. Cancer, however, was termed the “northern gate of the sun.”
Fig. 600.—Leo.
The next is Leo, the Lion, which came round in summer and at the period of much heat, so this fierce animal may have been chosen to represent that season. But mythology will have us credit the Nemæan Lion sent against Hercules by Juno as the origin of this constellation. The lion was, like the crab, placed amongst the stars when he was killed. He is a very brilliant constellation, and a very bright star called Regulus is to be seen in his chest—“Cor Leonis.” Another very fine star of the second magnitude is observable in the tail. The Lion consists of ninety-five stars, the principal ones being of the first and second magnitudes.
Fig. 601.—Virgo.
Virgo is supposed to be outlined by a very rich cluster of stars, and one of the first magnitude. The Virgin is by some supposed to be Astræa, the goddess, but is more likely referable to a girl gleaning, or holding an ear of corn in semblance of the harvest. This constellation contains more than one hundred stars. One of them in the wheat-ear is a particularly brilliant one, and noted for its “solitary splendour,” as no star of large magnitude is near it. The Arabs used to call it the Solitary Simak; Spica Virginis is the modern name.
Fig. 602.—Libra.
Libra, which follows, may either indicate the balance, or scales of justice, of Astræa, or the equal day and night at the autumnal equinox. Virgil mentions Astræa’s balance, and thus we have a classical authority for the very mythological view of the two foregoing constellations. Libra is not very distinct; it contains fifty-one stars, four of which are very bright.
Fig. 603.—Scorpio.
Scorpio, the Scorpion, according to classical writers, encountered Orion, who is also met with in the stellar universe. The scorpion stung Orion because he declared there was no living creature he could not overcome by force. On the other hand, this sign may have some reference to the unhealthy time of year, and the prevalence of disease about the time that Scorpio appeared. A beautiful star of reddish hue and of the first magnitude is prominent amongst the brilliant assembly of the Scorpion’s forty-four stars.
Fig. 604.—Sagittarius.
Sagittarius, the Archer, is, as one can see, a Centaur, and said to be Chiron, who was wounded by Hercules, and cured by being taken up to Heaven by Jupiter. This Chiron is represented as a great patron of the arts, and thus the fable may be said to exemplify the proverb, “Art is long, time is fleeting”; for readers of mythology will find much more in the legends than is apparent on the surface. But we can now only regard the Centaur from an astronomical, and not a philosophical standpoint. Sagittarius has no very brilliant stars. He is close to the Milky Way, and contains sixty-nine stars, five forming a sort of V in the bow, sometimes compared to a ladle or “dipper.”
Fig. 605.—Capricornus.
Capricornus, the Goat, is supposed to be Pan—“the great god Pan,” who turned himself into a goat. The sun was in Capricornus at mid-winter, so the “southern gate of the sun” was a title bestowed upon him. But now the constellation is later. It does not include any very striking stars, of which there are fifty-one in the “Goat.”
Fig. 607.—Aquarius.
Aquarius, or the Waterbearer, may have referred to wet weather, or as others declare, to Ganymede, the Cupbearer. There are four stars in the waterpot like a Y; and more than one hundred stars of small brilliancy are included in this constellation. But here again fancy must come to our assistance, for without a diagram the ordinary observer could not distinguish the Waterbearer.
Fig. 606.—Pisces.
Pisces, the Fishes, are not plainly defined. It is supposed that Venus and Cupid turned themselves into fish when the Titans assailed Heaven. This Constellation occupies a triangle in the sky.
The foregoing are the zodiacal constellations, and may be more easily remembered by repeating an old rhyme, which runs as follows:—
“The Ram, the Bull, the Heavenly Twins;
Then, next the Crab, the Lion shines,
The Virgin, and the Scales;
The Scorpion, Archer, and the Goat,
The Man who holds the Watering Pot,
The Fish with Glittering tails.”
The arrangement of the various Constellations at which we have so rapidly glanced, as well as of those that follow, has been the work of many different periods. Aratus and Ptolemy are the oldest enumerators, but modern research has added immensely to the store of knowledge. Many of the most prominent stars were named by Grecian and Arabian observers, and many of the names are still retained—such as Arcturus, Rigel, Capella, and others.
The Northern Constellations.
There are, altogether, thirty-five of these, as per list on next page. It is of course impossible to describe them all, but we will make a few remarks respecting those which will be distinguished most readily, and the manner of finding out particular stars. There are star maps published, and with a little attention and reading, a great many very pleasant evening excursions may be made across the sky, with or without a telescope. The following is the list of the northern constellations. We have put them in various types to indicate the most important.
| Name of Constellation. | “Author.” | No. of Stars. | |
|---|---|---|---|
| Ursa Major | The Great Bear | Aratus | 87 |
| Ursa Minor | The Lesser Bear | “ | 24 |
| Perseus | Perseus | “ | 59 |
| Auriga | The Waggoner | “ | 66 |
| Boötes | The Herdsman (Boötes) | “ | 54 |
| Draco | The Dragon | “ | 80 |
| Cepheus | Cepheus | “ | 35 |
| Canes Venatici | {The Greyhounds} | Hevelius | 25 |
| {Hunting Dogs | |||
| Cor Caroli | Heart of Charles | Halley | 3 |
| Triangulum | The Triangle | Aratus | 16 |
| Triangulum Minor | The Little Triangle | Hevelius | 10 |
| Musca | The Fly | Bode | 6 |
| Lynx | The Lynx | Hevelius | 44 |
| Leo Minor | The Lesser Lion | “ | 53 |
| Coma Berenices | Berenice’s Hair | Tycho Brahé | 43 |
| Camelopardalis | The Giraffe | Hevelius | 58 |
| Mons Menelaus | Mount Menalaus | “ | 11 |
| Corona Borealis | The Northern Crown | Aratus | 21 |
| Serpens | The Serpent | “ | 64 |
| Scutum Sobieski | Sobieski’s Shield. | Hevelius | 8 |
| Hercules | Hercules | Aratus | 113 |
| Serpentarius | The Serpent-bearer | “ | 74 |
| Taurus Poniatowski | Poniatowski’s Bull | Poczobat | 7 |
| Lyra | The Harp, or Lyre | Aratus | 22 |
| Vulpecula et Anser | Fox and Goose | Hevelius | 37 |
| Sagitta | The Arrow | Aratus | 18 |
| Aquila | The Eagle | “ | 71 |
| Delphinus | The Dolphin | “ | 18 |
| Cygnus | The Swan | “ | 81 |
| Cassiopeia | The Lady’s Chair | “ | 55 |
| Equuleus | The Little Horse | Ptolemy | 10 |
| Lacerta | The Lizard | Hevelius | 16 |
| Pegasus | Pegasus (Flying Horse) | Aratus | 89 |
| Andromeda | Andromeda | “ | 66 |
| Tarandus | The Reindeer | Lemounier | 12 |
| (There are a few others marked in continental maps.) | |||
The Great Bear, or “Charles’s Wain,” or the “Plough,” as Ursa Major is variously called, is of great value in indicating the pole star, which, when once known, can never be mistaken. This constellation has also been termed the “Dipper,” and is very conspicuous in the northern hemisphere. The three stars form the bear’s tail, or the handle of the “plough”; the others form the body, Charles’s Wain, or “Karl-Wagen,” the German term for peasant’s cart, is represented by the quadrangle forming the cart, and the other three stars are the horses.
The “Pointers” are the two end stars, and if a line be followed northwards from them it will lead close to Polaris, the principal star in the lesser bear. This pole star is of a very great brightness, and peeps out, almost isolated, with a pure lustre. The names of the pointers are Dubhe and Menak. The star at the tail-tip is Benetnasch, then Mizar and Alioth. Megrez and Phad are the remaining pair. We append a rough outline of the bear, for the information of those who have not yet noticed it.
The Lesser Bear is not so important as his elder brother as regards size, but he is very useful to astronomers. He resembles the Great Bear in appearance, but is smaller, and the positions of the stars are inverted. In the cut on page 555 (fig. 629) you see the little bear swinging round the polar star, which is at the tip of the Lesser Bear’s tail, so any one will be enabled to find him if they look for the polar star, and then count the three stars away from it, and the four in the body. The Great Bear’s tail points in the other direction. This movement of the earth’s axis by displacing the equinoctial points, alters the “declination” and “right ascension” of the stars (compare page 473). So Polaris is gradually approaching the actual polar point. In about 200 years he will have got as close as he can, and will then begin to recede from it, and in about 12,500 years after he will reach his most distant point.
Fig. 608.—The Great Bear.
Polaris, the Pole Star, was called “Cynosure” by the ancients, and thus we can understand the quotation, “Cynosure of neighbouring eyes,” when a person or object is very attractive. The pole star was the point to which all looked. There are some other very important stars in these constellations. For instance, in—
Perseus we have Algenib and Algol, of second magnitude.
Auriga we have Capella, of the first magnitude.
Boötes we have Arcturus, of the first magnitude.
Lyra we have Vega, a very large and bright star.
Aquila, Altair, also a very beautiful star.
In Cygnus there is Deneb, of the first magnitude.
These stars are also designated by the Greek letters—α, lyræ, or the first in the Lyre—that is, Vega; and so on for all, according to rank, as already explained.
Southern Constellations.
We must pass on to the southern constellations, of which there are forty-six; the principal ones are in capital letters:—
| Name of Constellation. | “Author.” | No. of Stars. | |
|---|---|---|---|
| * Phœnix | The Phœnix | Bayer | 13 |
| Apparatus Sculptoris | The Sculptor’s Tools | Lacaille | 12 |
| Eridanus Fluvius | The River Po | Aratus | 84 |
| * Hydrus | The Water-snake | Bayer | 10 |
| Cetus | The Whale | Aratus | 97 |
| Fornax Chemica | The Furnace | Lacaille | 14 |
| * Horologium | The Clock | ” | 12 |
| * Rheticulus Rhomboidialus | The Rhomboidal Net | ” | 10 |
| * Xiphias Dorado | The Sword-fish | Bayer | 7 |
| * Celapraxitels | The Engraver’s Tools | Lacaille | 16 |
| Lepus | The Hare | Aratus | 19 |
| Columba Noachi | Noah’s Dove | Halley | 10 |
| Orion | Orion | Aratus | 78 |
| Argo Navis | The ship Argo | ” | 64 |
| Canis Major | The Great Dog | ” | 31 |
| Equuleus Pictoris | The Easel | Lacaille | 8 |
| Monoceros | The Unicorn | Hevelius | 31 |
| Canis Minor | The Lesser Dog | Ptolemy | 14 |
| * Chameleon | The Chameleon | Bayer | 10 |
| Pyxis Nautica | The Mariner’s Compass | Lacaille | 4 |
| * Piscis Volans | The Flying Fish | Bayer | 8 |
| Hydra | The Snake | Aratus | 60 |
| Sextans | The Sextant | Hevelius | 41 |
| * Rober Carolinum | Charlie’s Oak | Halley | 12 |
| Antlia Pneumatica | The Air Pump | Lacaille | 3 |
| Crater | The Cup | Aratus | 31 |
| Corvus | The Crow | ” | 9 |
| *Crux Australis | The Southern Cross | Royer | 6 |
| Apis Musca | The South Fly | Bayer | 4 |
| * Avis Indica | The Bird of Paradise | ” | 11 |
| * Circinus | The Compass | Lacaille | 4 |
| Centaurus | The Centaur | Aratus | 35 |
| Lupus | The Wolf | ” | 24 |
| Norma | The Square | Lacaille | 12 |
| * Triangulum Australis | The Southern Triangle | Bayer | 5 |
| *Ara | The Altar | Aratus | 9 |
| * Telescopium | The Telescope | Lacaille | 9 |
| Corona Australis | The Southern Crown | Ptolemy | 12 |
| * Pavo | The Peacock | Bayer | 14 |
| * Indus | The Indian | ” | 12 |
| Microscopium | The Microscope | Lacaille | 10 |
| * Octans Hadliensis | Hadley’s Octant | ” | 43 |
| * Grus | The Crane | Bayer | 14 |
| Toucan | The Toucan | ” | 9 |
| Pisces Australis | The Southern Fish | Aratus | 24 |
| * Mons Mensa | Table Mountain | Lacaille | 30 |
We need only describe Orion and Canis Major, the principal groups. The former certainly constitutes the most glorious group, and it is visible to all the world, because the equinoctial passes through it.
Orion, as we have said, can be viewed from either hemisphere, and so can some others; but those marked with an asterisk in the foregoing list are not visible in the latitude of London.
Fig. 609.—Orion.
Orion is a very brilliant constellation, and contains two fine stars of the first magnitude, and some of the second. The former are Betelgeux and Rigel. Bellatrix is the third in order. The “belt” is formed of three bright stars, and the sword is visible as five stars just below. Canis Major possesses Sirius, a very fine star (the dog star). Canis Minor has two of the first and second magnitude, and Hydra has one of the first. The Southern Cross is a beautiful constellation, invisible in our latitude, but familiar to sailors in the Southern Seas.
The Stars’ Distance and Magnitude.
Fig. 610.—Polaris.
When we gaze up into the sky at night, we see the stars twinkling far away, and we may remark here that this twinkling of the stars is due to the atmosphere and the changes in its power of refraction, and of course the star’s light changes its direction. But if we ascend in a balloon into very high and rarefied strata of the air, we will find the twinkling less. We have given the number of the stars according to Flamstead, but the larger the telescope the greater will be the number of stars we shall see, numbers again being too far even for our perfected instruments.
But we can gain some idea of the magnitude of the stars when we consider the distance to arrive at, which is a most difficult task, for figures seem scarcely long enough to count the millions of miles, and no instrument can detect the parallax. Even supposing the parallax to be a very small fraction of a degree we should get a result equalling trillions of miles. No. 61 in Cygni was at one time continually observed by Professor Bessel, and he found that its distance—and it is the nearest star—is sixty-two and a half trillions of miles.
Fig. 611.—The Southern Cross.
Let us consider what this means. Light comes to us from the sun (91,000,000 of miles) in about eight minutes, and travels at the rate of something like 186,000 miles in a second. But even at that astounding rate the light from the star called 61 Cygni took ten years to reach the earth; and there are stars whose light has never yet reached the earth, although the gleam may have been travelling at 186,000 miles a second for thousands of years. And we may presume that though we still see the light of stars, some of them may be dead, but the light left is still progressing to us through space.
So we must conclude that some stars which look large, as do Vega and Sirius for instance, must be enormous “suns,” a great deal larger than our sun, and the stars are each the centre of invisible systems just as our sun is the centre of the “solar” system. Vega is a tremendous star, and shines with her own light as do all other visible stars; for reflected light, so very visible in the moon, which is close to us, would be quite invisible at such tremendous distances. So we must call these stars “suns,” and may add an apparently astonishing fact, that our own sun is merely one of the stars in the Galaxy, or “Milky Way”!
CHAPTER XLI.
THE STARS—(continued).
DOUBLE AND MULTIPLE STARS—COLOURED AND VARIABLE STARS—CLUSTERS, GROUPS, AND NEBULÆ—THE GALAXY, OR MILKY WAY—HOW TO FIND OUT THE PRINCIPAL STARS.
Although not very clearly visible to the naked eye, there are in the sky some pairs of stars very close together apparently; but when these double stars are examined with a good telescope we find that though we fancy they are two stars very close, in reality an immense distance separates them. By Vega, which we have already mentioned, there is apparently a star, which on examination will be found really to be two stars. It is also in the constellations of the Lyre, but of much lower magnitude than Vega. But in some instances there are three or four stars thus placed together, and the frequency of the occurrence of this fact establishes the farther fact that these combinations are not accidental—that the stars are interdependent and physically connected.
Fig. 612.—γ Leonis.
There are now at least six thousand double stars known,[27] and this is a very small proportion of the forty millions or so of suns which are believed to exist in space. But of these six thousand a larger proportion have been ascertained to be physically connected. More than six hundred of these pairs are double suns, while again there are other combinations of three and perhaps more. When two are thus connected we have what are termed binary systems, and when more are associated they are called triple and multiple stars. An example of the last-mentioned class is the small star above mentioned near Vega. It is ε Lyræ, and is a double of a double. In ordinary telescopes this will not be perceived, but with a high power the combination will be noticed. The same phenomenon is observable in one of the stars of Hercules and in Andromeda.
The revolution of these double suns, or binary systems, has been closely observed, and Professor Newcomb has given us a list of the binary systems of short period which are well determined. These are as follows:—
| 42 Coronæ | 26 years. |
| ζ Herculis | 35 years. |
| Struve 3,121 | 40 years. |
| η Coronæ | 40 years. |
| Sirius | 50 years. |
| ξ Cancri | 58 years. |
| ξ Ursæ Majoris | 63 years. |
| η Coronæ Borealis | 67 years. |
| α Centauri | 77 years. |
| μ Ophiuchi | 92 years. |
| λ Ophiuchi | 96 years. |
| ξ Scorpii | 98 years. |
Fig. 613.—Monocerotis.
It must be borne in mind that although these double stars appear close together from our standpoint, they may be far apart—one behind the other in a straight line. When such “pairs” exist they are known as optical pairs, or optically double stars, as distinguished from the actually physical “pairs” which revolve round the centre of a system. In Orion there has been discovered a regular system, and the θ in Orion, which appears in a common telescope a moderate star, and to the unaided eye only a speck of light, is really composed of seven stars—four are set in the form of a trapezium, as figured in the diagram in the margin by dots and asterisks. Two of these have been ascertained to possess attendants indicated by dots, and a seventh star was discovered by Lassell, and Humboldt remarks that in all probability this apparently tiny star in the constellation Orion constitutes a real system, for the five smaller stars have the same proper motion as the principal one.
Fig. 614.—Trapezium of Orion (Herschel).
Thus our imagination almost fails to grasp the infinity of the systems with our single sun, and with the distant double and even triple suns round which planets revolve perfectly independent of the other systems, as we are independent of them possessing heat and light from their own sun or suns as we receive it from ours, day and night seasons succeeding each other, and the wondrous varieties of the light produced by the appearance or withdrawal of a sun or two in the firmament of those most distant planets. These suns being double or triple would affect each other; the composition of the light given forth would produce—as we may assume—varying effects. We know something about the light of the stars by the spectroscope, and the colours of stars are due to the vapour which takes away a certain part of the light emitted, leaving the remainder to descend through the atmosphere to us.
Binary stars are most numerous of the doubles; for instance Castor, η Coronæ, Rigel, Polaris, Mivac, γ Leonis, γ Virginis, ξ Ursæ Majoris, α Hercules, 36 Andromedæ, λ Ophiuchi, and π Aquilæ. The illustration in the margin is Castor (or α Geminorum), the most northerly of the Twins. The η Coronæ is also figured, as are Polaris (see fig. 610 ante.), Boötes, Rigel, and γ Leonis.
The cuts herewith illustrate the relative positions at the periods named of the “doubles,” and of the revolution of suns around other suns as mentioned. As a consequence of their proper motion the binary stars appear to vary in their distances from each other, as in the topmost of the three cuts on the (opposite) page representing γ Virginis. The stars have gradually approached each other, and so are the stars in Castor approximating, and when they have closed, and have appeared almost as a single star, as they will do, they will take open order again.
Fig. 615.—η Coronæ. Fig. 616.—Boötes. Fig. 617.—Castor. Fig. 618.—Rigel.
The shortest time occupied by a double star in its revolution is thirty-five years, and we have already given some of those which have been ascertained. We will close this section with a few other examples. For instance, γ Virginis revolves in one hundred and fifty years, Castor in two hundred and forty years, 4 Aquarii in three hundred years, 37 Pegasi in five hundred years. There are numerous other instances up to a period of three thousand years, and about eight hundred of these binary systems are known. We have mentioned that there are two or more suns in the multiple systems. These suns are the cause of the different colours of the stars.
Colours of the Stars.
The question of star-colour follows naturally the consideration of the multiple stars; for although single stars have been observed of a ruddy colour, there are no instances of a blue or green one unattended by a companion. This colouring has been attributed to the contrast between multiple stars, for the colours are frequently complementary; but investigation has shown that this cannot be the case. For instances have been known in which, when two are thus associated, and one is concealed from us, the other is just as bright, and retains its former colour.
Of course in cases in which colour is apparent to the unaided vision, only the brightest stars betray colour. Antares, Betelgeuse and Aldebaran are red (orange) colour. Sirius and Canopus are white. Arcturus and Capella are yellowish, so is Pollux. Vega is bluish-white. These appearances are, of course, much more marked when the stars are examined through the telescope, and telescopic stars—which are stars unobservable without a glass—are very much coloured, and the multiple stars give us blue, green, violet, and other tints, besides those already mentioned.
Again, these coloured stars do not always remain the same colour. Sirius was once red; Mars was at times white. Spectrum analysis shows that the colours of many are due to absorption by the vapours of some of the rays; and the existence of certain vapours may cut off some, and at other times other vapours may exist and cut off other rays, and so the colours may be changed. Struve gives the following list of binary complements of “multiple” stars:—
| Pairs consisting of a blue and white principal star | 53 |
| Pairs consisting of a blue and a light yellow principal star | 52 |
| Pairs consisting of a blue and yellow or red principal star | 52 |
| Pairs consisting of a blue and green principal star | 16 |
Fig. 619.—Position of the two stars of γ Virginis.*
Fig. 620.—Position of the two stars of Castor.*
Fig. 621.—Position of the two stars of ξ Ursæ Majoris.
We need scarcely pursue this question farther, though many ideas concerning the coloured stars will arise in every thoughtful reader’s mind. Supposing that every system has its sun or suns, can we fancy the effects of a green or blue or violet sunlight—a light unmixed? To employ the words of Sir John Herschel—“It may be more easily suggested in words than conceived in imagination what variety of illumination two suns, a red and a green, or a yellow and blue one, must afford to a planet circulating round either—or what charming contrasts and graceful vicissitudes a red and a green day, for instance, alternating with a white one and with darkness, might arise from the presence or absence of one or other or both above the horizon.”
Lost and New Stars.
We may have perhaps read the “Lost Pleiad,” and wondered what has become of the star supposed to have dropped out of the cluster so well known in the constellation Taurus—the Pleiades. There are seven stars, of which six are visible to the average eye, and the ancients used to declare that one of the seven sisters (the daughters of Atlas and Pleione) hid herself because she had married a mortal, while all her sisters wedded gods. It is not improbable that one of the seven, formerly distinguishable with the unassisted eye, may have disappeared or been lost; but it is certain that strong eyesight can see more than seven now, and in the telescope there are about one hundred.
And it is a fact that some stars whose places have been carefully marked in the catalogue have subsequently disappeared. Many errors may have been made, and stars put down where no star existed, so a succeeding observer has not been able to find the star indicated. But, on the other hand, we may admit that stars have been lost to sight, and to compensate us for any such disappearances new stars are frequently observed, and these are very remarkable phenomena. About 121 B.C. Hipparchus perceived a new star, which was visible even in the daytime, and on subsequent occasions others came into existence—viz., in the years 945, 1264, and 1572. In the last-mentioned year Tycho Brahé suddenly perceived the new star, which was at first very brilliant. It grew fainter and fainter, after first gaining in intensity, and disappeared entirely in 1574; and at other times stars have been seen which remained only for a short time, and then disappeared.
The star discovered by Tycho Brahé was seen by him when walking across the fields one night, and he encountered peasants who were gazing at the new luminary. It was so bright that it threw a shadow from Brahé’s stick. The new arrival appeared in “Cassiopeia,” under the lady’s chair, forming, as pictured in the diagram, an irregular square. The strange star is the largest.
Fig. 622.—Cassiopeia.
Some stars exhibit extraordinary fluctuations, and one discovered by Mr. Birmingham in 1866, decreased rapidly and sank away to about the tenth magnitude, and then got brighter, and again diminished in splendour. The “Eta” Argûs has also been subjected to many fluctuations likewise, and such alterations have gained for these luminaries the name of “Variable Stars.”
Fig. 623.—Star-Map.
In the accompanying little chart there will be perceived two particular stars, named Algol, “the demon,” and Mira “the wonderful.” The latter is the most celebrated for its variable qualities, and its cycle of change occupies nearly one of our years. For a few days it appears very bright, and then fades away for about three months, to disappear for five months, and then it reappears again, increasing in brilliancy up to the second magnitude for another three months or so. Some people account for these phenomena by stating that the sides of the star being less luminous present the dark and light portions in rotation; but we can give no satisfactory explanation of the reason, unless it be caused by an aggregation of spots upon its surface, like sun-spots on our sun, or perhaps by eclipse.
Star-Clusters and Nebulæ.
Fig. 624.—Nebulæ in Pegasus.
Nebulæ and Star-clusters are numerous in the heavens. The most important are the Great Nebulæ in Orion and in Andromeda. But there are other very beautiful “patches” of luminous matter or cloud appearances composed of minute stars invisible to the naked eye. We annex specimens of the Nebulæ, one or two having been already inserted. There must be thousands of these star-clouds, and they have been classified by Sir John Herschel from Sir William’s discoveries as follows:—
(1) Clusters of stars, in which the stars are clearly distinguishable, divided again into regular and irregular clusters.
(2) Resolvable Nebulæ, which may be separated into distinct stars under powerful telescopes.
(3) Nebulæ, in which there is no appearance whatever of stars, divided into classes according to brightness, etc.
(4) Planetary Nebulæ.
(5) Stellar Nebulæ.
(6) Nebulous Stars.
Fig. 625.—Dumb bell Nebulæ.
We learn also from the foregoing authority that Nebulæ affect a certain district; that is, they have, as it were, a preference for it, and are not distributed in a random manner over the heavens, and are found in Leo, Leo Minor, Ursa Major, Canes Venatici, Coma, Böotes, and Virgo, and more sparingly in Aries, Taurus, Orion, Perseus, Draco, Hercules, Lyra, etc. Nebulæ are found associated with stars, as is the case with η Argus; these are called nebulous stars, and in the case of this particular star many very interesting investigations have been made. The Nebulæ are as equally variable as the stars they surround.
Fig. 626.—Nebulæ in Perseus.
What is termed the Nebular Hypothesis was put forward by La Place, and by it he endeavoured to account for the regular development of the stellar system, which is supposed to have originated from an immense nebular cloud. This immense mass would rotate and contract, and the outer portions would separate and develop in rings like Saturn’s rings. Then the rings break into separate portions, and each portion condenses into a planet, or the small “bits” travel round the sun like asteroids, and in this manner various systems were formed. This theory was considered to be quite exploded when stars were discerned in nebulæ by the more recent telescopes; but then the spectroscope came to our aid, and it was discovered that there were some nebulæ which are simply masses of glowing gas or aggregations of stones which are dashing against each other in so forcible a manner as to produce heat and luminosity. Mr. Lockyer appears to favour the latter theory as to nebulæ.
Fig. 627.—Nebulæ in Canes Venatici.
Mr. Proctor, however, has put forward a hypothesis that the star or meteor showers are the original cause of the sidereal system, and this rain of meteors has fallen for all time, gradually consolidating into orbs. The fact that the constituents of sun, earth and planets, comets and meteors being fundamentally the same lends probability to this hypothesis, which is fully explained by the author.
The Milky Way.
The Galaxy is familiar to all readers, and although visible all the year round, is perceived more plainly in August, September, and October, or at the beginning and ending of that period. This zone of stars was of course well known to the ancients, but it is to Galileo that we owe the first important information about the Galaxy; he decided that it was formed of stars. Sir John Herschel investigated the subject very closely, and to him much of the information concerning the Milky Way is due.
It is not very distinct in the north, but as it advances from Cepheus southwards by the Unicorn, it gets clearer, and opens out in Argo, and descends still south, becoming brighter near the Southern Cross. It then passes northward again, dividing into two branches, one of which dies out, and then over Sagittarius, and so on to Cygnus, then to Casseopeia and the starting-point. The number of stars in the Galaxy is about 18,000,000.
Fig. 628.—The Milky Way.
In this wonderful zone of stars the centre of our system, the sun, is placed. It was supposed to be divided as in the diagram above; the inner portion being the stars seen in their thickness, and the outer ring representing the stars viewed in the direction of the length and breadth. But afterwards, Herschel modified his opinions respecting the Milky Way, and since his death many astronomers—and Mr. Proctor more particularly—have devoted considerable time to an examination of this wonderful zone of stars; which, it must be remembered, is not a continuous stream; it is a series of luminous patches. On this point Professor Nichol says:—
“It is only to the most careless glance that the Milky Way appears a continuous zone. Let the naked eye rest thoughtfully on any part of it, and if circumstances are favourable, it will stand out rather as an accumulation of patches and streams of light in every conceivable variety of form and brightness; now side by side, now heaped on each other, again spanning across dark spaces ... and at other times darting off into the neighbouring skies in branches of capricious length and shape, which gradually thin away and disappear.”
The Milky Way has its greatest breadth in the “Swan,” and in the “Eagle” constellation it divides itself. In the “Southern Triangle” the zone is brightest, and in the “Southern Cross” the hole or space, termed by sailors the “Coal Sack,” is very distinct. It then contracts and expands, and there is in Argo another gap. Then it is lost for a space, then it branches out, and soon crosses the Equator, dilates, contracts, opens out again, and so returns to the “Swan” again.
Philosophers have frequently discoursed upon this phenomenon, but all statements must remain more or less speculative. From Kepler’s to the present time astronomers have been considering the Milky Way, and when the Nebular theory was given up, when the Galaxy was found to be composed of stars, there was, as we have noticed, the idea of the ring and the cloven disc. Mr. R. Proctor has likened the Galaxy to a coiled serpent, and considers the openings in the Milky Way as evidence that the stratum of stars is limited, and that here we can see beyond it. In fact, it would appear that it is a very complicated question; and as the zone itself is complicated “with outlying branches beyond the range of our most powerful telescopes,” so an actual knowledge of the Milky Way is beyond us at present. It is composed of most extraordinary aggregations of stars, which appear not only impossible to count, but each one to be independent of the other. Thus we must conclude our rapid survey of the Milky Way, and close with Mr. Proctor’s remark in his “Universe of Stars.” “The sidereal system,” he says, “is altogether more complicated, altogether more varied in structure than has hitherto been supposed. Within one and the same region co-exist stars of many orders of real magnitude, the greatest being thousands of times larger than the least. All the Nebulæ hitherto discovered, whether gaseous or stellar, irregular, planetary, ring-formed, or elliptic, exist within the limits of the sidereal system. They all form part and parcel of that wonderful system, whose nearer and brighter parts constitute the glories of our nocturnal heavens.”
And a little reflection will show how true this is. Not very long ago in the world’s life the solar system was supposed to consist of one sun with a few planets wandering around him. Then some more were found, and they were called “satellites.” For a long time man fancied he had reached the “ultima thule” of astronomy in these depths; but the whole idea was changed when it was discovered that beyond Mars there lie the asteroids and the host of bodies in this solar system which we cannot do more than allude to. Then when we consider that this “sun” of ours, which we think so enormous, and which keeps in subjection so many heavenly bodies, and illuminates them; when we reflect that there are in space, and visible, stars many times larger than our ruling star, each a sun, and that our sun would, if put where the great Sirius glows, be but a speck in the firmament, and his system invisible to our eyes, we may well wonder at the magnitude of the subject, and the Infinite Wisdom and Power “that telleth the number of the stars, and calleth them all by their names.”
How to read the Sky.
Fig. 629.—The “Swing” of the Lesser Bear.
A few particulars, to enable a reader to identify the most prominent stars, may be given as starting-points from which some few excursions into the spangled heavens may be attempted. But the suggestions must be considered with reference to the ever-varying directions of the supposed lines in consequence of the daily revolution of the sphere. We have illustrated this in the cut in the margin, wherein the Lesser Bear is shown as swinging round the Polar Star in different positions. Sometimes the lines of direction will be vertical, sometimes inclined, but all retaining their relative positions.
We have already learnt that the “pointers” of the Great Bear indicate the Polar Star in the Lesser Bear, and we can (roughly) estimate the distance between the pointers as 5°. This will give us the distance between the pointers and the Polar Star as 29°. By following an imaginary line through the two northern stars of the “Waggon” (the Bear) away from the “horses,” we shall find Capella about 50° away.
Fig. 630.—Diagram of the Pole Star.
If we pass from the first star next the waggon of “Charles’s Wain” to the Pole Star, and past it, we shall arrive at an irregular W. This is Cassiopeia, about as far beyond Polaris as the Bear is below it. When the latter is low, the former is at the zenith, and so on.
A line drawn from the Pole Star through the end star of the Great Bear leads to Arcturus. A line taken from Arcturus for about an equal distance will, with the Pole Star, make a triangle with Vega. The Polar Star may be called the Apex.
Regulus may be found southwards by drawing a line through the two first stars of the square in the Bear (opposite the pointers). From Vega, almost opposite the Pole Star, and through it about twice as far away on the other side, is Sirius, a brilliant “sun.” Procyon will be found to the westward of Regulus about 30°. From Procyon to the Pole Star a line will pass through Pollux and Castor.
Another line from the pole star through the middle of the three “horses” in the “Wain” will reach Spica Virginis about 70° beyond. So we can describe a large triangle with Spica, Regulus, and Arcturus, at the angles. Regulus is the apex, Spica and Arcturus a short base line.
Fig. 631.—Diagram of Sirius, etc.
From the pole star through Capella, passing between Betelgeux and Bellatrix, we shall describe a line leading to the three stars of Orion’s belt. Between it and the Pleiades is Aldebaran.
There are many other stars which could be indicated; but on a fine evening, if the observer will mark them upon a piece of paper, placing the pole star in the centre, he will be able to add to his star map very rapidly.
In the foregoing chapters of Astronomy we have seen how the earth and other planets move around the sun; we have glanced at the “fixed” stars and their groups, termed the constellations, and have noted the planets and their characteristics, with many other interesting facts. There is yet a great deal to be learnt, and much study will be required with daily (nightly) observations before the young reader will obtain success as a student of astronomy; but there is no study so interesting. We have seen what a very small portion of the universe is occupied by our solar system, and what a speck our earth is on the plain of creation. We find ourselves on the border-land of the incomprehensible, and we are lost in speculations upon the unseen.
CHAPTER XLII.
NEW ASTRONOMICAL APPLIANCES.
A CELESTIAL INDICATOR—ASTRONOMICAL OR COSMOGRAPHICAL CLOCK—A SIMPLE GLOBE—A SOLAR CHRONOMETER.
Having said something concerning astronomy, we will give a few instructions respecting the instruments not already described, and make some observations, supplementing our directions in the previous chapter, for many people will be glad to learn how to read the evening skies.
Here we have an apparatus which will prove useful to amateurs; it is a sort of celestial indicator by Mauperin, and will facilitate the finding of every star or constellation, when the apparatus has been made ready by pointing the rod, T, in the direction of the object it is desired to view. This rod is mounted upon a rod, S, and is movable upwards or downwards or sideways, and in the last-named movement it will carry with it an indicator, I, which slides over the chart or diagram of the heavens. The two arms of this indicator are always parallel to the plane of the rod, T, no matter in what position they may be on the chart or the inclination of the rod. The extremities of this rod are terminated by an eye-slit, and by a crescent respectively.
When the apparatus is set, all one has to do is to look through the eye aperture, O, and view the star which we have chosen in the centre of the crescent, C. This star will be found named in the space between the arms of the sliding indicator, I.
It is easy to perform the operation inversely—that is to say, to find in the sky, by means of the sighting-rod, T, the stars we have chosen on the chart between the prongs of the indicator. The chart represents exactly the heavens as we see them, and this new mode is opposed to the manner generally adopted with celestial charts, and is very important, for it obviates the necessity of holding the map above one’s head, its face downwards.
As we have already showed, it is not difficult to find the Polar Star and the Great Bear. The latter is readily recognised by its seven stars, and the Lesser Bear glides around the pole as shown in the diagram on the preceding page (fig. 629). Now let us see how the celestial indicator will work.
Let us take the apparatus into the open air, and place it upon its tripod stand. The upper portion will be found movable by loosening the screw, V. By another simple arrangement the table can be slanted, and by turning a screw we can entirely slope the side of the chart where midnight (minuit) is written. Being placed opposite the polar star we take the upper part of the chart by the button, G, and bring it before us by a horizontal rotative movement.
Fig. 632.—Celestial Indicator.
We now place the sliding indicator upon midday (midi), and keep it in that position while working the apparatus—that is, until we have caught sight of the polar star in the centre of the crescent, C, by means of the eye aperture, O. We have now obtained the meridian, and care should be taken to tighten the screw, V. Then the table is raised to its fixed place upon the support; it is regulated according to the latitude of the place, and the apparatus is then “oriented.”
The upper disc is an elliptical opening, or aperture, which contains for every moment the stars visible upon the horizon, and the circumference is furnished with a graduated scale of hours divided into five-minute divisions, and this is fixed upon the apparatus. The dotted line between the midday and midnight points gives the meridian.
The disc placed underneath is the celestial chart, on the circumference of which we shall find the days of the months. It can be moved around the rod, S, which represents the axis of the earth around which the heavens are supposed to revolve. When the stars have to be observed, the day of the month has to be brought to the time at which the observation is about to be made. We can easily read off the chart by looking through the eye-piece as already explained. Every five minutes it is necessary to move the chart one division, which indicates that five minutes have passed; (other stars are, of course, arriving). The apparatus can be packed away when done with, or the bearings taken, and then the trouble of getting it into position again need not be repeated.
A small lamp, L, throws its light upon the chart in such a way that the eyes of the observer are not incommoded, while the table is fully illuminated. It can be placed at L´ if necessary. The inclination varies according to the latitude of the place when the observations are made. There is an arrangement underneath which admits of this inclination according to longitude.
The apparatus can also be made available to ascertain what the aspect of the heavens will be upon any particular evening of the month. We have only to place the chart at the day and hour, and we shall then see upon it all the stars visible above the horizon. We can thus find out at what time the stars rise and set, and those which do not set—to find the hour at which they pass the meridian (the line drawn between midday and midnight upon the chart), and the time of their appearance on the horizon. When the sliding indicator, I, does not show a star that is discoverable in the sky, the observer may conclude that he is viewing a planet. This apparatus is well adapted for beginners in astronomy, as no deep preparatory study is necessary, and the tyro can read the sky as easily as he could read a book.
A Cosmographical Clock.
We have, in the foregoing chapters upon Astronomy, endeavoured to give the reader some idea respecting the inclination of the earth and its rotation, and writers have often endeavoured to devise an apparatus which shall show the position of the globe in space, its diurnal motion—even its inclination and the succession of seasons in its revolution round the sun. But such reproductions of simultaneous movements have hitherto been obtained only on a very large scale, which find their place well enough in the museum or lecture-room, but which it is quite impossible to utilize in our sitting-rooms, on our tables, or chimney-pieces. Besides, the usual apparatus employed is a very costly one, and only serves for occasional representation; it will not keep the facts constantly before the observer in the manner of a clock showing the time.
But for all who are interested in Astronomy, or in Cosmography, or even for a young person who desires merely to understand the reality of the earth’s motion and how our earth is placed in the universe—for any one who deems it of use that he or she should be able to see the signs and the seasons, and the days and years, and how the earth revolves, may obtain an astronomical or cosmographical clock, which will tell him or her how the “world wags”; a useful as well as an ornamental timepiece.
Fig. 633.—Cosmographical clock.
Now this is precisely the result which the talented inventor of the astronomical clock has arrived at. M. Mouret devoted a great portion of his life and all his available means to the realization of his great idea, and, sad to say, he died miserably in an attic the very day before his great and deserving effort brought him the reward for which he had so painfully striven and devoted himself to by a life of self-denial and labour.
M. Mouret communicated to his globe the astronomical movement, which our earth possesses, by the aid of clock-work, which conveys to it, second by second, at each stroke of the pendulum, the double movement of rotation and progression. The globe turns upon its axis in twenty-four hours, and thus one can perceive, without any mental effort, the rotation of our planet, and the portions of the globe which come under the influence of the sun in rotation, just as they do actually on the earth. Not the least interesting attribute of this ingenious arrangement is the fact that during breakfast or dinner one can see the displacement and revolution of the earth with reference to the sun to all people in the world. Here, on the meridian, all are at midday. There, on the left, near the circle which defines the limit between day and night, the sun is rising and day is beginning; opposite, on the right, the sun is setting and day is closing. Yonder is the Pacific Ocean in full daylight, while almost every continent is in darkness and the inhabitants wrapped in slumber. Now the Chinese are opening their eyes, and the Asiatic and European continents will soon be illuminated and awake. This is the movement of the world as it has ever been since time came into its calculations.
The ingenious inventor, who wished to make a clock of his apparatus, and not being able to change its place on the earth from day to day as the time changed, very cleverly reproduced the sun’s movement of declination by making it describe a double cone at the axis of the globe. At the equinoxes the poles are in a plane, and equal day and night are shown. At the winter solstice the north pole is inclined backwards at an angle 23° 28´, and our hemisphere is in the winter season. We have then only eight hours’ daylight and sixteen of darkness; six months later the pole is inclined towards the sun, and the southern pole is plunged in darkness. We have the long days and the southerners the long nights. An upright dial shows the time of the country in which the globe may happen to be, and one can ascertain at any moment what time it is anywhere else. A horizontal dial indicates the day of the month, and changes every day in a manner corresponding with the movement of the earth around the sun, reproduced by means of the arrangement with the double cone. The spectator is supposed to be turning his back to the sun.
We may add that these movements are all self-acting, and there is no need to interfere with the clock, which is wound up like ordinary timepieces. By an ingenious forethought the inventor provided that the sphere should be independent of the other movements, and it can be used for demonstration in the hands of the lecturer, and be explained with all its motions without in any way disarranging the clock-work. The globe must, of course, when replaced, be put exactly at the correct day and hour.
A Simple Terrestrial Globe.
A terrestrial globe without any mechanism, so long as its axis is parallel to that of the earth, exposed to the direct rays of the sun, represents our planet with its recurrence of day and night.
The figure (634) shows us a globe without any support. The axis is north and south, and makes, with the horizon, an angle equal to the latitude of Paris, if the support, A B, be horizontal. To make the axis of the globe parallel to the axis of our earth, the line, N S, must correspond to the meridian of the place; this can be done with the compass, for instance.
The solar rays always illuminate one-half of a sphere, no matter what its dimensions. If we look at the illustration we shall see that the line of separation between the light and dark portions of the globe corresponds with that in our earth. This globe, then, tells us the passage of light and darkness for the day, and even for the moment of the day, when it is turned as the earth moves. The place examined should be placed in the meridian of that place (Paris, for instance), and occupy the most elevated spot on the globe. The earth is then in just the same position, and daylight and darkness are shown exactly as they exist on the earth at the time.
If this globe be then observed for a few minutes, the sun will be seen rising and setting, as it were, in various places (we must remember we have concentrated the sun’s rays, not a lamp, upon the globe). The places on the right, if the observer be placed facing the sun, will come out of the shade, and those on the left will enter it. The former are then really enjoying the sunrise, and the others are actually witnessing his setting.
The globe represented, making the double revolution of our planet in the year, will reproduce all the actual phenomena of day and night as taking place on the earth itself if we stand at a little distance so as to observe it all at once.
Fig. 634.—A simple terrestrial globe.
Of course, the employment of this simple apparatus should not exclude more complicated ones, for the former can only be used on a fine bright day. But the advantage claimed for it is that in it we can imitate nature exactly. Illuminated, as it is, by the real sun, the portions of light and shade are indicated by the rays and not by a metallic circle.
In order that the line of demarcation may be exactly defined, it will be necessary that the sun’s rays be concentrated upon the globe, and that no lateral or vertical light be admitted. The curtains should therefore be so arranged, and the blinds pulled down to a certain point, and if the stand or support be painted black it will be found an advantage. If the globe be a small one, it will be sufficient to place the stand upon an ordinary table, without verifying the horizontal plane. With a large globe the arrangement must be very exact.
A Solar Chronometer.
M. Flechet’s chronometer, of which we give an illustration, is a kind of equitorial reduced to its most simple form. It is possible to ascertain the exact time by it very easily. It consists of a disc, AB, divided into twenty-four hours and fractions of hours. This disc turns upon itself around an arc, CD, which has a direction parallel to the axis of the world, and can be moved on a joint, E, according to the latitude of the place; F is a lens which can be moved and presented to the sun at any time, forming the centre of a concave and exactly spherical plate represented at GH.
Fig. 635.—Solar chronometer.
When the instrument is fixed so that the axis, CD, is parallel to the axis of the globe, the disc, AB, is turned so that the centre of the image of the sun, produced by the lens, shall fall at m. The real time is found by an examination of the position of the index, A, upon the hour graduations of the disc. A French writer, Ch. Delounay, has mentioned this instrument, and considers it easy of arrangement, exact in time, and very useful.
CHAPTER XLIII.
PHYSICAL GEOGRAPHY.—I. GEOLOGY.
GEOGRAPHY AND GEOLOGY—THE EARTH’S CRUST—ORIGIN OF THE EARTH—DENUDATION AND EXCAVATION BY WATER—ROCKS, GRAVEL, AND SAND—CLASSES OF ROCKS.
Fig. 636.—Cliffs worn by the sea.
When we were at school, and learnt the various countries of Europe, we had maps showing us the several divisions of one realm from another; the mountains, lakes, and other prominent features of the continent were learned and repeated, but we, maybe, seldom, perhaps never, bestowed a thought upon the formations of the mountains, and the manner in which rivers ran down into and through lakes to the ocean. There were the mountains, there were the lakes and rivers, and capes, and headlands, and there they are still, to all intents and purposes, the same to see, to climb up, to sail down, as the case may be. But the map of Europe has undergone a visible change. Territory has changed hands. Germany has gripped France, England has got Cyprus, Turkey has been dismembered, and Austria is annexing territory. This study is called Geography,—Political Geography,—for it marks the political boundaries. The knowledge of the formation of hills, headlands, lakes, rivers, seas, their causes, constitution, and effects, how they rose, how they exist and wax or wane during the course of centuries is Physical Geography, which we propose to consider.
This tree of knowledge includes some very important branches, almost parent stems. As a magnificent oak spreads forth its brawny arms, with smaller branches and twigs, each of these great branches being as large as an ordinary tree, so Physical Geography includes other arms such as Geology, Meteorology, Botany, and Physiology—even Astronomy in its comprehensive embrace. We find it is a difficult task to separate these kindred sciences from the great tree. We may have therefore to refer to earth, air, and water, and their various forms in hills and mountains, wind, vapour, rain, glaciers, and sea. We must learn how this earth has been gradually cooled, and what the various stages of its growth have done. We must consider plant and animal life upon our planet, and how the atmosphere affects them. All this is Physical Geography, and its satellite sciences of Geology, Meteorology, “Climatology,” Botany, and Physiology.
Fig. 637.—Disintegrated granite.
“Everything must have a beginning,” and the earth must have had a beginning, although the actual manner of the physical creation of the planet is a disputed fact. We are not about to discuss the religious side of the question, although we should undoubtedly find that Biblical and Geologic teaching run side by side towards the same end, and the testimony of the earth and sky bears witness to the Divine hand that created the universe, which we can trace back to the dim and distant ages when “the earth was without form and void, and darkness was upon the face of the deep.”
With this brief preface, let us consider some of these aspects, and pick up interesting facts from the ground.
Geology.
In the chemical and mineral sections of this volume we have heard something concerning the formation of the globe and its composition, its clays, rocks, etc. With these internal arrangements Chemistry and Mineralogy have dealt. Geology tells us about the external surface of the earth, its stones and rocks, and how they were formed, and generally something about the conformation of the crust of the earth, and its history.
When we speak of “crust” of the earth, we do not simply mean the exterior layer of gravel, clay, or stone. The crust is a thick one, and our crust extends just so far as we can cut into it. The surface of the sea can scarcely be termed a crust, but we must penetrate that ever-moving liquid boundary, and touch upon (and examine) what lies down below far beyond the “full fathom five” of the lead line. In this study we must not forget our Book of Nature, which is always open and inviting us to read. We shall see how things are produced, and how our physical surroundings will continue to be produced until the age of miracles returns, and Providence sees fit to interfere with the otherwise immutable laws which He in His wisdom has laid down for the universe.
It will be of no use to go back into space and imagine the world a red-hot fragment of matter, whirling through the heaven around the sun which, as a larger aggregation of burning atoms, kept it, as it now keeps it, in its place. The earth was a globe of liquid fire, or in gaseous state, and the atoms gradually cooled on the surface; the fire is still under our feet. The outer part would by degrees lose all its heat, while the interior remained hot; the planet must then have been surrounded by a steamy atmosphere, and enveloped in vapours condensed from the air through which no light of the sun could by any appreciable degree penetrate.
We can give an example of this, and it will be seen how the surface of the earth gradually became formed from the vaporous condition. If any one will take the pains to evaporate any saline solution in a capsule till it is about to crystallise, and observe attentively the pellicle of salt as it forms on the surface; first a partial film will show itself in a few places, floating about and joining with others, then when nearly the whole surface is coated, it will break up in some places and sink into the liquid beneath, another pellicle will form and join with the remains of the first, and as this thickens it will push up ridges and inequalities of the surface from openings and fissures in which little jets of steam and fluid will escape; these little ridges are chains of mountains, the little jets of steam those volcanic eruptions which were at that period so frequent, the surface of the capsule is the surface of the earth, and the five minutes which the observer has contemplated it, a million years.
The next effect of the cooling of the earth would be the gradual condensation of the vapour of water with which it was surrounded; this falling upon the earth formed seas and oceans, leaving only the higher portions exposed above its level. The clearing up of the dense dark clouds for the first time let in upon the earth’s surface the glorious and vivifying rays of the sun, and this great effect possibly accords with the earliest record in the Bible of the acts of creation: “And God said, let there be light, and there was light.”
This clearing up of the vapours and the subsequent rain no doubt gave rise to terribly grand electrical phenomena—thunderings and lightnings. By degrees the waters got their own way, and then many changes took place, land and water fighting, as it were, for the mastery, as they are fighting to this day.
But perhaps some reader may not think that the land and water of our earth are thus engaged. A very few minutes’ reflection will suffice to confirm our assertion. Look at the lofty crags in the Alps, for instance; what has shattered those peaks, and sent the masses toppling down in stone avalanches to the lower slopes, and then into the valleys?—Water. Water has been in the crevices, and was frozen there; in freezing it expanded and loosened the crags, which, forced asunder, gave an opening to more snow and ice, and so this powerful leverage, aided by the wind and storm, is disintegrating our mountains.
Fig. 638.—Breakers on the coast of Cornwall.
It is the same by the seashore; the cliffs are wearing away, and the sea approaches; at other places the sea recedes from the land, as coral formation and embryo chalk cliffs are rising under the surface of the ocean. Lakes dry up, and the meadow or farm arises on the site, while other old spots are submerged. No rest, no change of idea, but ever changing in physical appearance, Nature goes on her wondrous way, working now as steadily, as harmoniously, and as surely as she did before time was, and as she will continue to do when time shall be no more!
In our investigations into Geology we cannot enter into many technical details. Our object in these pages is recreation; but we shall, even under these circumstances, find plenty to interest, and sufficient to lead any one who wishes to pursue the study for himself. We will endeavour so to put the features of the stones and rocks before him that they will be recognized by the passer by. We will try to show how the earth has been built up, and how the great and terrible changes through which our little globe has passed have been effected. In our own islands, Great Britain and Ireland, we shall find traces of all the materials likely to be useful to us in our quest, As has been well said, “Geology is the Physical Geography of the past.”
Constitution of the Earth.
Fig. 639.—Shells in chalk.
The descent from rocks to stones, from stones to gravel, from gravel to sand, is evident to everyone, so we need not insist upon the fact that sand is powdered rock, and that an aggregation of sand particles makes stones. We have heard in the Mineralogy section that there are certain “earths”—silica, alumina, lime, etc. Of these “earths,” the two former constitute the greater portion of the ROCKS. Lime, also, is very evident, and in limestones fossils or organic remains are abundant. Now we must entirely put away from our minds the old idea that the earth we live on was created at once, or as it appeared to the first human beings. Our planet was prepared for man by degrees during millions of years. We conclude that the earth was originally composed of certain elements, and we find the same elements in the sun. Therefore, supposing (as is supposed) that the earth came from the sun, we have all the material of the globe in a fused state. As the earth cooled, rocks were formed by pressure, and then water came, and now we can read “books in the running brooks, and sermons in stones” at our leisure.
Fig. 640.—The streamlet.
Perhaps as someone reads this he may be walking by the seashore kicking the pebbles or seated upon the sands, the grains of which are so very tiny. He will probably find sand, shingle, and gravel within reach, and perhaps the curious-looking “pudding stone.” Now what can we learn from these stones or sand grains or that curious bit of conglomerate? Perhaps the reader may be at Ramsgate or Margate or another place where the “white cliffs of Albion” glisten in the sun. Take up a piece of chalk and examine it. It is soft and soils your hands, and you will throw it away, perhaps—but don’t. Take it home and put it under the microscope or a good magnifying glass. What do you see?
Fig. 641.—Cliffs showing strata.
Fig. 642.—Limestone with encrinites.
You will find the remains of animals—that is, shells and tiny bits of coral packed close together. Under the microscope they will become more separated, and the grains will be distinct fragments of shells, etc. If this little bit of chalk be composed of marine animals’ shells, of course the whole cliff is composed of the same kind of material. But how did the shells get into the chalk? Shells are chalk—carbonate of lime; lime was deposited at the bottom of the sea, and the infinite millions of minute animals formed themselves shells, and left them to be piled up by Nature’s forces into cliffs during countless ages.
Yes, but how did the lime get into the water to make the shells? We will endeavour to explain. Rain, when falling, takes some carbonic acid from the air, which we know contains it. This acts upon the lime in the rocks (lime is oxide of calcium, and calcium is an element in the earth), so we get a bi-carbonate of lime (soluble in water), which rises from the rocks in springs. These springs and their streams deposit lime, as we can see in caverns where we find stalactites and stalagmites. The lime is transmitted to the ocean, and absorbed by the crinoideans and molluscs which produced shells. These shells hardened and crystallized became limestone, and whole mountains are formed of this “organic” rock, which is used for so many purposes.
Fig. 643.—Chalk cliff.
We have spoken of Organic Rocks, but there are others, and we ought, perhaps, to have spoken of that kind before the chalk put them aside. Let us go back to our sandy shore again and look at the Sedimentary Rocks, which are the very first formation. We have all seen sandstone, and visitors to the South Devon Coast will remember the red cliffs near Dawlish and Teignmouth. These are red sandstone—not the very “old red” so pleasantly written of by Hugh Miller, but at any rate sandstone, and composed of grains of sand. When we were at Dawlish last year a piece of the sandstone had fallen on to the beach, and when the waves came up that stone was no doubt gradually washed away into sand, and then fell to the earth as sediment.
We said something a few pages back about the wear and tear which is always going on: the mountain is worn away—a mass falls, it is broken into smaller pieces; these are carried by a river; the mud is deposited, and the finer particles are ground and rounded into gravel, and finally sand. Beneath the current of the river, and at the bottom of a lake or sea, these sediments (mud, etc.) accumulate one on the top of the other in regular series called strata, and then the weight and pressure acting with the soluble mineral deposits always washing down, consolidate and bind the loose sand-grains into stone, which, in the course of ages, hardens. The stones thus formed from sediments such as gravel, mud, and sand, are termed Sedimentary Rocks; they have become rocks by enormous and continuous pressure. Thus:—
Sands have become “Sandstones”;
Gravel has become “Pudding-stone” (Conglomerate);
Mud and clay have become “Shale”;
Calcareous deposits have become “Limestone”;
Vegetable deposits have become “Coal.”
So we have sandy, clayey, limy, flinty, and corally rocks under long names respectively—Arenarious, Argillaceous, Calcareous, Silicious; and we may add Bitumenous and Ferruginous—Irony Rocks—to the list.
Speaking of sediments, it is curious to note the different colours of the Arve and the Rhone which meet near Geneva. The white sedimentary Arve can be traced for a long distance beside, not mixing with, the blue Rhone. The same effect can be traced where the latter river enters the Lake of Geneva. So the land is being perpetually carried away and deposited; and where water gains on land there is somewhere else always a corresponding elevation to compensate it. Thus places disappear, and the sea washes over the site, as on the Kentish Coast, where Earl Godwin’s land was inundated, and new land is reclaimed or is elevated from the sea to make up the balance.
“There rolls the deep, where grew the tree;
Oh, earth, what changes hast thou seen!
There, where the long street roars, has been
The stillness of the central sea.”
We have spoken of sedimentary and organic rocks. There is yet another kind called igneous, or fiery rocks—those upraised by volcanic action. Of the igneous rocks the crystalline have been evidently in a fused condition. Granite is an example; lava or basalt is the usual term for volcanic rock, and the basaltic caves of Staffa and the Giants’ Causeway bear testimony to the igneous or volcanic origin of the surroundings. The pillars and fantastic rocks of Ireland and Scotland which are so remarkable, are simply lava, which was erupted in a molten state, now cooled and contracted into blocks of curious regularity of form.
Fig. 644.—Trap rock (Staffa).
Granite, already referred to, is another igneous rock, and must have been forced upwards; for as an igneous rock granite has cooled beneath the crust of the earth, throwing out arms, while melting, into other formations, and frequently being found in mountains. There is another kind of igneous rocks formed by the continuous accumulation of the ashes, etc., vomited forth from volcanoes. Masses of mountain are thus produced in the course of years, and the material thus formed is called tufa, or tuff, when consolidated; and this (now solidified) is what caused the destruction of Pompeii and Herculaneum.
Fig. 645.—Eruption of granite.
So we have two classes of Igneous Rocks, the Crystalline and Fragmental (or the “Plutonic” and “Volcanic”), including basalts, pitchstones, pumice, trachytes, granite, syenite, etc.; and, on the other hand, tuff and “volcanic” breccia, with felstones, porphyries, etc., which have been classed as intermediary.
Of these three classes of rocks, the sedimentary and the organic compose the greater portion of the earth. We will now glance at the crust of the earth and its various formations.
Volcanic eruption.