THE REASON WHY.
CHAPTER I.
1. Why should we seek knowledge?
Because it assists us to comprehend the goodness and power of God.
And it gives us power over the circumstances and associations by which we are surrounded: the proper exercise of this power will greatly promote our happiness.
2. Why does the possession of knowledge enable us to exercise power over surrounding circumstances?
Knowledge enables us to understand that, in order to live healthily, we require to breathe fresh and pure air. It also tells us that animal and vegetable substances, undergoing decay, poison the air, though we may not be able to see, or to smell, or otherwise discover the existence of such poison. Knowing this, we become careful to remove from our presence all such matters as would tend to corrupt the atmosphere. This is only one of the countless instances in which knowledge gives us power over surrounding circumstances.
3. Name some other instances in which knowledge gives us power.
Knowledge of Geography and of Navigation enables the mariner to guide his ship across the trackless deep, and to reach the sought-for port, though he had never before been on its shores.
Knowledge of Chemistry enables us to separate or to combine the various substances found in nature. Thus we obtain useful and precious metals from what at first appeared to be useless stones; transparent glass from pebbles, through which no light could pass; soap from oily substances; and gas from solid bodies.
"Give instruction to a wise man, and he will be yet wiser; teach a just man, and he will increase in learning."—Proverbs ix.
Knowledge of Medicine enables the physician to overcome the ravages of disease, and to save suffering patients from sinking prematurely to the grave.
Knowledge of Anatomy and of Surgery enables the surgeon to bind up dangerous fractures and wounds, and to remove, even from the internal parts of bodies, ulcers and diseased formations that would otherwise be fatal to life.
Knowledge of Mechanics enables man to increase his power by the construction of machines. The steam-ship crossing the ocean in opposition to wind and tide, the railway locomotive travelling at 60 miles an hour, and the steam-hammer beating blocks of iron into useful shapes, are evidences of the power which man acquires through a knowledge of mechanics.
Knowledge of Electricity enables man to stand in comparative safety amid the awful war of the elements. Lightning, the offspring of electricity, has a tendency to strike upon lofty objects by which it may be attracted. By its mighty powers churches or houses may be instantly levelled with the dust. But man, knowing that electricity is strongly attracted by particular substances, raises over lofty buildings rods of steel communicating with bars that descend into the ground. The lightning, rushing with indescribable force toward the steeple, is attracted by the bar of steel, and conducted harmlessly to the earth. Man may thus be said to take even lightning by the hand, and to divert its destroying force by the aid of Knowledge. And in countless other instances "Knowledge is Power."
CHAPTER II.
Because the air contains oxygen, which is necessary to life.
5. Why is oxygen necessary to life?
Because it combines with the carbon of the blood, and forms carbonic acid gas.
"Be not as the horse, or as the mule, which have no understanding: whose mouth must be held with the bit and bridle."—Psalm xxxii.
6. Why is this combination necessary?
Because we are so created that the substances of our bodies are constantly undergoing change, and this resolving of solid matter into a gaseous form, is the plan appointed by our Creator to remove the matter called carbon from our systems.
7. Why do our bodies feel warm?
Because, in the union of oxygen and carbon, heat is developed.
8. What is this union of oxygen and carbon called?
It is called combustion, which, in chemistry, means the decomposition of substances, and the formation of new combinations, accompanied by heat; and sometimes by light, as well as heat.
9. What is formed by the union of oxygen and carbon?
Carbonic acid gas.
10. What becomes of this carbonic acid gas?
It is sent out of our bodies by the compressure of the lungs, and mingles with the air that surrounds us.
11. Is this carbonic acid gas heavier or lighter than the air?
Pure carbonic acid gas is the heaviest of all the gases. That which is sent out of the lungs is not pure, because the whole of the air taken into the lungs at the previous inspiration has not been deprived of its oxygen, and the nitrogen is returned. Therefore the breath sent out of the lungs may be said to consist of air, with a large proportion of carbonic acid gas.
12. What is the composition of air in its natural state?
It consists of oxygen, nitrogen, and carbonic acid gas, in the proportions of oxygen 20 volumes, nitrogen 79 volumes, and carbonic acid gas 1 volume. It also contains a slight trace of watery vapour.
13. What is the state of the air after it has once been breathed?
It has parted with about one-sixth of its oxygen, and taken up an equivalent of carbonic acid. And were the same air to be breathed six times successively, it would have parted with all its oxygen, and could no longer sustain life.
"A prudent man forseeth the evil, and hideth himself; but the simple pass on, and are punished."—Proverbs xxvii.
14. Is the impure air sent out of the lungs lighter or heavier than common air?
At first, being rarefied by warmth, it is lighter. But, if undisturbed, it would become heavier as it cooled, and would descend.
15. Why is it proper to have beds raised about two feet from the ground?
Because at night, the bed-room being closed, the breath of the sleeper impregnates the air of the room with carbonic acid gas, which, descending, lies in its greatest density near to the floor.
16. What are the chief sources of carbonic acid gas?
The vegetable kingdom (as will be hereafter explained), the combustion of substances composed chiefly of carbon, the breathing of animals, and the decomposition of carbonic compounds.
17. Is breathing a kind of combustion?
It is. In the breathing of animals, the burning of coals, or of wood, or candles, &c., similar changes occur. The oxygen of the air combines with the carbon of the substance said to be burnt, and forms carbonic acid gas, which unfits the air for the purposes of either breathing or of burning, until it has been renewed by admixture with the air.
It is one of the elementary bodies, and is very abundant throughout nature. It abounds mostly in vegetable substances, but is also contained in animal bodies, and in minerals. The form in which it is most familiar to us is that of charcoal, which is carbon almost pure.
19. What is meant by an elementary body?
An elementary body is one of those substances in which chemistry is unable to discover more than one constituent. For instance, the chemist finds that water is composed of oxygen and hydrogen. Water is therefore a compound body. But carbon consists of carbon only, and therefore it is called a simple, or elementary body.
"Where no wood is, there the fire goeth out: so where there is no tale-bearer, the strife ceaseth."—Proverbs xxvi.
20. Why is it dangerous to burn charcoal in rooms?
Because, being composed of carbon that is nearly pure, its combustion gives off a large amount of carbonic acid gas.
21. What is the effect of carbonic acid gas upon the human system?
It induces drowsiness and stupor, which, if not relieved by ventilation, would speedily cause death.
22. What is the reason that people feel drowsy in crowded rooms?
Because the large amount of carbonic acid gas given off with the breaths of the people, makes the air poisonous and oppressive.
23. What other causes of drowsiness are there?
The candles, gas, or fires that may be burning in the rooms where people are assembled. Three candles produce as much carbonic acid gas as one human being; and it is probable that one gas-light produces as much carbonic acid gas as two persons.
24. Have people ever been poisoned by their own breaths?
In the reign of George the Second, the Rajah of Bengal took some English prisoners in Calcutta, and put 146 of them into a place which was called the "Black Hole." This place was only 18 feet square by 16 feet high, and ventilation was provided for only by two small grated windows. One hundred and twenty-three of the prisoners died in the night, and most of the survivors were afterwards carried off by putrid fevers. Many other instances have occurred, but this one is the most remarkable.
CHAPTER III.
Oxygen is one of the most widely diffused of the elementary substances. It is a gaseous body.
"Stand in awe and sin not: commune with your own heart upon your bed and be still"—Psalm iv.
26. Why do persons who are walking, or riding upon horseback feel warmer than when they are sitting still?
Because as they breathe more rapidly, the combustion of the carbon in the blood is increased by the oxygen inhaled, and greater heat is developed.
27. Why does the fire burn more brightly when blown by a bellows?
Because it receives, with every current of air, a fresh supply of oxygen, which unites with the carbon and hydrogen of the coals, causing more rapid combustion and increased heat.
28. Why does not the oxygen of the air sometimes take fire?
Because oxygen, by itself, is incombustible. The wick of a candle, which retains the slightest spark, being immersed in oxygen, will instantly burst into a brilliant flame; and even a piece of iron wire made red-hot, and dipped in oxygen, will burn rapidly and brilliantly. Oxygen, though non-combustible of itself, is the most powerful supporter of combustion.
29. Why do we know that oxygen will not burn of itself?
Because when we immerse a burning substance into a jar of oxygen, it immediately burns with intense brilliancy; but directly it is withdrawn from the oxygen, the intensity of the flame diminishes, and the oxygen which remains is unaffected.
30. Why do we know that oxygen is necessary to our existence?
Because animals placed in any kind of gas, or in any combination of gases, where oxygen does not exist, die in a very short time.
It is found in the air, mixed with nitrogen; in water combined with hydrogen; in the tissues of vegetables and animals; in our blood; and in various compounds called, from the presence of oxygen, oxides.
32. Why is the oxygen of the air mixed so largely with nitrogen?
Because oxygen in any greater proportion than that in which it is found in the atmosphere, would be too exciting to the animal system. Animals placed in pure oxygen die in great agony from fever and excitement, amounting to madness.
"As vinegar is to the teeth, and as smoke to the eyes, so is the sluggard to him that sent him."—Proverbs x.
Nitrogen is an elementary body in the form of gas.
It is chiefly found in the air, of which it constitutes 79 out of 100 volumes. It may be mixed with oxygen in various proportions; but in the atmosphere it is uniformly diffused. It is found in most animal matter, except fat and bone. It is not a constituent of the vegetable acids, but it is found in most of the vegetable alkalies.
Acids are a numerous class of chemical bodies. They are generally sour. Usually (though there are exceptions) they have a great affinity for water, and are easily soluble therein; they unite readily with most alkalies, and with the various oxides. All acids are compounds of two or more substances. Acids are found in all the kingdoms of nature.
Alkalies are a numerous class of substances that have a great affinity for, and readily combine with, acids, forming salts. They exercise peculiar influence upon vegetable colours, turning blues green, and yellows reddish brown. But they will restore the colours of vegetable blues which have been reddened by acids; and, on the other hand, the acids restore vegetable colours that have been altered by the alkalies. Alkalies are found in all the kingdoms of nature.
37. Could animals live in nitrogen?
No; they would immediately die. But a mixture of oxygen and nitrogen, in equal volumes, constitutes nitrous oxide, which gives a pleasurable excitement to those who inhale it, causing them to be merry, almost to insanity; it has, therefore, been called laughing gas.
38. Why does nitrous oxide produce this effect?
Because it introduces into the body more oxygen than can be consumed. It, therefore, deranges the nervous system, and being a powerful stimulant, gives an unnatural activity to the nervous centres and the brain.
"Lord, make me know mine end, and the measure of my days, that I may know how frail I am."—Psalm xxxix.
39. In what proportions are the atmospheric gases found in the blood?
The mean quantity of the gases contained in the human blood has been found to be equal to 1-10th of its whole volume. In venous blood, the average quantity of carbonic acid is about 1-18th, that of oxygen about 1-85th, and that of nitrogen about 1-100th of the volume of the blood. In arterial blood their quantities have been found to be carbonic acid about 1-14th, oxygen about 1-38th, and nitrogen about 1-72nd.
40. Then is nitrogen taken into the blood from the air?
Such a supposition is highly improbable. It is probably derived from nitrogenised food, just as carbonic acid is derived from carbonised food.
Venous blood is that which is returning through the veins of the body from the organs to which it has been circulated.
Arterial blood is that which is flowing from the heart through the arteries to nourish the parts where those arteries are distributed.
43. What is the difference between venous and arterial blood?
Venous blood contains more carbonic acid, and less oxygen and nitrogen than arterial blood.
It will not burn, nor will it support combustion.
45. What is the difference between "burning" and "supporting combustion?"
Oxygen gas will not burn of itself, but it aids the decomposition by fire of bodies that are combustible. It is therefore called a supporter of combustion. But hydrogen gas, though it burns of itself will extinguish a flame immersed in it. It is therefore said to be a body which will burn, but will not support combustion.
"As coals are to burning coals, and wood to fire; so is a contentious man to kindle strife."—Proverbs xxvi.
46. What becomes of the nitrogen that is inhaled with the air?
It is thrown off with the breath, mixed with carbonic acid gas, and flies away to be renewed by a fresh supply of oxygen.
47. Where does nitrogen find a fresh supply of oxygen?
In the atmosphere. Nitrogen is said to possess a remarkable tendency to mix with oxygen, without having a positive chemical affinity for it. That is to say, neither the oxygen nor the nitrogen undergoes any change by the union, except that of admixture. The oxygen and the nitrogen still possess their own peculiar properties. Oxygen and nitrogen are found in nearly the same proportions in all climates, and at all altitudes.
48. In combustion does any other result take place besides the union of oxygen and carbon forming carbonic acid gas?
Yes. Usually hydrogen is present, which in burning unites with oxygen, and forms water.
CHAPTER IV.
Hydrogen is an elementary gas, and is the lightest of all known bodies.
50. Will hydrogen support animal life?
It will not. It proves speedily fatal to animals.
51. Will hydrogen support combustion?
Although it will burn, yielding a feeble bluish light, it will, if pure, extinguish a flame that may be immersed in it. Hydrogen will therefore burn, but will not support combustion.
52. Why will hydrogen explode, if it will not support combustion?
When hydrogen explodes it is always in combination with oxygen, or with the common air, which contains oxygen. Two measures of hydrogen and one of oxygen form a most explosive compound.
"As smoke is driven away, so drive them away: as wax melteth before the fire, so let the wicked perish at the presence of God."—Psalm xlvi.
53. Why does hydrogen explode, when mixed with oxygen, upon being brought in contact with fire?
Because of its strong affinity for oxygen, with which, upon the application of heat, it unites to form water.
54. Where does hydrogen chiefly exist?
In the form of water, where it exists in combination with oxygen. Eleven parts of hydrogen, and eighty-nine of oxygen, form water.
55. Is hydrogen found elsewhere?
It is never found but in a state of combination; united with oxygen, it exists in water; with nitrogen, in ammonia; with chlorine, in hydro-chloric acid; with fluorine, in hydro-fluoric acid; and in numerous other combinations.
56. Is the gas used to illuminate our streets, hydrogen gas?
It is; but it is combined with carbon, derived from the coals from which it is made. It is therefore called carburetted hydrogen, which means hydrogen with carbon.
57. How is hydrogen gas obtained from coals?
It is driven out of the coals by heat, in closed vessels, which prevent its union with oxygen.
58. What becomes of the water which is formed by the burning of hydrogen in oxygen?
It passes into the air in the form of watery vapour. Frequently it condenses, and may be seen upon the walls and windows of rooms where many lights or fires are burning. Sometimes, also, portions of it become condensed in the globes of the glasses that are suspended over the jets of gas. A large volume of these gases forms only a very small volume of water.
59. What becomes of the carbonic acid gas which is produced by combustion?
It is diffused in the air, which should be removed by adequate ventilation.
"I will both lay me down in peace and sleep: for thou, Lord, only, makest me dwell in safety."—Psalm iv.
60. What proportion of carbonic acid gas is dangerous to life?
Any proportion over the natural one of 1 per cent. may be regarded as injurious. But toxicologists state that five per cent. of carbonic acid gas in the atmosphere is dangerous to life.
Persons who study the nature and effects of poisons and their antidotes.
62. Which kind of combustible used for lighting tends most to vitiate the air?
Assuming all the lights to be of the same intensity, the degree in which the substances burnt would vitiate the atmosphere may be gathered from the number of minutes each would take to exhaust a given quantity of air. This has been found to be: rape oil, 71 minutes; olive oil, 72; Russian tallow, 75; town tallow, 76; sperm oil, 76; stearic acid, 77; wax candles, 79; spermaceti candles, 83; common coal gas, 98; canal coal gas, 152. Thus it is shown that rape oil is most destructive of the atmosphere, and that coal gas is the least destructive.
63. Is an escape of hydrogen gas from a gas-pipe dangerous to life?
It is dangerous, first, by inhalation. There are no less than six deaths upon record of persons who were killed by sleeping in rooms near to which there was a leakage of gas.
It is dangerous, secondly, by explosion.
In 1848, an explosion of gas occurred in Albany-street, Regent's-park, London. The gas accumulated in a shop for a very short time only. It had been escaping from a crack in the meter for about one hour and twenty minutes. The area of the room was about 1,620 cubic feet. When the gas exploded, it blew out the entire front of the premises, carried two persons through a window into an adjoining yard, and forced another person on to the pavement on the opposite side of the street, where she was killed. The effect of the explosion was felt for more than a quarter of a mile on each side of the house, and most of the windows in the neighbourhood were shattered. The iron railings over the area of the house directly opposite were snapped asunder; and a part of the roof, and the back windows of another house, were carried to a distance of from 200 to 300 yards. The pavement was torn up for a considerable length, and the damage done to 103 houses was afterwards reported to amount to £20,000. Other serious explosions have taken place. The explosions of "coal damp," which frequently occur in mines, are of a similar character.
"O Lord, our Lord, how excellent is thy name in all the earth! who hast set thy glory above the heavens."—Psalm viii.
64. What proportion of hydrogen gas with atmospheric air will explode?
According to the researches of Sir Humphrey Davy, seven or eight parts of air, to one of gas, produce the greatest explosive effect; while larger proportions of gas are less dangerous. A mixture of equal parts of gas and air will burn, but it will not explode. The same is the case with a mixture of two of air, or three of air, and one of gas; but four of air and one of gas begin to be explosive, and the explosive tendency increases up to seven or eight of air and one of gas, after which the increased proportion of gas diminishes the force of the explosion.
65. What is the best method of preventing the explosion of gas?
Observe the rule, never to approach a supposed leakage with a light. Fortunately the gas, which threatens our lives, warns us of the danger by its pungent smell. The first thing to be done is to open windows and doors, and to ventilate the apartment. Then turn the gas off at the main, and wait a short time until the accumulated gas has been dispersed.
66. Does hydrogen gas rise or fall when it escapes?
Being twelve times lighter than common air it rises, and therefore it would be better for ventilation to open the window at the top than at the bottom. But all gases exhibit a strong tendency to diffuse themselves, and therefore they do not rise or fall in the degree that might be anticipated.
67. What proportion of hydrogen in the air is dangerous to life, if inhaled?
One-fiftieth part has been found to have a serious effect upon animals. The effects it produces upon the human system are those of depression, headache, sickness, and general prostration of the vital powers. It is therefore advisable to observe precautions in the use of gas.
"From the place of his habitation he looketh upon all the inhabitants of the earth."—Psalm xxxiii.
68. What proportion of gas in the air may be recognised by the smell?
By persons of acute powers of smelling it may be recognised when there is one part of gas in five hundred parts of atmospheric air; but it becomes very perceptible when it forms one part in a hundred and fifty. Warning is, therefore, given to us long before the point of danger arrives.
69. What other sources of hydrogen are there in our dwellings?
It arises from the decomposition of animal and vegetable substances, containing sulphur and hydrogen. These give off a gas called sulphuretted hydrogen, from which the fætid effluviam of drains and water-closets chiefly arise. We should, therefore, take every precaution to secure effective drainage, and to keep drain-traps in proper order.
70. May the use of gas for purposes of illumination be considered highly dangerous?
Not if it is intelligently managed. The appliances for the regulation of gas are so very simple and perfect, that accidents seldom arise except from neglect. In England 6,000,000 tons of coal are usually consumed in the manufacture of gas, producing 60,000,000,000 cubic feet of gas. And yet accidents are of very uncommon occurrence.
CHAPTER V.
Heat is a principle in nature which, like light and electricity, is best understood by its effects. We popularly call that heat, which raises the temperature of bodies submitted to its influence.
Caloric is another term for heat. It is advisable, however, to use the term caloric when speaking of the cause of heat, and of heat as the effect of the presence of caloric.
"While the earth remaineth, seed-time and harvest, and cold and heat, and summer and winter, and day and night, shall not cease."—Gen. viii.
73. What is the source of caloric?
The sun is its chief source. But caloric, in some degree, exists in every known substance.
74. What are the effects of caloric?
Heat which, in proportion to its intensity, acts variously upon all bodies, causing expansion, fusion, evaporation, decomposition, &c.
75. Why is caloric called a repulsive agent?
Because its chief effects are to expand, fuse, evaporate, or decompose the substances upon which it acts.
76. What is an attractive agent, in contradistinction to a repulsive agent?
Chemical attraction, or affinity, is an attractive agent—as when bodies seek of their own natures to unite and form some new body.
77. When is a body said to be hot?
When it holds so much caloric that it diffuses heat to surrounding objects.
78. When is a body said to be cold?
When it holds less caloric than surrounding objects, and absorbs heat from them.
79. How may caloric be excited to develop heat?
By any means which cause agitation, or produce an active change in the condition of bodies. Thus friction, percussion, sudden condensation or expansion, chemical combination, and electrical discharges, all develope heat.
80. Why do "burning glasses" appear to set fire to combustible substances?
Because they gather into one point, or focus, several rays of caloric as they are travelling from the sun, and the accumulation of caloric developes that intensity of heat which constitutes fire.
In optics, it is the point or centre at which, or around which, divergent rays are brought into the closest possible union.
"Yet man is born to trouble, as the sparks fly upward.—I would seek unto God, and unto God would I commit my cause."—Job v.
It is a violent chemical action attending the combustion of the ingredients of fuel with the oxygen of the air.
83. What are the properties of fire?
It imparts heat, which has the effect of expanding both fluids and solids.
It cannot exist without the presence of combustible materials.
It has a tendency to diffuse itself in every direction.
It cannot exist without oxygen or atmospheric air.
84. What elements take part in the maintenance of a fire?
Hydrogen, carbon, and oxygen. Hydrogen and carbon exist in the fuel, and oxygen is supplied by the air.
85. How does the combustion of a fire begin?
A match made of phosphorous and sulphur (highly inflammable substances) is drawn over a piece of sand-paper; the friction of the match induces the presence of caloric, which developes heat, and ignites the match, the burning of which is sustained by the oxygen of the air. The flame is then applied to paper or wood, and the heat of the flame is sufficient to drive out hydrogen gas, which unites with the oxygen of the air, and burns, imparting greater heat to the carbon of the coals, which assumes the form of carbonic acid gas by union with oxygen, and in a little while all the conditions of combustion are established.
86. What are the properties of heat?
It may exist without fire or light.
It is not sensible to vision.
It makes an impression upon our feelings.
It acts powerfully upon all bodies.
It has no weight.
It attends, or is connected with, all the operations of nature.
It radiates from all bodies in straight lines, and in all directions.
It strikes most powerfully in direct lines.
Its rays may be collected into a focus, just as the rays of the sun.
It may be reflected from a polished surface.
It is more easily conducted by some substances than by others.
"For my days are consumed like smoke, and my bones are burned as an hearth."—Psalm cii.
Animal heat is derived from the slow combustion of carbon in the blood of animals with the oxygen of the air which the animals breathe.
Latent heat (or more properly latent caloric) is that which exists, in some degree, in all bodies, though it may be imperceptible to the senses.
89. Is there latent caloric in ice, snow, water, marble, &c?
Yes; there is some amount of caloric in all substances.
A blacksmith may hammer a small piece of iron until it becomes red hot. With this he may light a match, and kindle the fire of his forge. The iron has become more dense by the hammering, and it cannot again be heated to the same degree by similar means, until it has been exposed in fire, to a red heat. Is it not possible that, by hammering, the particles of iron have been driven closer together, and the latent heat driven out? No further hammering will force the atoms nearer, and therefore no further heat can be developed. But when the iron has again absorbed caloric, by being plunged in a fire, it is again charged with latent heat. Indians produce sparks by rubbing together two pieces of wood. Two pieces of ice may be rubbed together until sufficient warmth is developed to melt them both. The axles of railway carriages frequently become red hot from friction.
Yes; whenever oxygen combines with carbon to form carbonic acid gas, an extrication of heat takes place, however minute the amount. Such a combination occurs much more extensively during the germination of seeds and the impregnation of flowers, than at any other time. In the germination of barley heaped in rooms, previous to being converted into malt, it is well known that a considerable amount of heat is developed.
91. Has any investigation of this subject ever been carefully made?
Yes. Lamarck, Senebier, and De Candolle, found the flowers of the Arum Maculatum, between three and seven o'clock in the afternoon, as much as 7 deg. Reaum. warmer than the external air. Schultz found a difference of 4 deg. to 5 deg. between the heat of the spathe of the Canadian pinnatifolium and the surrounding air, at six to seven o'clock p.m. Other observations have established differences of as much as 30 deg. between the temperature of the spathe of the Arum cordifolium, and that of the surrounding atmosphere.
"And there are diversities of operations, but it is the same God which worketh in all."—Corinthians xii.
92. Have plants sometimes a temperature lower than that of the surrounding air?
Yes. It has not only been found that under particular circumstances the heat of certain parts of plants is elevated to a very remarkable degree, but that, under nearly all circumstances, they have a temperature different from that of the external air, being warmer in winter, and cooler in summer.
CHAPTER VI.
93. How many kinds of combustion are there?
There are three, viz., slow oxydation, when little or no light is evolved; a more rapid combination, when the heat is so great as to become luminous; and a still more energetic action, when it bursts into flame.
94. Why does phosphorous look luminous?
Because it is undergoing slow combustion.
95. Why do decayed wood, and putrifying fish, look luminous?
Because they are undergoing slow combustion. In these cases the heat and light evolved are at no one time very considerable. But the total amount of heat, and probably of light, generated through the lengthy period of this slow oxydation, amounts to exactly the same as would be evolved during the most rapid combustion of the same substances.
It is gaseous matter burning at a very high temperature.
97. Why, when we put fresh coals upon a fire, do we hear the gas escaping from the coals without taking fire?
Because, the fire being slow, the temperature is not high enough to ignite the gas.
"I will praise thee, O Lord, with my whole heart; I will show forth thy marvellous work."—Psalm ix.
98. What is the gas which escapes from the coals?
Carburetted hydrogen.
99. Why, if we light a piece of paper, and lay it where the gas is escaping from the coals, will it burst into flame?
Because the lighted paper gives a heat sufficient to ignite the gas; and because also hydrogen requires the contact of flame to ignite it.
100. Why, when the coals have become heated, will the hydrogen burst into flame?
Because the carbon of the coals, and the oxygen of the air, have begun to combine, and have greatly increased the heat, and have produced a rapid combustion, so nearly allied to flame, that it ignites the hydrogen.
101. What temperature is required to produce flame?
That depends upon the nature of the combustible you desire to burn. Finely divided phosphorous and phosphorated hydrogen will take fire at a temperature of 60 deg. or 70 deg.; solid phosphorous at 140 deg.; sulphur at 500 deg.; hydrogen and carbonic oxide at 1,000 deg. (red heat); coal gas, ether, turpentine, alcohol, tallow, and wood, at about 2,000 deg. (incipient white heat). When once inflamed they will continue to burn, and will maintain a very high temperature.
Smoke consists of small particles of carbon of hydrogen gas, and other volatile matters, which are driven off by heat and carried up the chimney.
103. Is it not a waste of fuel to allow this matter to escape?
It is, as it might all be burnt up by better management.
104. How may the waste be avoided?
By putting on only a little coals at a time, so that the heat of the fire shall be sufficient to consume these volatile matters as they escape.
"And the strong shall be as tow, and the maker of it as a spark, and they shall both burn together, and none shall quench them."—Isaiah i.
105. Why is there so little smoke when the fire is red?
Because the hydrogen and the volatile parts of the coal have already been driven off and consumed, and the combustion that continues is principally caused by the carbon of the coals, and the oxygen of the air.
106. Will carbon, burnt in oxygen, produce flame and smoke?
It burns brightly, but it produces neither flame nor smoke.
107. Why do not charcoal and coke fires give flame?
Because the hydrogen has been driven off by the processes by which charcoal and coke are made.
108. What is a conductor of heat?
A conductor of heat is any substance through which heat is readily transmitted.
109. What is a non-conductor of heat?
A non-conductor is any substance through which heat will not pass readily.
110. Name a few good conductors.
Gold, silver, copper, platinum, iron, zinc, tin, stone, and all dense solid bodies.
111. Name a few non-conductors.
Fur, wool, down, wood, cotton, paper, and all substances of a spongy or porous texture.
112. How is heat transmitted from one body to another?
By Conduction, Radiation, Reflection, Absorption and Convection.
113. What is the Conduction of heat?
It is the communication of heat from one body to another by contact. If I lay a penny piece upon the hob, it becomes hot by conduction.
114. What is the Radiation of heat?
The transmission of heat by a series of rays. If I hold my hand before the fire, the rays of heat fall upon it, and my hand receives the heat through radiation.
"Sing praises to the Lord, which dwelleth in Zion, declare among the people his doings."—Psalm ix.
115. What is the Reflection of heat?
The reflection of heat is the throwing back of its rays towards the direction whence they came. In a Dutch oven the rays of heat pass from the fire to the oven, and are reflected back again by the bright surface of the tin. There is, therefore, considerable economy of heat in ovens, and other cooking utensils constructed upon this plan.
116. What is the Absorption of heat?
The absorption of heat is the taking of it up by the body to which it is transmitted or conducted. Heat was conveyed to my hand by radiation, and taken up by my hand by absorption.
117. What is the Convection of heat?
The convection of heat is the transmission of it through a body or a number of bodies, or particles of bodies, by those substances which first received it; as when hot water rises from the bottom of a kettle and imparts heat to the cold water lying above it.
CHAPTER VII.
118. Why does not a piece of wood which is turning at one end, feel hot at the other end?
Because wood is a bad conductor of heat.
119. Why is wood a bad conductor of heat?
Because the arrangement of the particles of which it is composed does not favour the transmission of caloric.
120. Why do some articles of clothing feel cold, and others warm?
Because some are bad conductors of heat, and do not draw off much of the warmth of our bodies; while others are better conductors, and take up a larger portion of our warmth.
"The fining pot is for silver, and the furnace for gold: but the Lord trieth the hearts."—Proverbs xvii.
121. Which feels the warmer, the conductor or non-conductor?
The non-conductor, as it does not readily absorb the warmth of our bodies.
122. What substances are the best conductors of heat?
Gold, silver, copper, and most substances of close and hard formation, &c.
123. What substances are the worst conductors of heat?
Fur, eider down, feathers, raw silk, wood, lamp-black, cotton, soot, charcoal, &c.
124. Why has the toasting-fork a wooden handle?
Because wood is not so good a conductor as metal, therefore the wood prevents the heat from being transmitted by conduction to our hands.
125. Why has the coffee-pot a wooden handle?
Because the metal of the coffee-pot would otherwise conduct the heat to the hand; but wood, being a bad conductor, prevents it.
126. Why does hot water in a metal jug feel hotter than in an earthenware one?
Because metal, being a good conductor, readily delivers heat to the hand; but earthenware, being an indifferent conductor, parts with the heat slowly.
127. How can we ascertain that wood prevents the conduction of heat to the hand?
By passing the top of the finger along the wooden handle of the coffee-pot, until it reaches the point where the wood meets the metal. The wooden handle will be found to be cool, but the metal will feel very hot.
128. Of what use are kettle-holders?
Being made of bad conductors, such as wood, paper, or woollen cloth, they will not readily conduct the heat from the kettle to the hand.
"Wisdom is the principal thing; therefore get wisdom: and with all thy getting get understanding."—Proverbs iv.
129. Will a kettle-holder, being a bad conductor, sometimes conduct heat to the hand?
Yes. But so slowly that the hand will not feel the inconvenience of too much heat.
130. Why does hot metal feel hotter than heated wool, though they may both be of the same degree of temperature?
Because metal gives out heat more rapidly than wool, by which it is made more perceptible to our feelings.
131. Which would become cold first—the metal or the wool?
The wool, because, although the metal conducts heat more rapidly, to a substance in contact with it, it does not radiate heat as well as a black and rough substance.
132. Why do iron articles feel intensely cold in winter?
Because iron is one of the best conductors, and draws off heat from the hand very rapidly.
133. What is the cause of the sensation called cold?
When we feel cold, heat is being drawn off from our bodies.
134. What is the cause of the sensation called heat?
When we feel hot, our bodies are absorbing heat from external causes.
The condition here implied is that of health, and of ordinary circumstances. A person in a condition of fever, suffering from intense heat arising from a diseased state of the blood, could not be said to be absorbing heat. Nor could such a description apply to a person who, by a very rapid walk, has raised the temperature of his body considerably above its natural state, by the internal combustion which has already been described. A person feeling hot in bed, from excessive clothes, feels hot from the development of heat internally, which is not conducted away with sufficient rapidity to maintain the natural temperature of the body.
135. If a person, sitting before a fire-place, without a fire, were to set one foot upon a rug, and the other upon the stone hearth, which would feel the colder?
The foot on the stone, because stone is a good conductor, and would conduct the warmth of the foot away from it.
"The earth is the Lord's, and the fulness thereof; the world, and they that dwell therein."—Psalm xxiv.
136. What does the hearth-stone do with the heat that it receives?
It delivers it to the surrounding air, and to any other bodies with which it may be in contact—and as it parts with heat, it takes up more from any body hotter than itself.
137. When there is no fire in a room, what is the relative temperature of the various things in the room?
They are all of the same temperature.
138. If all the articles in the room are of the same temperature, why do some feel colder than others?
Because they differ in their relative powers of conduction. Those that are the best conductors feel coldest, as they convey away the heat of the hand most rapidly.
If you lay your hand upon the woollen table cover, or upon the sleeve of your coat or mantle, it will feel neither warm nor cold, under ordinary circumstances. But if you raise your hand from the table cover, or coat, and lay it on the marble mantel piece, the mantel-piece will feel cold. If now you return your hand from the mantel-piece to the table cover or coat, a sensation of warmth will become distinctly perceptible. This will afford a good conception of the relative powers of conduction of wool and marble.
139. How long does a substance feel cold or hot to the touch?
Until it has brought the part touching it to the same temperature as itself.
140. When do substances feel neither hot nor cold?
When they are of the same temperature as our bodies.
141. Why, under these circumstances, do they feel neither hot nor cold?
Because they neither take heat from, nor supply it to, the body.
142. Which would feel the warmer, when the fire was lighted, the hearth-rug or the hearth-stone?
The hearth-stone, because it is a good conductor, and would not only receive heat readily, but would part with it as freely (thereby making its heat perceptible). But the hearth-rug, being a bad conductor, would part with its heat very slowly, and it would therefore be less perceptible.
"Fire and hail; snow and vapour; stormy wind fulfilling his word."—Psalm cxlviii.
143. Would the hearth-stone feel hotter than the hearth-rug though both were of the same temperature?
It would feel hotter than the hearth-rug, because it would part with its heat so rapidly that it would be the more perceptible.
144. But if the hearth-stone and the hearth-rug were both colder than the hand, which would feel the colder of the two?
Then the hearth-stone would feel the colder, because, being a good conductor, it would take heat from the hand more freely than the hearth-rug, which is a bad conductor.
145. Why would the hearth-stone feel comparatively hotter in the one case, and colder in the other?
Because, being a good conductor, it would conduct heat rapidly to the hand when hot, and take heat rapidly from the hand when cold.
CHAPTER VIII.
146. Which are the better conductors of heat, fluids or solids?
Generally speaking, solids, especially those of them that are dense in their substance.
147. Why are dense substances the best conductors of heat?
Because the heat more readily travels from particle to particle until it pervades the mass.
148. Why are fluids bad conductors of heat?
Because of the want of density in their bodies; and because a portion of the imbibed heat always passes off from fluids by evaporation.
"He casteth forth his ice like morsels: who can stand before his word,"—Psalm cxlvii.
149. Why are woollen fabrics bad conductors of heat?
Because there is a considerable amount of air occupying the spaces of the texture.
150. Is air a good or a bad conductor?
Air is a bad conductor, and it chiefly transmits heat, as water does, by convection.
151. Is water a good or a bad conductor?
Water is an indifferent conductor, but it is a better conductor than air.
152. Why, when we place our hands in water, which may be of the same temperature as the air, does the water feel some degrees colder?
Because water, being a better conductor than air, takes up the warmth of the hand more rapidly.
153. Why, when we take our hands out of water do they feel warmer?
Because the air does not abstract the heat of the hand so rapidly as the water did, and the change in the degree of rapidity with which the heat is abstracted produces a sensation of increased warmth.
154. Why do we see blocks of ice wrapped in thick flannel in summer time?
Because the flannel, being a non-conductor, prevents the external heat from dissolving the ice.
Flannel wrapped around a warm body keeps in its heat; and wrapped around a cold body, prevents heat from passing into it.
155. How do we know that air is not a good conductor of heat?
Because, in still air, heat would travel to a given point much more rapidly, and in greater intensity, through even an indifferent solid conductor, than it would through the air.
156. How do we know that water is not a good conductor of heat?
Because in a deep vessel containing ice, and with heat applied at the top, some portion of the water may be made to boil before the ice, which lies a little under the surface, is melted.
"As snow in summer, and as rain in harvest; so honour is not seemly for a fool."—Prov. xxvi.
157. Why would you apply the heat at the top, in this experiment?
Because in heating water it expands and rises. The boiling of water is caused by the heated water ascending from the bottom, and the colder water descending to occupy its place. If the heat were not applied at the top, it would be distributed quickly by convection, but not by conduction.
158. Why are bottles of hot water, used as feet-warmers, wrapped in flannel?
Because the flannel, being a bad conductor, allows the heat to pass only gently from the bottle, and preserves the warmth for a much longer time.
159. Why are hot rolls sent out by the bakers, wrapped up in flannel?
Because the flannel, being a bad conductor, does not carry off rapidly the heat of the rolls.
160. Why is it said that snow keeps the earth warm?
Because snow is a bad conductor, and prevents the frosty air from depriving the earth of its warmth.
161. Why are snow huts which the Esquimaux build found to be warm?
Because snow, being a bad conductor, keeps in the internal heat of the dwelling, and prevents the cold outer air from taking away its warmth.
162. Why is snow, being composed of congealed water (and water being a better conductor than air), so good a non-conductor?
Because in the process of congealation it is frozen into crystalline forms, which, being collected into a mass, form a woolly body, thus proving the truthfulness of the Bible simile, which says, God "giveth snow like wool."
"He giveth snow like wool: he scattereth the hoar frost like ashes."—Psalm cxlvii.
FIG. 1.—CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE.
163. Why does it frequently feel warmer after a frost has set in?
Because, in the act of congealation a great deal of heat is given out, and taken up by the air, and thus the severity of the cold is in some degree moderated.
164. Why is it frequently colder when a thaw takes place?
Because, in the process of thawing, a certain amount of heat is withdrawn from the air, and enters the thawed ice.
165. What benefit results from these provisions of Nature?
They moderate both the severity of frosts, and the rapidity of thaws, which, in changeable climates, would be seriously detrimental to life, and to vegetation.
166. Why are furs and woollens worn in the winter?
Because, being non-conductors, they prevent the warmth of the body from being taken up by the cold air.
167. Why are the skins of animals usually covered with fur, hair, wool, or feathers?
Because their coverings, being non-conductors of heat, preserve the warmth of the bodies of the animals.
"He sendeth out his word, and melteth them: he causeth his wind to blow, and the waters to flow."—Psalm cxlvii.
168. How is the greater warmth of animals provided for in the winter?
It is observed that, as winter approaches, there comes a short woolly or downy growth, which, adding to the non-conducting property of their coats, confines their animal warmth.
In small birds during winter, let the external colour of the feathers be what it may, there will be found a kind of black down next their bodies. Black is the warmest colour, and the purpose here is to keep in the heat, arising from the respiration of the animal.
169. How is warmth provided for in animals that have no such coats?
They are furnished with a layer of fat, which lies underneath the skin. Fat consists chiefly of carbon, and is a non-conductor.
170. Why are summer breezes said to be cool?
Because, as they pass over the heated surface of the body, they bear away a part of its heat.
171. Why is a still summer air said to be sultry?
Because, being heated by the sun's rays, and being a bad conductor, it does not relieve the body by carrying off its heat.
172. Why does fanning the face make it feel cooler?
Because, by inducing currents of air to pass over the face, a part of the excessive heat is taken up and carried away.
173. Why does perspiration cool the body?
Because it takes up a part of the heat, and, evaporating, carries it into the air.
174. Why does blowing upon hot tea cool it?
Because it directs currents of air over the surface of the tea, and these currents take up a part of the heat and bear it away.
175. Why does air in motion feel cooler than air that is still?
Because each wave of air carries away a certain portion of heat and being followed by another portion of air, a further amount of heat is borne away.
"Though I walk in the valley of the shadow of death I will fear no evil, for thou art with me."—Psalm xxiii.
176. Is the atmosphere ever as hot as the human body?
Not in this country. On the hottest day it is 10 or 12 deg. cooler than the temperature of our bodies.
177. What is the highest degree of artificial heat which man has been known to bear?
A man may be surrounded with air raised to the temperature of 300 deg. (the boiling point being 212), and yet not have the heat of his body raised more than two or three degrees above its natural temperature of from 97 deg. to 100 deg.
178. Why may man endure this degree of heat for a short time without injury?
Because the skin, and the vessels of fat that lie underneath it, are bad conductors of heat.
And because perspiration passing from the skin and evaporating, would bear the heat away as fast as it was received.
Because, also, the vital principle (life) exercises a mysterious influence in the preservation of living bodies from physical influences.
179. Is the air ever hot enough, in any part of the world, to destroy life?
Yes. The hot winds of the Arabian deserts, which are called simooms, scatter death and desolation in their track, withering trees and shrubs, and burying them under waves of hot sand. When camels see the approach of a simoom they rush to the nearest tree or bush, or to some projecting rock, where they place their heads in an opposite direction to that from which the wind blows, and endeavour to escape its terrible violence. The traveller throws himself on the ground on the lee side of the camel, and screens his head from the fiery blast within the folds of his robe. But frequently both man and beast fall a prey to the terrible simoom.
180. Why are these hot winds so terrible in their effects?
Because, being in motion, they search their way to every part of the body, and passing over it leave some portion of their heat behind, which is again followed by additional heat from every fresh blast of wind.
"The fear of the Lord is the beginning of knowledge: but fools despise wisdom and instruction."—Proverbs i.
CHAPTER IX.
The radiation of heat is a motion of the particles, in a series of rays, diverging in every direction from a heated body.
182. What is this phenomena of Radiation understood to arise from?
From a strongly repulsive power, possessed by particles of heat, by which they are excited to recede from each other with great velocity.
183. What is the greatest source of Radiation?
The sun, which sends forth rays of both light and heat in all directions.
184. When does a body radiate heat?
When it is surrounded by a medium which is a bad conductor.
185. When we stand before a fire, does the heat reach us by conduction or by radiation?
By radiation.
186. What becomes of the heat that is radiated from one body to another?
It is either absorbed by those bodies, or transmitted through them and passed to other bodies by conduction, or diffused by convection, or returned by reflection.
187. How do we know that heat is diffused by radiation?
If we set a metal plate (or any other body, though metal is best for the experiment) before the fire, rays of heat will fall upon it. If we turn the plate at a slight angle, and place another object in a line with it, we shall find that the plate will reflect the rays it has received by radiation, on to the object so placed; but if we place an object between the fire and the plate, we shall find that the rays of heat will be intercepted, and that the latter can no longer reflect heat.
"The fear of the Lord is the beginning of wisdom: a good understanding have all they that do his commandments."—Psalm cxi.
188. Does the agitation of the air interfere with the direction of rays of heat?
It has been found that the agitation of the air does not affect the direction of rays of heat.
189. Why, then, if a current of air passes through a space across which heat is radiating, does the air become warmer?
Because it takes up some portion of the heat, but it does not alter the direction of the rays.
This is clearly illustrated by reference to rays of light which are seen under many circumstances. But they are never bent, moved, nor in any way affected by the wind.
190. Why will not a current of air disturb the rays of heat, just as it would a spider's web, or threads of silk?
Because heat is an imponderable agent, that is, something which cannot be acted upon by the ordinary physical agencies. It has no weight, presents no substantial body, and is, in these latter respects, similar to light and electricity.
191. What other sources of radiation of heat are there besides the sun and the fire?
The earth, and all minor bodies, are, in some degree, radiators of heat.
192. What substances are the best radiators?
All rough and dark coloured substances and surfaces are the best radiators of heat.
193. What substances are the worst radiators of heat?
All smooth, bright, and light coloured surfaces are bad radiators of heat.
Dr. Stark, of Edinburgh, has proved, by a series of experiments, the influence which the colours of bodies have upon the velocity of radiation. He surrounded the bulb of a thermometer successively with equal weights of black, red, and white wool, and placed it in a glass tube, which was heated to the temperature of 180 deg. by immersion in hot water. The tube was then cooled down to 50 deg. by immersion in cold water; the black cooled in 21 minutes, the red in 26 minutes, and the white in 27 minutes.
"Say unto wisdom, Thou art my sister; and call understanding thy kinswoman."—Proverbs vii.
194. If you wished to keep water hot for a long time, should you put it into a bright metal jug, or into a dark earthenware one?
You should put it into a bright metal jug, because, being a bad radiator, it would not part readily with the heat of the water.
195. Why would not the dark earthenware jug keep the water hot as long as the bright metal one?
Because the particles of earthenware being rough, and of dark colour, they radiate heat freely, and the water would thereby be quickly cooled.
CHAPTER X.
196. But if (as stated in the Lessons upon Conduction) metal is a better conductor of heat than stone or earthenware, why does not the metal jug conduct away the heat of the water sooner than the earthenware jug?
It would do so, if it were in contact with another conductor; but, being surrounded by air, which is a bad conductor, the heat must pass off by radiation, and as bright metal surfaces are bad radiators, the metal jug would retain the heat of the water longer than the earthenware one.
197. Supposing a red-hot cannon ball to be suspended by a chain from the ceiling of a room, how would its heat escape?
Almost entirely by radiation. But if you were to rest upon the ball a cold bar of iron, a part of the heat would be drawn off by conduction. Warm air would rise from around the ball, and, moving upwards, would distribute some of the heat by convection. And some of its rays, falling upon a mirror, or any other bright surface, might be diffused by reflection.
"I will teach you by the hand of God; that which is with the Almighty will I not conceal."—Job xxvii.
198. Do some substances absorb heat?
Yes; those substances which are the best radiators are also the best absorbers of heat.
199. Why does scratching a bright metal surface increase its power of radiation?
Because every irregularity of the surface acts as a point of radiation, or an outlet by which the heat escapes.
200. Why does a bright metal tea-pot produce better tea than a brown or black earthenware one?
Because bright metal radiates but little heat, therefore the water is kept hot much longer, and the strength of the tea is extracted by the heat.
201. But if the earthenware tea-pot were set by the fire, why would it then make the best tea?
Because the dark earthenware tea-pot is a good absorber of heat, and the heat it would absorb from the fire would more than counterbalance the loss by radiation.
202. How would the bright metal tea-pot answer if set upon the hob by the fire?
The bright metal tea-pot would probably absorb less heat than it would radiate. Therefore it would not answer so well, being set upon the hob, as the earthenware tea-pot.
203. Why should dish covers be plain in form, and have bright surfaces?
Because, being bright and smooth, they will not allow heat to escape by radiation.
204. Why should the bottoms and back parts of kettles and saucepans be allowed to remain black?
Because a thin coating of soot acts as a good absorber of heat, and overcomes the non-absorbing quality of the bright surface.
"And the foolish said unto the wise, Give us of your oil, for our lamps are gone out."
205. But why should soot be prevented from accumulating in flakes at the bottom and sides of kettles and saucepans?
Because, although soot is a good absorber of heat, it is a very bad conductor; an accumulation of it, therefore, would cause a waste of fuel, by retarding the effects of heat.
206. Why should the lids and fronts of kettles and saucepans be kept bright?
Because bright metal will not radiate heat; therefore, the heat which is taken up readily through the absorbing and conducting power of the bottom of the vessel, is kept in and economised by the non-radiating property of the bright top and front.
207. Does cold radiate as well as heat?
It was once thought that cold radiated as well as heat. But a mass of ice can only be said to radiate cold, by its radiating heat in less abundance than that which is emitted from other bodies surrounding it. It is, therefore, incorrect to speak of the radiation of cold.
CHAPTER XI.
208. Why, if you hold a piece of looking-glass at an angle towards the sum, will light fall upon an object opposite to the looking-glass?
Because the rays of the sun are reflected by the looking-glass.
209. Why, when we stand before a mirror, do we see our features therein?
Because the rays of light that fall upon us are reflected upon the bright surface of the mirror.
210. Why, if a plate of bright metal were held sideways before a fire, would heat fall upon an object opposite to the plate?
Because rays of heat may be reflected in the same manner as the rays of light.
"But the wise answered saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves."—Matt. xxv.
211. Why would not the same effect arise if the plate were of a black or dark substance?
Because black and dark substances are not good reflectors of heat.
212. What are the best reflectors of heat?
Smooth, light-coloured, and highly polished surfaces, especially those of metal.
213. Why does meat become cooked more thoroughly and quickly when a tin screen is placed before the fire?
Because the bright tin reflects the rays of heat back again to the meat.
214. Why is reflected heat less intense than the primary heat?
Because it is impossible to collect all the rays, and also because a portion of the caloric, imparting heat to the rays, is absorbed by the air, and by the various other bodies with which the rays come in contact.
215. Can heat be reflected in any great degree of intensity?
Yes; to such a degree that inflammable matters may be ignited by it. If a cannon ball be made red hot, and then be placed in an iron stand between two bright reflectors, inflammable materials, placed in a proper position to catch the reflected rays, will ignite from the heat.
There is a curious and an exceptional fact with reference to reflected heat, for which we confess that we are unable to give "The Reason Why." It is found that snow, which lies near the trunks of trees or the base of upright stones, melts before that which is at a distance from them, though the sun may shine equally upon both. If a blackened card is placed upon ice or snow under the sun's rays, the frozen body underneath it will be thawed before that which surrounds it. But if we reflect the sun's rays from a metal surface, the result is directly contrary—the exposed snow is the first to melt, leaving the card standing as upon a pyramid. Snow melts under heat which is reflected from the trees or stones while it withstands the effect of the direct solar rays. In passing through a cemetery this winter (1857), when the snow lay deep, we were struck with the circumstance that the snow in front of the head-stones facing the sun was completely dissolved, and, in nearly every instance, the space on which the snow had melted assumed a coffin-like shape. This forced itself so much upon our attention that we remained some time to endeavour to analyse the phenomena; and it was not until we remembered the curious effect of reflected heat that we could account for it. It is obvious that the rays falling from the upper part of the head-stone on to the foot of the grave would be less powerful than those that radiated from the centre of the stone to the centre of the grave. Hence it was that the heat dissolved at the foot of the grave only a narrow piece of snow, which widened towards the centre, and narrowed again as it approached the foot of the head-stone, where the lines of radiation would naturally decrease. Such a phenomena would prove sufficient to raise superstition in untutored minds.
"The light of the righteous rejoiceth, but the lamp of the wicked shall be put out."—Proverbs xiii.
216. Are good reflectors of heat also good absorbers?
No; for reflectors at once send back the heat which they receive, while absorbers retain it. It is obvious, therefore, that reflectors cannot be good absorbers.
217. How do fire-screens contribute to keep rooms cool?
Because they turn away from the persons in the room rays of heat which would otherwise make the warmth excessive.
218. Why are white and light articles of clothing cool?
Because they reflect the rays of heat.
White, as a colour, is also a bad absorber and conductor.
219. Why is the air often found excessively hot in chalk districts?
Because the soil reflects upon objects near to it the heat of the solar rays.
220. How does the heat of the sun's rays ultimately become diffused?
It is first absorbed by the earth. Generally speaking, the earth absorbs heat by day, and radiates it by night. In this way an equilibrium of temperature is maintained, which we should not otherwise have the advantage of.
221. Does not the air derive its heat directly from the sun's rays?
Only partially. It is estimated that the air absorbs only one-third of the caloric of the sun's rays—that is to say, that a ray of solar heat, entering our atmosphere at its most attenuated limit (a height supposed to be about fifty miles), would, in passing through the atmosphere to the earth, part with only one-third of its calorific element.
"As for the earth, out of it cometh bread; and under it is turned up as it were fire."—Job xxviii.
222. What becomes of the remaining two-thirds of the solar heat?
They are absorbed chiefly by the earth, the great medium of calorific absorption; but some portions are taken up by living things, both animal and vegetable. When the rays of heat strike upon the earth's surface, they are passed from particle to particle into the interior of the earth's crust. Other portions are distributed through the air and water by convection, and a third portion is thrown back into space by radiation. These latter phenomena will be duly explained as we proceed.
223. How do we know that heat is absorbed, and conducted into the internal earth?
It is found that there is a given depth beneath the surface of the globe at which an equal temperature prevails. The depth increases as we travel south or north from the equator, and corresponds with the shape of the earth's surface, sinking under the valleys, and rising under the hills.
224. Why may we not understand that this internal heat of the earth arises, as has been supposed by many philosophers, from internal combustion?
Because recent investigations have thrown considerable and satisfactory light upon the subject. It has been ascertained that the internal temperature of the earth increases to a certain depth, one degree in every fifty feet. But that below that depth the temperature begins to decline, and continues to do so with every increase of depth.
Yes. They both absorb and radiate heat, under varying circumstances. The majestic tree, the meek flower, the unpretending grass, all perform a part in the grand alchemy of nature.
"Consider the lilies of the field, how they grow; they toil not, neither do they spin."
When we gaze upon a rose it is not its beauty alone that should impress us: every moment of that flower's life is devoted to the fulfilment of its part in the grand scheme of the universe. It decomposes the rays of solar light, and sends the red rays only to our eyes. It absorbs or radiates heat, according to the temperature of the ærial mantle that wraps alike the flower and the man. It distills the gaseous vapours, and restores to man the vital air on which he lives. It takes into its own substance, and incorporates with its own frame, the carbon and the hydrogen of which man has no immediate need. It drinks the dew-drop or the rain-drop, and gives forth its sweet odour as a thanksgiving. And when it dies, it preaches eloquently to beauty, pointing to the end that is to come!
CHAPTER XII.
226. How do we know that plants operate upon the solar and atmospheric heat?
A delicate thermometer, placed among the leaves and petals of flowers, will at once establish the fact, not only that flowers and plants have a temperature differing from that of the external air, but that the temperature varies in different plants according to the hypothetical, or supposed requirements, of their existences and conditions.
227. What is the chief cause of variation in the temperature of flowers?
It is generally supposed that their temperature is affected by their colours.
228. Why is it supposed that the colour of a flower influences its temperature?
Because it is found by experiment that the colours of bodies bear an important relation to their properties respecting heat, and hold some analogy to the relation of colours to light.
If when the ground is covered with snow, pieces of woollen cloth, of equal size and thickness, and differing only in colour, are laid upon the surface of the snow, near to each other, it will be found that the relation of colour to temperature will be as follows:—In a few hours the black cloth will have dissolved so much of the snow beneath it, as to sink deep below the surface; the blue will have proved nearly as warm as the black; the brown will have dissolved less of the snow; the red less than the brown; and the white the least, or none at all. Similar experiments may be tried with reference to the condensation of dew, &c. And it will be uniformly found that the colour of a body materially affects its powers of absorption and of radiation.
"And yet I say unto you, that even Solomon, in all his glory, was not arrayed like one of these."—Matt. vi.
229. Why do we know that these effects are not the result of light?
Because they would occur, in just the same order, in the absence of light.
230. Why are dark coloured dresses usually worn in winter, and light in summer?
Because black absorbs heat, and therefore becomes warm; while light colours do not absorb heat in the same degree, and therefore they remain cool.
231. Why do iron articles, even when near fire, usually feel cool?
Because they are bad absorbers, and do not take up heat freely, unless they are in contact with a hot body.
232. How is heat diffused through the atmosphere?
By convection. The warmth radiating from the surface of the earth warms the air in contact with it; the air expands, and becoming lighter, flies upwards, bearing with it the caloric which it holds, and diffusing it in its course.
233. How do the waters of the ocean become heated?
Chiefly by convection. Nearly all the heat which the sun sheds upon the ocean is borne away from its surface by evaporation, or is radiated back into the atmosphere. But the ocean gathers its heat by convection from the earth. It girdles the shores of tropical lands where, being warmed to a high degree of temperature, it sets across the Atlantic from the Gulf of Mexico, and exercises an important influence upon the temperature of our latitude.
234. What is the cause of winds?
Currents of air, and winds, are the result of convection. The air, heated by the high temperature of the tropics, ascends, while the colder air of the temperate and the frigid zones blows towards the equator to supply its place.
"Give unto the Lord the glory due unto his name; worship the Lord in the beauty of holiness."—Psalm xxix.
235. What is the cause of sea breezes?
Sea breezes are also the result of convection. The land, under the heat of the day's sunshine, becomes of a high temperature, and the expanded air on its surface flies away towards the ocean. As the sun goes down, the earth cools again, and the air flies back to find its equilibrium.
Many countries by the sea are subjected to these periodical breezes, known as either "land" or "sea breezes," according to their direction. About eight o'clock in the morning an ærial current begins to flow from the sea towards the land, and continues until about three o'clock in the day; then the current takes a reverse direction, flowing from the land to the sea. This it continues to do throughout the night, until the time of sunrise, when a temporary calm ensues.
236. Why does a soap bubble ascend in the air?
Because, being filled with warm air, it is lighter than the surrounding medium, and therefore ascends.
237. Why does the bubble fall after it has been in the air some time?
Because the air contained in it has become cool, and, as it contains carbonic acid gas, it is heavier than the air.
238. What became of the warmth at first contained in the bubble?
It has been distributed in the air through which the bubble passed.
239. What does this simple illustration of the distribution of warmth explain?
It explains the law of convection, or heat distribution, over the surface of the globe.
240. Why does air ascend the chimney?
Because, being heated, it becomes lighter than the surrounding medium, and therefore flies upwards, through the outlet provided for it.
241. Why does air fly from the doors and windows towards the fire-place?
Because, as the warm air flies away, cold air rushes in to occupy its place.
"How much better is it to get wisdom than gold? and to get understanding rather to be chosen than silver."—Proverbs xvi.
242. What does this example of the motion of the air in our rooms explain?
It explains the movement of volumes of air by convection, and illustrates the origin of breezes and winds.
243. What is the chief effect of this law of convection?
Under its influence air and water are the great equalisers of solar heat, rendering the earth agreeable to living things, and suited to the laws of their existence.
Owing, also, to this law of convection, the constituents of the air are equalised. The breath of life, supplied by the purer oxygen of the "sunny south," is diffused in salubrious gales over the wintry climes of the north. And the waters, evaporated from the bosom of the central Atlantic Ocean and the Pacific, are borne across vast continents, and poured down in fertilising showers upon distant lands.
To the educated mind, nothing is too simple to merit attention. To the ignorant, few things are sufficiently attractive to excite curiosity. Knowledge enables us to estimate the varied phenomena that are hourly arising around us, and to see, even in the most trifling effects, illustrations of those great causes and consequences that govern with mighty power the material world. Man, sitting by his fire-side, is enabled to witness the operation of some of nature's grandest laws: light and heat are around him; conduction, radiation, reflection, absorption, and convection of heat are all going on before him; little winds are sweeping by his footstool, and warm currents, with miniature clouds folded in their arms, are passing upward before his view. Chemical changes are going on; the solid rock of coal disappears, flying away as an invisible gas. The little "hills are melted," and hard stones have been converted into "fervent heat." Although some of these changes are imperceptible to the eye, they are manifest to the educated mind; and the pleasures of philosophical observation are as sweet as a poet's dreams.
CHAPTER XIII.
"Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house."—Matt. v.
244. Why will a piece of paper, held three or four inches over the flame of a candle, become scorched?
Because the hot air and gas produced by the burning of the candle ascends rapidly.
245. Why will a piece of paper held about an inch below the flame of a candle scarcely become warmed?
Because the heat ascends; and only a little of it falls upon the paper, and that by radiation.
Fig. 2.—DIAGRAM SHOWING THE COMBUSTION OF A CANDLE.
246. Why does the lower part of the flame of a candle (D) burn of a blue colour?
Because the hydrogen of the tallow, having a stronger affinity for the oxygen of the air than carbon has, ignites first. Pure hydrogen burns with a bluish flame.
247. Why does the middle of the flame (C) look dark?
Because it is occupied with gaseous vapours, derived from the tallow, which have not yet ignited.
248. Why does the upper part of the flame (B) produce a bright yellow light?
Because it is in this part of the flame that the hydrogen of the candle, and the oxygen of the air, combine, and there is just sufficient carbon mixed with the hydrogen to improve its illuminating power.
249. Why is there a fringe of pale light (A) around the upper part of the flame?
Because some of the carbon escapes in a state of incandesence, and as soon as it reaches the air it combines with oxygen, and so forms carbonic acid gas.
If any dark body, such as the blade of a knife, be held between the eye and the flame of the candle, so as to shut off the light of the more luminous part, the pale fringe around the flame will be found distinctly perceptible. Incandesence means heated to whiteness.
"How oft is the candle of the wicked put out? and how oft cometh their destruction upon them?"—Job xxi.
250. Why does the flame terminate in a point?
Because cold air rushes towards the flame in every direction, and is carried upward. At the point where the flame terminates the cold currents have so reduced the temperature that combustion can no longer be sustained.
251. Why, if you hold anything immediately over the flame, will the flame lengthen?
Because, by preventing the rapid escape of the heated air, you maintain a temperature which increases the combustion at the point of the flame.
252. Why should persons whose clothes take fire, throw themselves down?
Because flame spreads most rapidly in an upward direction.
253. Why should persons whose clothes are on fire roll slowly about when they are down?
Because they thereby press out the fire.
254. Why does pressing a flame or a spark put it out?
Because it prevents the contact of the flame or spark with the oxygen of the air.
Extinguishers put out the flame of candles in the same manner. A person dies from "suffocation" through the absence of oxygen; and it is literally practicable to "suffocate" a fire.
255. Why does the wick turn black as it burns?
Because it consists principally of carbon.
256. Why, when the point of the wick turns out and meets the air, does it exhibit a bright spark?
Because the carbon of the wick comes into immediate contact with the oxygen of the air.
257. Why does holding a candle "upside down" put it out?
Because the melted grease runs down too rapidly, and at too low a temperature to undergo combustion. It therefore reduces the heat, and extinguishes the flame.
"Lord, what is man that thou takest knowledge of him! or the son of man, that thou makest account of him."—Psalms cxliv.
258. Why is it more difficult to blow out the flame of a candle with a cotton wick than one with a rush wick?
Because the cotton wick imbibes more of the combustible materials, and holds in its loose texture the inflammable gases in a state ready for combustion.
259. Why does blowing sharply at a candle flame put it out?
Because the breath drives away the vapour of the grease which, becoming gaseous, supports the flame.
And because too rapid a flow of cold air reduces the temperature below the point at which combustion can be maintained.
260. Why will a gentle puff of breath, if given speedily after the flame is extinguished, rekindle it?
Because the oxygen of the air combines with the carbon and hydrogen that are still escaping from the heated wick, and re-lights it.
261. Why will not a similar puff rekindle the flame of a rushlight?
Because its wick retains but little heat, and holds a comparatively small amount of combustible matter in a volatile state.
262. Why is a fire, when it is very low, sometimes put out by blowing it?
Because the too rapid flow of cold air reduces the temperature of the burning mass.
263. Why will a piece of paper twisted like an extinguisher put out a candle?
Because, before the flame of the candle can ignite the paper, the oxygen contained within it is consumed, and the flame is suffocated.
"When his candle shined upon my head, and when by his light I walked through darkness."—Job xxix.
264. Why do tallow candles require snuffing?
Because the oxygen of the air cannot reach the wick through the body of flame—therefore the unconsumed carbon accumulates upon the wick.
265. Why do composite and wax candles not require snuffing?
Because their wicks are made by a series of plaits, by which they are bent to meet the oxygen of the air, and consumed.
266. Why does setting a glass upon a lamp increase its brilliancy, though it shortens the flame?
Because it conducts an increase of air to the flame, and the greater supply of oxygen causes the escaping vapour of oil to be all rapidly consumed.
267. Why does a candle burn dimly when the wick has become loaded with carbon?
Because the carbon radiates the heat, and disperses it, and reduces the heat of the flame below that temperature which is essential to its luminosity.
268. What differences characterise the combustion of carbon and of hydrogen?
The combustion of carbon takes place without the production of flame. The charcoal (or carbon in any other form) being heated to redness, enters directly into combination with the oxygen of the surrounding air, and the carbonic acid gas, being invisible, passes away unobserved.
But in the combustion of hydrogen the heat developed is so intense as to render the gas itself luminous, just as iron may be heated to a red or white heat.
269. What has become of the candle when it has been burnt?
It has been resolved partly into carbonic acid gas which, though unperceived, has diffused itself through the surrounding air; and partly into water, which escaped in the form of thin vapour.
270. Has any part of the candle been consumed or lost?
No; there is no such thing as "loss" in the operations of nature. Every particle of the candle, now invisible, exists either in the form of gas, vapour, or water, with, perhaps, a few solid particles that may be called ashes, but which are too minute to excite attention.
"I know that whatsoever God doeth, it shall be for ever: nothing can be put to it, nor anything taken from it; and God doeth it that men should fear before him."—Eccles. iii.
The economy of nature should teach us a very impressive lesson—nothing is suffered to be wasted, not even the slightest atom. As soon as any body has fulfilled its purpose in one state of being, it is passed on to another. The candle, existing no longer as a candle, is flying upon the wings of the air as carbonic acid gas, and as water. These probably find their way to the garden or the field, where the carbonic acid gas forms the food of the plant, and the water affords it a refreshing drink. And can it be supposed that the Almighty Being, who has thus economised the existence of the material creation, should be less mindful of the immaterial soul of man? There is an eternity before us, the certainty of which is evidenced even by the laws of the material creation.
CHAPTER XIV.
Coal is a "vegetable fossil."
272. What is meant by a vegetable fossil?
It is a substance originally vegetable, which, by pressure and other agencies within the earth, has been brought to a condition approaching that of mineral or earthy matter.
273. Why do we know that coal is of vegetable origin?
By the chemical components of its substance; and also by the vegetable forms that are found abundantly in coal beds.
Professor Buckland, in his Bridgewater Treatise, speaking of the impressions of plants found in the coal mines, says; "The finest example I have ever witnessed is that of the coal mines of Bohemia. The most elaborate imitations of living foliage upon the painted ceilings of Italian palaces bear no comparison with the beauteous profusion of extinct vegetable forms with which the galleries of these instructive coal mines are overhung. The roof is covered as with a canopy of gorgeous tapestry, enriched with festoons of most graceful foliage, flung in wild irregular profusion over every part of its surface. The effect is heightened by the contrast of the coal-black colour of these vegetables with the light ground-work of the rock to which they are attached. The spectator feels himself transported, as if by enchantment, into the forests of another world; he beholds trees, of forms and characters now unknown upon the surface of the earth, presented to his senses almost in the beauty and vigour of their primeval life; their scaly stems and bending branches, with their delicate apparatus of foliage, are all spread forth before him, little impaired by the lapse of countless ages, and bearing faithful records of extinct systems of vegetation which began and terminated in times of which these relics are the infallible historians."
"Surely every man walketh in a vain show; surely they are disquieted in vain: he heapeth up riches, and knoweth not who shall gather them."—Ps. xxxix.
274. What are the chemical components of coal?
They consist of carbon, hydrogen, oxygen, and nitrogen. The proportions of these elements vary in different kinds of coal. Carbon is the chief component; and the proportions may be stated to be, generally, carbon, 90 per cent.; hydrogen, from 3 to 6 per cent.; the other elements enter into the compound in such small proportions, that, for all ordinary purposes, it is sufficient to say that coal consists of carbon and hydrogen, but chiefly of carbon.
Charcoal consists almost entirely of carbon. It is made from wood by the application of heat, without the admission of air. The hydrogen and oxygen of the wood are expelled, and that which remains is charcoal, or carbon in one of its purest states.
Animal charcoal, like vegetable charcoal, consists of carbon in a state approaching purity. It is made from the bones of animals, heated in iron cylinders. It is commonly called ivory black.
277. What is the purest form of carbon known?
The purest form of carbon is the diamond, which may be said to be absolutely pure.
Hence we derive another of the beautiful lessons of science—a lesson which teaches us to despise nothing that God has given. The soot which blackens the face of a chimney-sweep, and the diamond that glistens in the crown of the monarch, consist of the same element in merely a different atomic condition. What a lesson of humility this teaches to Pride! The haughty beauty as she walks the ball-room, inwardly proud of the radiance of her gems as they rise and fall upon her breast, little thinks or knows that every breath that is expired around her wafts away the like element of which her treasures are composed. That even in our own flesh and bones the same abounding substance lies hid; and that the buried tree of the primitive world, and the little flower of to-day, are both the instruments of giving this singular element to man!
Coke is coal, divested of its hydrogen and other volatile parts, by a similar process to that by which charcoal is produced. It forms the residue after hydrogen gas has been made from coals. It consists almost entirely of carbon.
"Oh that men would praise the Lord for his goodness, and for his wonderful works to the children of men."—Psalm cvii.
279. Why do burning coals produce yellow flame?
Because the hydrogen which they contain is combined with some proportion of carbon, which imparts a bright yellow colour to the flames.
280. Why do some of the flames of a fire appear much whiter than others?
Because the quality of coals, and the conditions under which they are burnt, are liable to variation. Some coals yield a heavy hydrogen, called bi-carburetted hydrogen, which burns with a much brighter flame than carburetted hydrogen.
281. Why does bi-carburetted hydrogen burn with a whiter flame than the common coal gas?
Because it is combined with a larger proportion of carbon, to which it owes its increased luminosity.
282. Why do some of the flames of a fire appear blue?
Because the hydrogen which is escaping where those flames occur is pure hydrogen, destitute of carbon.
283. Why does the fire sometimes appear red, and without flame?
Because the volatile gases have been driven off and consumed, and combustion is continued by the carbon of the coals and the oxygen of the air.
284. What effect has the burning of a fire upon the composition of the air?
It is found that in burning 10lb. of coal the oxygen contained in 1,551 cubic feet of air is altogether absorbed. It is therefore necessary to keep the atmosphere of a room, in which a coal fire is burning, fresh and pure, to supply 155 cubic feet of fresh air for every pound of coal that is consumed.
"O Lord how manifold are thy works, in wisdom hast thou made them all: the earth is full of thy riches."--Psalm civ.
285. Why does wood which is "green" hiss and steam when it is burnt?
Because it contains a large amount of water, which must be evaporated before combustion can proceed.
286. What is the effect of this evaporation?
A great deal of heat is unprofitably expended in driving off the water of the fuel.
287. Why does poking a fire cause it to burn more brightly?
Because it opens avenues through which the air may enter to supply oxygen.
288. Why do "blowers" improve the draft of air through a fire?
Because, by obstructing the passage of the current of air over the fire, they cause additional air to pass through it, and therefore a greater amount of oxygen is carried to the coals.
Unconsumed particles of coal, rendered volatile by heat, and driven off.
Carbon in minute particles, driven off with other volatile matters and deposited on the walls of chimneys.
291. Why do fresh coals increase the quantity of smoke?
Because they contain volatile matters which are easily driven off; and because, also, they reduce momentarily the heat, so that those matters that first escape cannot be consumed.
292. Why do charcoal and coke fires burn clearly and without flame?
Because the hydrogen has been previously driven off from those substances.
293. Why is it difficult to light charcoal and coke fires?
Because they contain no hydrogen to produce flame, and assist combustion.
"He hath made his wonderful works to be remembered: the Lord is precious and full of compassion."—Psalm cxl.
A new plan of kindling fires has lately been recommended. Coals are to be laid in the bottom of the fire-place to a considerable depth, then the paper and wood are to be laid on, and then a little coals and cinders over them. This plan of "laying in" the fire is precisely the reverse of that which has been pursued for many years. The theory is, that when the coals in the bottom are ignited, a more even combustion is kept up, whilst the smoke and gas which would otherwise escape, and become as so much waste fuel, is burnt up, and produces heat. We have heard the plan strongly recommended by persons who have tried it, and who testify to the great economy of fuel to which it conduces.
CHAPTER XV.
294. Why does paper ignite more readily than wood?
Because its texture is less dense than that of wood; its particles are therefore more readily heated and decomposed.
295. But if articles of loose texture are bad conductors of heat, why do they so easily ignite?
The fact that they are bad conductors assists their ignition. The heat which would pass from particle to particle of the dense substance of iron, and be conducted away, accumulates in the interspaces of paper, and ignites it.
296. Why does wood ignite less readily than paper?
Because its substance is denser than that of paper; it therefore requires a higher degree of heat to inflame its substance.
297. Why does wood, when ignited, burn longer than paper?
Because, being a denser substance, it submits a larger number of particles, within a given space, to the action of the heat, and the formation of gases.
298. Why do we, in lighting a fire, first lay in paper, then wood, and lastly coals?
Because the paper is more easily ignited than wood, and wood than coals; therefore the paper assists the ignition of the wood, and the wood assists the ignition of the coals.
"It is a good thing to give thanks unto the Lord, and to sing praises unto thy name, O Most High."—Psalm xcii.
299. Why will not wood ignite by the flame of a match?
It will do so, unless there is a great disproportion between the size of the wood and the flame of a match. A thin piece of wood will ignite, but a square block will not, because the heat of the flame is insufficient to raise the temperature of a large surface to the point that will drive out its gases.
300. Why do we place the paper under the wood, and the wood under the coals?
Because heat and flame, when surrounded by air, have a strong tendency to spread themselves upwards.
301. Would it be possible to light the coals by putting the paper and the wood upon the top?
It would be possible; but the loss of heat would be so great, that a much larger quantity of paper and wood would be required.
302. Why does a poker laid across the top of a dull fire revive it?
Because the poker radiates the heat it receives from the fire downward upon the fuel.
Because, also, it divides the ascending air, and thereby creates currents.
The amount of good which the poker does to the fire is very slight indeed. Generally, the housewife stirs the fire first, and blows or brushes away the ashes that prevent the influx of air. She then places the poker upon the top, and the popular mind supposes that the poker "draws" the fire. The custom of placing a poker over the fire is of very remote antiquity. It was once believed that forming a cross, by placing the poker over the bars, protected the fire from the hostility of malignant witches!
303. Why should fire-places be fixed as low as possible in rooms?
Because heat ascends, and when the fire-places are high the lower parts of the room are inadequately warmed. Also, as currents of air fly towards the fire, elevated fire-places cause drafts about the persons of the inmates to a much greater extent than they would if they were lower down.
"Unto thee, O God, do we give thanks: for that thy name is near thy wondrous works declare."—Psalm lxxv.
304. Why, if a piece of paper be laid with its flat surface upon the fire, will it "char," but not ignite?
Because, as in the case of the proper candle-extinguisher, the carbonic acid gas accumulating beneath it prevents its igniting.
305. Why, if you direct a current of air towards the paper, will it burst into a blaze?
Because the carbonic acid gas is displaced by a current of air containing oxygen.
306. Why does water extinguish fire?
Because it saturates the fuel, and prevents the gases thereof from combining with the oxygen of the air.
307. As water contains oxygen, why does not the oxygen of the water support the fire?
Because the affinity between the hydrogen and oxygen of the water is so strong that fire cannot separate them.
Water may be decomposed by heat, as will be hereafter explained. But the heat of an ordinary fire is insufficient. There is, however, some reason for believing that, in cases of very large fires, such as the accidental burning of houses, &c., when the supply of water thrown upon the fire is very deficient, the water does become decomposed, and add to the fury of the flames.
308. Why does the blacksmith sprinkle water upon the coals of his forge?
The blacksmith uses small coals because the small pieces thereof are more easily ignited than large lumps would be, and they convey heat better by completely surrounding the articles put into the fire. He sprinkles water on the coal dust to hold its particles together by cohesion, until the heat forms it into a cake. A strong blast of hot hair drives the vapour of the water away, and leaves a porous mass to the action of the fire.
309. Why, when the blacksmith thrusts a heated iron into a tankard of water, do we recognise a peculiar smell?
Because the intense heat disengages a small volume of the gases of which water is formed.
"Oh the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out."—Rom. xi.
310. Which gas do we (in this instance) recognise by the smell?
The hydrogen gas. Oxygen gas possesses no odour.
311. What is Spontaneous Combustion?
Spontaneous combustion is that which occurs in various bodies when they become highly heated by chemical changes.
312. Why is heat developed during chemical changes?
Because, as all bodies contain latent caloric, the disturbance of the atoms of which those bodies are composed, during the new combinations that constitute chemical changes, frequently sets the caloric free, and an accumulation of caloric produces spontaneous combustion.
313. Does a match ignite spontaneously when drawn over a rough surface?
No. Because in this case the combustion arises from heat applied by friction.
314. Does phosphorous ignite spontaneously when held in a warm hand?
Phosphorous will ignite when held in a warm hand, but it does not then produce spontaneous combustion, because it ignites through the agency of applied heat.
315. But if a piece of dry phosphorous be sprinkled with powdered charcoal it will ignite, without the application of heat. Why is this?
Because the carbon (charcoal) absorbs oxygen from the air, and conveys it to the phosphorous. Here are chemical changes which develope heat, and produce spontaneous combustion.
316. Why do hay-stacks sometimes take fire?
Because the hay, having become damp, decays, and passes on to a state of fermentation, in which chemical changes occur, during which heat is evolved. Hay, taking fire under these circumstances, would exhibit spontaneous combustion.
"Who hath woe? who hath sorrow? who hath contentions? who hath babbling? who hath words without cause? who hath redness of the eyes? * * * They that tarry long at the wine."—Prov. xxiii.
317. What substances are liable to produce spontaneous combustion?
All substances which contain sugar, starch, and other components liable to fermentation. All bodies that evolve, under low degrees of temperature, inflammable gases. And all organic bodies undergoing decay.
Grain, cotton, hemp, flax, coals, oily and greasy substances.
318. What is the Ignis Fatuus (sometimes called "Will-o'-the-Wisp", "Corpse Candles," and "Jack-o'-Lantern")?
It is a flame produced by spontaneous combustion, caused by the decay of animal or vegetable bodies, which evolve phosphoretted hydrogen gas, under circumstances attended by a low degree of heat, sufficient to ignite the gases. It is mostly seen over marshy places, and burial-grounds.
Many a "Ghost Story" has owed its origin to these singular but harmless appearances. People, ignorant of the cause, have been terrified at the effect. To the fancy of an affrighted mortal, the simple flame of the Ignis Fatuus has assumed the form of a departed friend, and even found a supernatural voice. If, excited by a momentary daring, the beholder moved towards the light upon which he gazed, it fled from him. If he turned from it and walked away, it followed him, step by step. The darkness of a lonely road, or the sacred solitude of a burial-place, have been sufficient accessories to authenticate the appearance of a spirit. And yet how simple the phenomenon? Matters so volatile as those which produce the Ignis Fatuus would naturally be driven back by the motion in the air caused by an advancing body; and, on the other hand, a body moving from them would create a current in which the Ignis Fatuus would follow. Poisonous gases, escaping from decaying bodies, pass into the air and take fire. They are thereby converted into harmless compounds. Thus we see that the "ghost" which terrifies the mind of the ignorant, becomes a "guardian angel" to the educated.
319. Has spontaneous combustion ever occurred in living bodies?
It has occurred in numerous instances to persons habituated to the excessive use of spirits.
320. Why should spontaneous combustion occur in the case of the drunkard?
Because spirituous drinks contain a large proportion of ALCOHOL, one of the constituents of which is hydrogen. The vital energies of the drunkard, being destroyed by excess, chemical agencies obtain an ascendancy, and it is supposed that the hydrogen of the alcohol combines with the phosphorous of the body to form phosphoretted hydrogen, which ignites spontaneously, and literally consumes the living temple.
"Drought and heat consume the snow waters; so doth the grave those which have sinned."—Job xxiv.
Cases of spontaneous combustion are of rare occurrence. But they are sufficiently well authenticated by high medical authority, in many parts of the world, to present an awful warning to the inveterate drunkard. The cases of which we have read the particulars present details of the most appalling description. How signally the Almighty displeasure at intemperance is expressed, when the very drink which imparts the mad pleasure of intoxication is made the direct instrument by which the drunkard is destroyed!
CHAPTER XVI.
321. Why does friction produce heat?
Because all bodies contain latent heat, that is, heat that lies hid in their substance, and the rubbings of two bodies against each other draws the latent heat to the excited surfaces.
322. Why does the rubbing of two surfaces together attract latent heat to those surfaces?
Because it is a law of nature that heat shall always attend motion; and it is generally found that the intensity of heat bears a specific relation to the velocity of motion.
323. What are the sources of heat?
The rays of the sun, the currents of electricity, the action of chemicals, and the motion of substances.
Because its latent heat is partly drawn off by the surrounding air.
Because the heat, once latent in the water, but drawn off by the air, has returned to it, and restored the water to its former condition.
"So teach us to number our days, that we may apply our hearts unto wisdom." Psalm xc.
326. Why does water become steam?
Because a larger amount of heat has entered into it than can remain latent in water. The water therefore expands and rises in the form of vapour, or water attenuated by heat.
327. How many degrees of heat are latent, or hidden, in the different states of water?
In thawing ice, 140 deg. of caloric become latent; and in converting the water into steam, 1,000 deg. more of caloric are be taken up. Therefore, ice requires to take up 1,140 deg. of latent caloric before it becomes steam.
328. What is the most modern theory of heat?
It is this—that caloric, which produces heat, is an extremely subtile fluid, of so refined a nature that it possesses no weight, yet is capable of diffusing itself among the particles of the most solid bodies.
It is also believed that—all bodies are subject to the action of two opposing forces: one, the mutual attraction of their particles; the other, the repulsive force of caloric—and that bodies exist in the æriform, fluid, or solid state, according to the predominance of either the one or the other of these opposing forces.
329. How do we measure the quantity of caloric in any substance?
It is impossible to determine the amount of caloric which any body contains. Our sensations would obviously be deceptive, since, if we dipped the right hand in snow, and held the left hand before the fire, and then immersed both hands in cold water, the water would feel warm to the right hand and cold to the left hand.
But, as caloric uniformly expands substances that are under its influence, one of the bodies most sensitive to calorific effects has been selected to be the indicator of the amount of caloric. This substance is quicksilver; and the scale of measurement, and the apparatus for exhibiting the rise or fall of the quicksilver, constitute the thermometer.
330. If it is impossible to measure the amount of caloric in any substance, how can it be said that ice absorbs 140. deg. in becoming water?
Those figures simply record the amount of calorie indicated by the thermometer. The instrument will show with sufficient accuracy the relative amount of caloric in various bodies, or in the same bodies under different circumstances, but it can never determine the precise amount of caloric in any one body.
"Great is the Lord, and greatly to be praised in the city of our God, in the mountain of his holiness."—Psalm xlviii.
331. Why, if a hot and a cold body were placed near to each other, would the cold one become warmer, and the hot one cooler?
Because free caloric (that is, caloric that is not latent,) always exhibits a tendency to establish an equilibrium. If twenty bodies, of different temperatures, were placed in the same atmosphere, they would all soon arrive at the same temperature. The caloric would leave the bodies of those of the highest, and find its way to those of the lowest temperature.
It travels in parallel rays in all directions with a velocity approximating to that of light; and it passes through various bodies with a rapidity proportionate to their power of conduction.
333. Why does melted metal run like a stream of fluid?
Because caloric has passed into its substance, and, repelling its particles, has separated them to that degree which produces fluidity.
334. How do we know that it is caloric passing into the substance of the metal which produces this effect?
Because, as soon as a bar of metal begins to be heated, it expands and lengthens. It continues to do so, until the heat arrives at that point which causes the metal to melt.
335. Why does the iron of an ironing-box sometimes become too large for the box to receive it?
Because caloric has passed into the substance of the iron, and repelled its particles, by which it has become expanded.
336. Why does the iron enter the box when it has become partially cooled?
Because a portion of the caloric has left the iron, the particles of which have drawn closer together, and contracted the mass.
"Cast thy burden upon the Lord, and he shall sustain thee; he shall never suffer the righteous to be moved."—Psalm lv.
This effect is frequently observed by females in domestic life, who, when they are ironing, or using the Italian irons, find that the heated metal has been too much expanded to enter the box or tube. They find it necessary to wait until the cooling of the iron has had the effect of reducing its dimensions. The expansion of bodies by heat is one of the grandest and most important laws of nature. We are indebted to it for some of the most beautiful, as well as the most awful, phenomena. And science has gained some of its mightiest conquests through its aid. Yet frequently, though quite unthought of, in the hands of the humble laundress, will be found a most striking illustration of this wonderful force of caloric.
337. Are there any instances in which the abstraction of latent heat will reduce the hulk of bodies?
Yes, there are several. But the most familiar one is that which is exhibited by mixing a pint of the oil of vitriol with a pint of water. A considerable amount of heat will be evolved; and it will be found that the two pints of fluid will not afterwards fill a quart measure.
338. Is there any latent heat in air?
Yes: a considerable amount. In a pint measure of air, though in no way evident to our perceptions, there lurks sufficient caloric to raise a piece of metal several inches square to glowing redness.
339. How do we know that caloric exists in the air?
It has been positively demonstrated by the invention of a small condensing syringe, by which, through the rapid compression of a small volume of air, a spark is emitted which ignites a piece of prepared tinder.
340. What is the cause of the spark when a horse's shoe strikes against a stone?
The latent heat of the iron or the stone is set free by the violent percussion. The same effect takes place when flint strikes against steel, as in the old method of obtaining a light with the aid of the tinder-box.
"The waters are laid as with a stone, and the face of the deep is frozen."—Job xxxviii.
What an eloquent lecture might be delivered upon the old-fashioned tinder-box, illustrated by the one experiment of "striking a light." In that box lie, cold and motionless, the Flint and Steel, rude in form and crude in substance. And yet, within the breast of each, there lies a spark of that grand element which influences every atom of the universe; a spark which could invoke the fierce agents of destruction to wrap their blasting flames around a stately forest, or a crowded city, and sweep it from the face of the world; or which might kindle the genial blaze upon the homely hearth, and shed a radiant glow upon a group of smiling faces; a spark such as that which rises with the curling smoke from the village blackmith's forge—or that which leaps with terrific wrath from the troubled breast of a Vesuvius. And then the tinder—the cotton—the carbon: What a tale might be told of the cotton-field where it grew, of the black slave who plucked it, of the white toiler who spun it into a garment, and of the village beauty who wore it—until, faded and despised, it was cast amongst a heap of old rags, and finally found its way to the tinder-box. Then the Tinder might tell of its hopes; how, though now a blackened mass, soiling everything that touched it, it would soon be wedded to one of the great ministers of nature, and fly away on transparent wings, until, resting upon some Alpine tree, it would make its home among the green leaves, and for a while live in freshness and beauty, looking down upon the peaceful vale. Then the Steel might tell its story, how for centuries it lay in the deep caverns of the earth, until man, with his unquiet spirit, dug down to the dark depths and dragged it forth, saying, "No longer be at peace." Then would come tales of the fiery furnace, what Fire had done for Steel, and what Steel had done for Fire. And then the Flint might tell of the time when the weather-bound mariners, lighting their fires upon the Syrian shore, melted silicious stones into gems of glass, and thus led the way to the discovery of the transparent pane that gives a crystal inlet to the light of our homes; of the mirror in whose face the lady contemplates her charms; of the microscope and the telescope by which the invisible are brought to sight, and the distant drawn near; of the prism by which Newton analysed the rays of light; and of the photographic camera in which the sun prints with his own rays the pictures of his own adorning. And then both Flint and Steel might relate their adventures in the battle-field, whither they had gone together; and of fights they had seen in which man struck down his fellow-man, and like a fiend had revelled in his brother's blood. Thus, even from the cold hearts of flint and steel, man might learn a lesson which should make him blush at the "glory of war;" and the proud, who despise the teachings of small things, might learn to appreciate the truths that are linked to the story of a "tinder-box."
CHAPTER XVII.
341. Since all bodies expand by heat and contract by cold, why does water, when it reaches the freezing point, expand?
Because, in freezing, water undergoes crystallization, in which its particles assume a new arrangement occupying greater space.
342. Why does water never freeze to a great depth?
Because the covering of ice which is formed upon the surface of the water prevents the cold air from continuing to draw off the caloric of the water.
"For he saith to the snow, Be thou on the earth; likewise to the small rain, and to the great rain of his strength."—Job xxxvii.
343. Why has this exceptional law of the expansion of water, when freezing, been ordained?
Because, but for this, deep waters might be frozen through their whole depth. This would destroy the myriads of fish and other living things that inhabit the water. Parts of the earth, now clad in verdure, would be lost in eternal winter; and even in the most temperate zones it would take months to effect a thaw; and thawing would be attended with such floods and subterranean commotion as are terrible to contemplate.
344. Why are bed-room windows sometimes covered with crystalline forms on winter mornings?
Because the vapour of the breaths of the inmates has condensed upon the window-panes, and formed water. The water has frozen with the cold, and exhibits the beautiful crystalline forms into which its particles are arranged.
Here we have another domestic illustration of the great laws of nature. It is the same law which locks the arctic regions in ice and decorates our window-panes. This beautiful phenomenon is usually witnessed by us on frosty mornings when we rise from our beds. It has a story which the observer of nature may read in its sparkling eyes. It tells that, although without the air is biting cold, God has wrapped a mantle around the face of nature to keep it from injury; and that the earth and the waters, though looking chilled and dead, have still the warmth of life preserved in their bosoms.
Dew is watery vapour diffused in the air, condensed by coming in contact with bodies colder than the atmosphere.
346. Why does the air become charged with watery vapour?
Because, during the day, under the influence of the sun's rays, vapours are exhaled from all the moist and watery surfaces of the earth. These vapours are held in suspension in the atmosphere until, by a change in the temperature of the earth, and of bodies on the surface of the earth, they are condensed, and deposited in translucid drops.
347. What causes the decline of temperature that favours the deposition of dew?
The earth, which during the day received heat from the solar rays, radiates the heat back into the air, and therefore becomes itself colder. All the various objects upon the face of the earth also radiate heat in a greater or lesser degree. And dew will be found to be deposited upon the surfaces of such bodies in proportion to the fall of their temperature through radiation.
"The Lord is my shepherd, I shall not want. He maketh me to lie down in green pastures."—Psalm xxiii.
348. Why is there little or no dew when the nights are cloudy?
Because clouds act as secondary radiators; and when the earth radiates its heat towards the clouds, the clouds again radiate it back to the earth.
Fig. 3.—ILLUSTRATING THE FORMATION OF DEW.
If plates of glass be laid over grass-beds, as in the engraving [Fig. 3], no dew will be deposited on the grass underneath the glass plates, although all around the grass will be completely wetted. The explanation is that the glasses, being radiators of heat, act in the same manner as the clouds, returning the heat to the bodies underneath them, and preventing the formation of dew thereon.
![[Fig. 3]](#i-087.jpg){kind=link}
349. Why does dew form most abundantly on cloudless nights?
Because the heat which is radiated by the earth does not return to it. The temperature of the earth, and the air immediately upon its surface, is therefore lowered, and dew is formed.
It has been observed that sheep that have lain on the grass during the formation of dew have their backs completely saturated with it, but that underneath the line where their bodies turn to the earth, their coats will be dry. In the same manner glass globes suspended in the air, on dew forming nights, will be found loaded with globules of dew upon the top, but there will be no appearance of moisture underneath.
"Dost thou know the balancings of the clouds, the wondrous works of him which is perfect in knowledge."—Job xxxvii.
350. Why are star-lit nights usually colder than cloudy nights?
Because heat is radiated from the earth, and passes away into the utmost regions of the atmosphere.
351. Why is there little dew under branches of thick foliage?
Because the foliage acts as a screen, which prevents the radiated heat of the earth from passing away.
352. Why is there no dew formed on windy nights?
Because, as winds generally consist of dry air, they absorb and bear away the atmospheric moisture.
353. Why are valleys and low places chiefly subject to dew?
Because the elevated lands around them prevent the disturbance of the air in which the moisture is held.
354. What bodies are most likely to be covered with dew?
All bodies that are good radiators of heat, such as wool, swansdown, grass, leaves of plants, wood, &c.
355. What bodies are likely to receive little dew?
All bad radiators of heat, such as polished metal surfaces, smooth stones, and polished surfaces generally. Dew will be found to lie more abundantly upon rough and woolly leaves than upon smooth ones.
356. At what period of the night is the largest amount of dew usually formed?
It is generally supposed that dew is formed most copiously in the mornings and evenings. But such is not the case. It is deposited at all hours of the night, but most plentifully after midnight.
357. Why is dew formed most plentifully after midnight?
Because, as radiation has been going on for some time, the temperature of the earth, and of various bodies upon it, has been considerably reduced.
"Out of whose womb came the ice? and the hoary frost of heaven, who hath gendered it?"—Job xxxviii.
358. In what parts of the world is the maximum of dew formed?
In warm lands near the sea, or in the vicinity of rivers or lakes, as the localities of the Red Sea, the Persian Gulf the coast of Coromandel, in Alexandria, and Chili.
359. In what parts of the world is the minimum of dew formed?
It is quite absent in arid regions, in the interior of continents, such as Central Brazil, the Sahara, and Nubia.
360. Why is dew seldom formed at sea?
Because of the defective radiating quality of the surface of water.
361. Why is a heavy dew regarded as the precursor of rain?
Because a heavy formation of dew indicates that the air is saturated with moisture.
Hoar-frost is frozen dew.
363. Why is hoar-frost said to foretell rain?
Because it shows that the air is saturated with moisture, and the temperature of the air being low, the vapours are likely to condense, and produce showers.
Honey-dew is the name applied to a sweet and sticky moisture occasionally deposited upon the leaves of plants. It is, however, an error to call it dew, as it is procured by a class of insects termed aphides.
Fogs are clouds formed near the earth's surface; but London fogs are distinguished from clouds by the fact that they embrace in their vaporous folds the smoke and volatile matters imparted to the air by the operations of man. This is also the case with fogs generally that arise near large towns.
"Hath the rain a father? or who hath begotten the drops of dew?"—Job xxxviii.
366. Why are certain coasts liable to almost perpetual fogs?
Because of local or geographical agencies which contribute to their production. The coasts of California are almost constantly wrapped in fog; and, almost as constantly, the western coast of the American continent, as far south as Peru. Newfoundland, Nova Scotia, and Hudson's Bay, are all subject to dense and frequent fogs arising from the condensation of vapour from the water flowing from the hot Gulf-stream, coming in contact with the colder air.
Dry fogs are characterised by a dull opaque appearance of the atmosphere. They are most common in certain parts of North America, though they sometimes occur in Germany and in England. They are generally referred to the electrical state of the atmosphere, but the theory of them is still a matter of doubt.
The term mist is generally applied to vapours that rise over marshy places, or the surfaces of water, and roll or move over the land.
369. What is the difference between a mist and a fog?
Fogs, as they are known to us, generally arise over the land, and are usually mingled with the smoke of large towns. Mists generally arise over water, or wet surfaces.
370. Why do mists and fogs disappear at sunrise?
Because the condensed vapours are again expanded and dispersed by the heat of the sun's rays.
371. Why do fogs frequently rise in the morning and fall again in the evening?
Because, warmed by the sun's rays, they become more rarefied, and fly away at an altitude where they appear to be altogether dispelled; but at night, when the earth cools by radiation, the vapours near the earth again condense, and settle in the form of fog.
372. Why do fogs sometimes rest upon a given locality for several days together, and then disappear?
They are probably kept near to the surface of the earth by a superstratum of cold air. A cold air lying above, or a cold air lying below, might equally contribute to keep a fog near the surface of a particular part of the earth, until a flow of wind, or a fall of rain, altered the atmospheric condition.
"He bindeth up the waters in his thick clouds; and the cloud is not rent under them."—Job xxvi.
There are many interesting facts connected with the history of dew. It has attracted the attention of natural philosophers in all ages. But its true theory was never understood until recently. The ancients imagined that dews were shed from the stars; and the alchemists and physicians of the middle ages believed that the dew distilled by night possessed penetrating and wonder-working powers. The ladies of those times sought to preserve their beauty by washing in dew, which they regarded as a "celestial wash." They collected it by placing upon the grass heaps of wool, upon the threads of which the magic drops clustered.
CHAPTER XVIII.
Clouds are volumes of vapour, usually elevated to a considerable height.
Fig. 4—CIRRO-CUMULUS, OR SONDER CLOUD.
From the evaporation of water at the earth's surface.
375. Why do we not see them ascend?
We do, sometimes, in the form of what we call mists, but generally the vapours that rise and contribute to the formation of clouds are so thin that they are invisible.
"With clouds he covereth the light, and commandeth it not to shine by the cloud that cometh betwixt."—Job xxxvi.
376. Why, if they are invisible when they rise, do they became visible when they have ascended?
Because the vapours become cooled in passing through the air, and form a denser body.
377. Why, when they are condensed, do they not follow the course of gravitation, and descend?
Because the vapours form into minute vesicles, which we may call vapour bubbles, and these, being warmed by the sun, are specifically lighter than the air.
Because, also, the lower parts of clouds do partially descend, but again becoming more rarefied by meeting with a warmer atmosphere, they again ascend, and are thus poised upon the air.
Because, also, there is always a degree of atmospheric motion upward, caused by the convection of heat from the earth's surface. And, although there must also be downward movements of the air to supply the place of that which has ascended, still the heat of the ascending air, combined with its upward movement, expands and floats the vapour of the clouds.
378. At what height do clouds usually fly?
They fly at every degree of altitude; but clouds of specific character are said to fly at given altitudes, or to occupy certain ranges of altitude. We will give their probable altitudes when speaking of the specific clouds.
Fig. 5.—CIRRUS, OR CURL CLOUD.
"Who giveth rain upon the earth, and sendeth waters upon the fields."—Job v.
379. How many descriptions of clouds are there?
There are seven.
1. The Cirrus ([Fig. 5]), estimated range of altitude from 10,000 to 24,000 feet.
![[Fig. 5]](#i-093.jpg){kind=link}
2. The Cumulus ([Fig. 7]), from 3,000 to 10,000 feet.
![[Fig. 7]](#i-095.jpg){kind=link}
3. The Stratus, an extended continuous level sheet of cloud, increasing from beneath. They fly very low.
4. The Nimbus ([Fig. 10]), 1,500 to 5,000 feet.
![[Fig. 10]](#i-098.jpg){kind=link}
5. The Cirro-cumulus ([Fig. 4]), from 3,000 to 20,000 feet.
![[Fig. 4]](#i-091.jpg){kind=link}
6. The Cirro-stratus ([Fig. 6]), from 5,000 to 10,000 feet.
![[Fig. 6]](#i-094.jpg){kind=link}
7. The Cumulo-stratus ([Fig. 9]), from 3,000 to 10,000 feet.
![[Fig. 9]](#i-096.jpg){kind=link}
Fig. 6—CIRRO-STRATUS, OR WANE CLOUD.
The estimated heights given must be looked upon as very conjectural, although they have been derived from the best existing authorities. It is sufficient to know that the range of the altitude of the various clouds is from that of the Nimbus, or thunder cloud, 1,500 feet, to that of the Cirrus, 24,000 feet, the others being intermediate. The first three of the clouds above enumerated constitute what are called the primary forms. The remaining four are called secondary forms, because they arise, as their names generally indicate, out of combinations of the primary forms. Although, from the frequent mingling of clouds, it is not always practicable to identify them by the adopted classification, still, as there is generally a prevalence of one type of cloud over another, the observer would be able to distinguish a "Cirrus sky," or "Cirro-cumulus sky,"&c. Upon some occasions the typical characters of the clouds are beautifully defined; and the contemplation of their forms, and the laws of their formation, affords infinite pleasure to the observer. The advantages of scientific knowledge are such, that whether you look downwards, to the earth, or upwards to the sky, you have still the writing of God to read.
380. What produces the various shapes of clouds?
1. The state of the atmosphere.
2. The electrical condition of the clouds.
3. The movements of the atmosphere.
4. The season of the year.
"Behold, he withholdeth the waters, and they dry up; also he sendeth them out, and they overturn the earth."—Job xii.
381. What are the dimensions of clouds?
A single cloud has been estimated to have as many as twenty square miles of surface, and to be above a mile in thickness, while others are no larger than a house, or a man's hand.
Fig. 7.—CUMULUS, OR PILE CLOUD.
382. How are clouds affected by winds?
If cold winds blow upon the clouds, the cold condenses the vapour, turning the clouds into rain. But if warm dry winds blow upon the clouds, they rarefy the vapour to a greater degree, and temporarily disperse the clouds.
383. How do winds affect the shapes of clouds?
When winds are mild and gentle, the clouds break into small patches, and rise to a considerable height. But when the winds are cold and blustering, the clouds fly low, and roll along in heavy masses.
384. Why are east winds usually dry?
Because in coming towards England they pass over vast continents of land, and comparatively little ocean. Hence they are not loaded with vapours.
385. Why do west winds generally bring rain?
Because they come across the Atlantic, and are heavily charged with vapour.
386. Why are north winds generally cold and dry?
Because they come from the arctic ocean, over vast areas of ice and snow.
"Terrors are turned upon me: they pursue my soul as the wind; and my welfare passeth away as a cloud."—Job xxx.
387. Why are south winds warm and rainy?
Because they come from the southern regions, heated by the hot earth and sands, and as they cross the sea they absorb a large amount of vapour.
Fig. 9.—CUMULO-STRATUS, OR TWAIN CLOUD.
388. Why are clouds said to indicate the changes of the weather?
Because, as it is the state of the clouds that, to a great extent, determines the state of the weather, the formation of the clouds must predicate approaching changes.
389. What do cirrus clouds foretell?
Cirrus clouds foretell fine weather, when they fly high, and are thin and light.
They foretell light rain when, after a long continuance of fine weather, they form fleecy lines stretched across the sky.
They foretell a gale of wind when, for some successive days, they gather in the same quarter of the heavens, as if denoting the point from which to expect the coming gale. ([Fig. 5]).
390. What do cumulus clouds foretell?
Cumulus clouds, when they are well defined, and advance with the wind, foretell fine weather.
When they are thin and dull, and float against the wind, or in opposition to the lower currents, they foretell rain.
When they increase in size, and become dull and grey at sunset, they predict a thunder-storm. ([Fig. 7].)
"When he made a decree for the rain, and a way for the lightning and the thunder."—Job xxviii.
391. What do stratus clouds foretell?
Stratus clouds foretell damp and cheerless weather.
392. What do nimbus clouds foretell?
Nimbus clouds foretell rain, storm, and thunder. ([Fig. 10].)
393. What do cirro-cumulus clouds foretell?
Cirro-cumulus clouds, in summer, foretell increasing heat attended by mild rain, and a south wind; but in winter they commonly precede the breaking up of a frost, and the setting in of foggy and wet weather. ([Fig. 4].)
394. What do cirro-stratus clouds foretell?
Cirro-stratus clouds foretell rain or snow, according to the season of the year.
These clouds extend in long horizontal streaks, thinning away at their base, and in parts becoming wavy or patchy.
When they are thus defined in the heavens they are a certain indication of bad weather. ([Fig. 6].)
395. What do cumulo-stratus clouds foretell?
Cumulo-stratus clouds usually foretell a change of weather—from rain to fine, or from fine to rain. ([Fig. 9].)
Fig. 10.—NIMBUS, OR STORM CLOUD.
"Behold, I will put a fleece of wool in the floor; and if the dew be on the fleece only, and it be dry upon all the earth beside, then shall I know that thou wilt save Israel." * * *
CHAPTER XIX.
396. Why are cloudy days colder than sunny days?
Because the clouds intercept the solar rays in their course towards the earth.
397. Why are cloudy nights warmer than cloudless nights?
Because the clouds radiate back to the earth the heat which the earth evolves?
Because, also, the clouds radiate to the earth the heat they have derived from the solar rays during a cloudy day.
398. Why is the earth warmer than the air during sunshine?
Because the earth freely absorbs the heat of the solar rays; but the air derives comparatively little heat from the same source.
399. Why does the earth become colder than the air after sunset?
Because the earth parts with its heat freely by radiation; but the air does not.
400. Why do glasses, mats, or screens, prevent the frost from hitting plants?
Because they prevent the radiation of heat from the plants, and also from the earth beneath them.
401. Why are the screens frequently covered with dew on their exposed sides?
Because they radiate heat from both their surfaces. A piece of glass, laid horizontally over the earth, would radiate heat both upwards and downwards. But on its lower surface it would receive the radiated heat of the earth, while from its upper surface it would throw off its own heat and become cool. Therefore dew would be deposited upon the upper, but not on the under surface.
402. Why does dew rest upon the upper surfaces of leaves?
Because the under surfaces receive the radiated warmth of the earth.
"And it was so: for he rose up early on the morrow, and thrust the fleece together, and wringed the dew out of the fleece, a bowl full of water."
403. Why are cultivated lands subject to heavier dews than those that are uncultivated?
Because cultivation breaks up the hard surface of the earth, and thus its radiating power is increased.
404. Why is the gravel walk through a lawn comparatively dry while the grass of the lawn is wet with dew?
Because gravel is a bad radiator, but grass is a good radiator.
405. What benefit results from this arrangement?
In cultivated lands, where moisture is required, it is induced by the very necessity which demands it; while in rocky and barren places, where it would be of no good, dew does not form.
406. Why does little dew form at the base of hedges and walls, and around the trunks of trees?
Because those bodies in some degree counteract the radiation of heat from the earth; and they also radiate heat from their own substances.
407. Why do heavy morning dews and mists usually come together?
Because they both have their origin in the humidity of the atmosphere. The temperature of the earth having fallen, dew has been deposited; but, at the same time, the condensation of the vapour in the air has formed a screen over the surface of the earth, which has checked the further radiation of heat, and, consequently, the further formation of dew. The sun rises, therefore, upon an atmosphere charged with visible vapour at the earth's surface, and his first sloping rays, having little power to warm the atmosphere, the mist continues visible for some time.
408. What effect have winds upon the formation of dew?
Winds, generally, and especially when rapid, prevent the formation of dew. But those winds that are moist, and contribute to the formation of clouds, indirectly aid the formation of dew through the formation of clouds, and also by the moisture they impart to the air.
"And Gideon said unto God, * * * Let it now be dry only upon the fleece, and upon all the ground let there be dew."
409. Why does the humidity of the atmosphere sometimes form clouds, and at others form fogs, mists, dews, &c.?
The result depends upon the varying temperature, motion, and direction of the atmosphere.
A warm light atmosphere, of a few day's duration, will elevate the vapours to the region where they are formed into clouds.
A chill air, lying upon the surface of the warmer earth, will occasion mists or fogs.
A cold earth, acting upon the vapours contained in a warmer atmosphere, will condense them and occasion dews.
410. Why are frosty mornings usually clear?
Because, in the cold atmosphere which preceded the frost, there was but little evaporation; and now that the frost has set in, the vapours that existed have become frozen in the form of hoar-frost.
411. Why are clear nights usually cold?
Because the "screen" afforded by the clouds does not exist; therefore the heat of the earth escapes, while the vapours of the air are abstracted from it by condensation into dew, thereby imparting great clearness to the nights.
412. Why are hoar-frosts, or, as they are termed, "white frosts," so frequent, and "black frosts" so unusual?
Because white, or hoar frosts, result from the coldness of the earth, which, from its great radiating power, is always varying. But black-frosts result from the coldness of the air, which is liable to less variation of temperature than the earth.
A black-frost results from the coldness of the atmosphere, which is at the time overshadowed by a dull cloud, giving a darkness to everything, and a leaden appearance to the frozen surface of water.
414. Why are black-frosts said to last?
Because as they result from the temperature of the air, which is less likely to vary than that of the earth, there is a probability that the coldness thereof will last for some time.
"And God did so that night: for it was dry upon the fleece only, and there was dew on all the ground,"—Judges vi.
415. What benefits result from the radiation of heat, &c.?
But for the radiation of heat, we should be subjected to the most unequal temperatures. The setting of the sun would be like the going out of a mighty fire. The earth would become suddenly cold, and its inhabitants would have to bury themselves in warm covering, to wait the return of day. By the radiation of heat, an equilibrium of temperature is provided for, without which we should require a new order of existence.
The amount of heat which our earth receives from the sun, and the economy of that heat by the laws of radiation, reflection, absorption, and convection, are exactly proportionate to the necessities of our planet, and the living things that inhabit it. It is held by philosophers that any change in the orbit of our earth, which would either increase or decrease the amount of heat falling upon it, would, of necessity, be followed by the annihilation of all the existing races. The planets Mercury and Venus, which are distant respectively 37 millions of miles, and 63 millions of miles, from the great source of solar heat, possess a temperature which would melt our solid rocks; while Uranus (1,800 millions of miles), and Neptune (whose distance from the sun has not been determined), must receive so small an amount of heat, that water, such as ours, would become as solid as the hardest rock, and our atmosphere would be resolved into a liquid! Yet, poised in the mysterious balance of opposing forces, our orb flies unerringly on its course, at the rate of 63,000 miles an hour; preserving, in its wonderful flight, that precise relation to the sun, which takes from his life-inspiring rays the exact degree of heat, which, being shared by every atom of matter, and every form of organic existence, is just the amount needed to constitute the heat-life of the world!
CHAPTER XX.
Rain is the vapour of the clouds which, being condensed by a fall of temperature, forms drops of water that descend to the earth.
It is the return to the earth in the form of water, of the moisture absorbed by the air in the form of vapour.
417. Does rain ever occur without clouds?
It sometimes, but rarely happens, that a sudden transition from warmth to cold will precipitate the moisture of the air, without the formation of visible clouds.
"Canst thou lift up thy voice to the clouds, that abundance of waters may cover thee?"—Job xxxviii.
418. Why are drops of rain sometimes large and at other times small?
Because the drops, in falling, meet and unite, and also gather moisture in their descent. The greater the height from which a rain drop has descended, the larger it is, provided that its whole course lay through a rainy atmosphere.
The size of the drops is also influenced by the amount of moisture in the atmosphere, the degree of cold, and the rapidity of the change of temperature, by which the drops are produced.
419. In what seasons of the year are rains most prevalent?
Throughout Central Europe rains are most prevalent in summer, but in Southern Europe the preponderance is on the side of winter rains.
420. In what months of the year does it rain most frequently in this country?
It rains more frequently from September to March, than from March to September; but the heaviest rains occur from March to September.
421. Why are there more rainy days from September to March?
Because the temperature of the air is more frequently lowered to that degree which precipitates its vapours.
Months in the order of their comparative wetness:—1. October. 2. February. 3. July. 4. September. 5. January. 6. December.
Months in the order of their comparative dryness:—1. March. 2. January. 3. May. 4. August. 5. April. 6. November.
422. In what part of the world does the greatest quantity of rain fall?
The greatest quantity of rain falls near the equator, and the amount decreases towards the poles.
"Who can number the clouds in wisdom? or who can stay the bottles of heaven."—Job xxxviii.
423. In what part of the world do the heaviest rains occur?
The heaviest rains occur in the tropics, during the hot season. The drops of rain in the tropical regions are so large, and the force with which they descend so great, that their splash upon the skin causes a smarting sensation.
424. In what parts of the world do the least rains occur?
There are some parts of the earth which are rainless, such as Egypt, the desert of Sahara, the table lands of Persia and Montgolia, the rocky flat of Arabia Petræ, &c.
425. How many rainy days are there in a year?
The frequency of rainy days is greatest in countries near the sea, and their number decreases the further we journey from the sea-border towards the inland. In England it rains on an average 152 to 155 days in the year.
426. In what part of England does the greatest amount of rain fall?
In the town of Keswick, in Cumberland, where 63 inches of rain fall in a year; Kendal, in Westmoreland, 58 inches; Liverpool, 34 inches; Dublin, 25 inches; Lincoln, 24 inches; London, 21 inches.
427. Why do the heaviest rains occur at the tropics?
Because the hot air absorbs a large amount of vapour, and rises into the higher regions of the atmosphere, where the vapours are suddenly condensed into heavy rains, by cold currents from the poles.
428. Why does the greatest quantity of rain fall at the equator?
Because the hot air absorbs a large amount of vapour, and as the atmosphere is usually calm, there is an absence of currents, by which the saturated air would be removed. In this, which is called "the Region of Calms," rain falls almost daily.
429. Why are some parts of the earth rainless?
Because, being situated in tropical or torrid latitudes, and at a distance from the ocean, the atmosphere above them is always in a dry state.
"Thou, O God, didst send a plentiful rain, whereby thou didst confirm thine inheritance, when it was weary."—Psalm lxviii.
430. When is air said to be saturated with vapour?
When it cannot take up a larger quantity than that which it already holds.
When common salt is dissolved in water, until the water can take up no more, the water is then said to be saturated with salt.
431. What proportion of water is air capable of sustaining in the form of vapour?
The amount of water held in suspension by the air averages the following proportion: one thousand cubic feet of air contain as much vapour as, were it condensed to water, would yield about two fifths of a pint.
But one thousand cubic feet of air are capable of holding half-a-pint of water; and this may be regarded as the point of saturation.
Thus, in a room ten feet square and ten feet high, the air, at the point of saturation, would hold in the form of vapour, half-a-pint of water. It must not be forgotten, however, that the point of saturation necessarily varies with the temperature of the air.
432. Why are cloudy days and nights not always wet?
Because the air has not reached the state of saturation.
433. Why does rain purify the air?
Because it produces motion in the particles of the air, by which they are intermixed. And it precipitates noxious vapours, and cleanses the face of the earth from unhealthy accumulations.
434. Why are mountainous localities more rainy than flat ones?
Because the mountains attract the clouds; and because the clouds that are flying low are borne against the sides of the mountains and directed upwards, where they meet with cold currents of air.
435. Why does more rain fall by night than by day?
Because by night the temperature of the air, heated during the day, falls to that degree which condenses its vapours into rain.
"As the hart panteth after the water brooks, so panteth my soul after thee O God."—Psalm xlii.
436. Why do bunches of dried sea-weed indicate the probability of coming rain?
Because they readily imbibe moisture, and when they become soft and damp they show that the air is approaching the point of saturation.
437. Why does the weather-toy, called the "weather-cock," foretell the probability of rain?
Because it is made with a piece of cat-gut which swells with moisture, and as it swells, shrinks. The cat-gut is so applied that when it shrinks, it turns a rod which sends the man out of the house, and when it dries it sends the woman out. Therefore, when the man appears, it is a sign of wet, and when the woman appears it is a sign of dry weather.
There is another toy, called the Capuchin, which is made upon the same principle. The figure lifts a hood over its head when wet is approaching, and takes it off when the weather is becoming dry. In this case, a piece of cat-gut is also employed. Various weather-toys may be made upon this principle—among others, a little umbrella, which will open on the approach of wet, and close on the return of fine weather.
A gentleman once made a wooden horse, which he declared should of itself walk across a room, without machinery of any kind. The assertion was discredited; but the horse was placed in a room close to the wall on one side. The room was locked, and otherwise fastened, so that no one could interfere with the experiment. After a time the door was opened, and it was found that the horse had actually crossed the floor, and stood on the opposite side. The horse was made from wood of a peculiar kind, liable to great expansion in wet weather, and cut in a manner to produce the greatest elongation. The fore hoofs were so made that where they were set they would remain, so that the contracting parts should draw up from behind. It is easy to understand how, in this way, the wooden horse crossed the apartment.
438. Why does ladies' hair drop out of curl upon the approach of damp weather?
Because the hair absorbs moisture, which causes its spirals to relax and unfold.
439. Why is it said in mountainous countries that rain is coming, because the mountains are "putting their night-caps on?"
Because the clouds descend when they are heavy with vapour, and being attracted to the mountain tops they are said to "cap the mountains."
"Hast thou entered into the treasures of the snow; or hast thou seen the treasures of the hail."—Job xxxviii.
CHAPTER XXI.
Snow is congealed vapour, which would have formed rain; but, through the coldness of the air, has been frozen in its descent into crystalline forms. ([Fig. 1].)
![[Fig. 1]](#i-053.jpg){kind=link}
Because it reflects all the component rays of light.
442. Why is snow said to be warm, while white garments are worn for coolness?
Snow is warm by virtue of its light and woolly texture. But it is also warm on account of its whiteness; for, had it been black, it would have absorbed the heat of the sun, which would have thawed the snow. Instead of which, it reflects heat; and the reflected heat falls upon bodies above the snow, while the warmth of the earth is preserved beneath it. White clothing is cool, because it reflects from the body of the wearer the heat of the sun. White snow is warm, because it reflects the sun's heat upon bodies.
There are few persons but have felt the effect of the sun's rays reflected by the white snow on a clear wintry day. And, as regards the warmth of snow towards the earth, by preventing the radiation of heat, it has been found that a thermometer buried four inches deep in snow has shown a temperature of nine degrees higher than at the surface.
443. Why are lofty mountains always covered with snow?
Because the upper regions of the atmosphere are intensely cold.
444. Why are the upper regions of the atmosphere intensely cold?
Because the atmosphere retains but little of the heat of the sun's rays as they pass to the earth. Because at high altitudes the air is greatly rarefied. And because the radiation of heat from the earth does not materially affect such high regions.
"He causeth the vapours to ascend from the ends of the earth: he maketh lightnings for the rain: he bringeth the wind out of his treasuries."—Ps. xxxv.
445. What is meant by the snow line?
The snow line is the estimated altitude in all countries where snow would be formed. Even at the equator, at an altitude of 15,000 to 16,000 feet from the level of the sea, snow is found upon the mountain summits, where it perpetually lies. As we proceed north or south from the equator the snow line lessens in altitude. Had we in England a mountain 6,000 feet high, it would be perpetually crowned with snow.
446. Why do we hear of red snow?
Red snow is the name given to the snow in the arctic regions upon which a minute vegetable (probably the Protoccus nivalis) grows, imparting to the snow a red colour. Recent microscopic investigations have shown it to consist of a minute vegetable cell, which secretes a red colouring matter.
Snow is found to be of greater importance to man than is generally supposed. But, although in this country we are enabled to recognise the hand of Providence in the gift, there are latitudes wherein the blessing thus conferred is more deeply felt. In such countries as Canada, Sweden, and Russia, the falling of snow is looked for with glad anticipations, quite equalling those which herald the "harvest-home" of England, or the "vintage" of France. No sooner is the ground covered with snow, than cranky old vehicles that had been jolting over rough roads, and sticking fast in deep ruts of mud, are wheeled aside, and swift sledges take their place. Towns distant from each other find an easy mode of communication; the markets are enlivened, and trade thrives. Snow supplies a kind of railroad, covering the entire face of the country, and sledges glide over it, almost with the speed of the locomotive.
Sleet is snow which, in falling, has met with a warmer current of air than that in which it congealed. It therefore partially melts and forms a kind of wet snow.
Hail is also the frozen moisture of the clouds. It is probably formed by rain drops in their descent to the earth, meeting with an exceedingly cold current of air by which they become suddenly frozen into hard masses.
It is also supposed that the electrical state of the air and of the clouds influences the formation of hail.
"If the clouds be full of rain, they shall empty themselves upon the earth."—Eccles. xi.
449. Why is it supposed that the electrical state of the air and the clouds affects the formation of hail?
Because hail is more common in the summer than at other seasons, and is frequently attended by storms of thunder and lightning.
450. Why do hail-storms most frequently occur by day?
Because the clouds, being charged with vapour to saturation, favour the formation of hail by sudden electrical or atmospheric changes. In the gradual cooling of night, the clouds would expend themselves in rain.
Astonishing facts respecting hail-storms are upon record. In 1719 there fell at Kremo, hailstones weighing six pounds. In 1828 there was a fall of ice at Horsley, in Staffordshire, some of the pieces of which were three inches long, by one inch broad; and other solid pieces were about three inches in circumference. Hail storms are most frequent in June and July, and least frequent in April and October. Hail clouds float much lower in the sky than other clouds; their edges are marked by frequent heavy folds; and their lower edges are streaked with white, the other portions being massive and black. ([Fig. 10].)
CHAPTER XXII.
Light, according to Newton, is the effect of luminous particles which dart from the surfaces of bodies in all directions. According to this theory, the solar light which we receive would depart from the sun and travel to the earth.
According to Huyghens, light is caused by an infinitely elastic ether, diffused through all space. This ether, existing everywhere, is excited into waves, or vibrations, by the luminous body.
The theory of light is so undetermined that neither the views of Newton, nor those of Huyghens, can be said to be exclusively adopted. Writers upon natural philosophy seize hold of either or both of those theories, as they present themselves more or less favourably in the explanation of natural phenomena. In "The Reason Why," as we have to speak of the effects of light rather than of its cause, we shall avoid, as far as possible, the doubtful points. But let no one be discouraged by the fact that the theory of light, as, indeed, of all the imponderable agents, is imperfectly understood. Rather let us rejoice that there are vast fields of discovery yet to be explored; and that light, the most glorious and inspiring element in nature, invites us from the sun, the moon, and the stars, and from the face of every green leaf and variegated flower, to search out the wonders of its nature, and further to exemplify the goodness and wisdom of God.
"And God said, Let there be light: and there was light."
452. What is the distance of the sun from the earth?
Ninety five millions of miles.
453. At what rate of velocity does light travel?
At the rate of 192,000 miles in a second, through our atmosphere; and 192,500 miles in a second through a vacuum.
454. How long does light take to travel from the sun to the earth?
Eight minutes and thirteen seconds.
455. What is the constitution of the sun?
It is a spherical body, 1,384,472 times larger than the earth.
456. From what does the luminosity of the sun arise?
From a luminous atmosphere, or, as M. Arago named it, photosphere, which completely surrounds the body of the sun, and which is probably burning with great intensity.
457. What are the minor sources of light?
Light may be produced by chemical action, by electricity, and by phosphoresence, in the latter of which various agencies unite.
A ray of light is the smallest portion of light which we can recognise.
A medium is a body which affords a passage for the rays of light.
A beam of light is a group of parallel rays.
461. What is a pencil of light?
A pencil of light is a body of rays which come from or move towards a point.
"And God saw the light, that it was good: and God divided the light from the darkness."—Gen. i.
462. What is the radiant point?
The radiant point is that from which diverging rays of light are emitted.
The focus is the point to which converging rays are directed.
Diverging, starting from a point, and separating. Converging, drawing together towards a point.
464. What is the constitution of a ray of light?
A ray of white light, as we receive it from the sun, is composed of a number of elementary rays, which, with the aid of a triangular piece of glass, called a prism, may be separated, and will produce under refraction the following colours:—
1. An extreme red ray—a mixture of red and blue, the red predominating.
2. Red.
3. Orange—red passing into and combining with yellow.
4. Yellow—the most luminous of all the rays.
5. Green—yellow passing into and combining with the blue.
6. Blue.
7. Indigo—a dark and intense blue.
8. Violet—blue mingled with red.
9. Lavender grey—a neutral tint.
10. Rays called fluorescent, which are either of a pure silvery blue, or a delicate green.
465. Why is a ray of light, which contains these elementary rays, white?
Because the colour of light is governed by the rapidity of the vibrations of the ether-waves. When a ray of light is refracted by, or transmitted through a body, its vibrations are frequently disturbed and altered, and thus a different impression is made upon the eye.
Light which gives 37,640 vibrations in an inch, or 458,000,000,000,000 in a second of time, produces that sensation upon the eye which makes the object that directs the vibrations appear red. Yellow light requires 44,000 vibrations in an inch, and 535,000,000,000,000 in a second of time. And the other colours enumerated (see [464]) all require different velocities of vibration to produce the colours by which they are distinguished.
"The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light."—Matt. v.
Accepting the theory of vibrations, and applying it to the elucidation of the phenomena of light—it is unnecessary, we think, to believe that a ray of white light contains rays in a state of colour. It is said that if we divide a circular surface into parts, and paint the various colours in the order and proportions in which they occur in the refracted ray, and then spin the circle with great velocity, the colours will blend and appear white. But such is not the case; the result is in some degree an illusion, arising out of the sudden removal of the impression made upon the eye by the colours; and if a piece of white paper be held by the side of the coloured circle in motion, the latter will be found to be grey. When it is remembered that in colouring a white surface with thin colours, the white materially qualifies the colours, it must be admitted that the experiment fails to support the assertion that the colours of the spectrum produce white. But there can be no difficulty in understanding that a ray of light undergoing refraction, becomes divided into minor rays, which differing in their degrees of refrangibility, vary also in the velocity of their vibrations, and produce the several sensations of colour.
466. Why is a substance white?
Because it reflects the light that falls upon it without altering its vibrations.
467. Why is a substance black?
Because it absorbs the light and puts an end to the vibrations.
Because it imparts to the light that falls upon it that change in its vibratory condition, which produces on our eyes the sensation of redness.
Because it reflects the light without altering its vibrations.
470. Why is the primrose yellow?
Because, though it receives white light, it alters its vibrations to 44,000 in an inch, and 535,000,000,000,000 in a second, and this is the velocity of vibration which produces upon the eye a sensation of yellow.
"But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness."—Matt. v.
471. Why are there so many varieties of colour and tint in the various objects in nature?
Because every surface has a peculiar constitution, or atomic condition, by which the light falling upon it is influenced. In tropical climates, where the brightness of the sun is the most intense, there the colours of natural objects are the richest; the foliage is of the darkest green; the flowers and fruits present the brightest hues; and the plumage of the birds is of the most gaudy description. In the temperate climates these features are more subdued, still bearing relation to the degree of light. And at a certain depth of the ocean, where light penetrates only in a slight degree, the objects that abound are nearly colourless.
It has been held by many philosophers (and the theory is so far conclusive that it cannot be dispensed with) that there is an analogy between the vibratory causes of sound, and the vibratory causes of colour. Any one who has seen an Æolian harp, and listened to the wild notes of its music, will be aware that the wires of the harp are swept by accidental currents of air; that when those currents have been strong, the notes of the harp have been raised to the highest pitch, and as the intensity of the currents has fallen, the musical sounds have deepened and softened, until, with melodious sighing, they have died away. No finger has touched the strings; no musical genius has presided at the harp to wake its inspiring sounds; but the vibration imparted to the air, as it swept the wires, has alone produced the chromatic sounds that have charmed the listener. If, then, the varied vibrations of the air are capable of imparting dissimilar sensations of sounds to the ear, is it not only possible, but probable, that the different vibrations of light may impart the various sensations of colours to the eye?
CHAPTER XXIII.
472. What is the refraction of light?
When rays of light fall obliquely upon the surface of any transparent medium, they are slightly diverted from their course. This alteration of the course of the rays is called refraction, and the degree of refraction is influenced by the difference between the densities of the mediums through which light is transmitted.
"Let your light so shine before men, that they may see your good works, and glorify your father which is in heaven."—Matt. v.
473. If a ray of light falls in a straight line upon a transparent surface, is it then refracted?
In that case the ray pursues its course—there is no refraction.
474. Is the direction in which the rays are bent, or refracted, influenced by the relative densities of the media?
A ray of light falling slantingly upon a window, in passing through it is slightly brought to the perpendicular; and if it then falls upon the surface of water, it is still further brought to the perpendicular in passing through the water.
475. Is light refracted in passing from a dense medium to a thinner one?
It is; but the direction of the refraction is just the opposite to the instance just given; a ray of light passing through water into air, does not take a more perpendicular course, but becomes more oblique.
Fig. 11.
476. Why, if a rod or a spoon be set in an empty basin, will it appear straight, or of its usual shape?
Because the rays of light that are reflected from it all pass through the same medium, the air.
477. Why if water be poured into the basin will the rod or spoon appear bent?
Because the rays of light that pass through the water are reflected in a different degree to those that pass through the air.
"Evening, and morning, and at noon, will I pray, and cry aloud; and he shall hear my voice."—Psalm lv.
Place in the bottom of an empty basin ([Fig. 11].) a shilling; then stand in such a position at the point B that the line of sight, over the edge of the basin, just excludes the shilling from view. Then request some one to pour water into the basin, until it is filled to C ([Fig. 12].), keeping your eye fixed upon the spot. The shilling will gradually appear, and will soon come entirely in view. Not only will the shilling be brought in view, but also portions of the basin before concealed. This is owing to the rays of light passing from the bottom through the water in a direction more perpendicular than they would have done through the air; but on leaving the water they become more oblique, and hence they convey the image of the shilling over the edge of the basin, which otherwise would have obstructed the view.
![[Fig. 11]](#i-112.jpg){kind=link}
![[Fig. 12]](#i-113.jpg){kind=link}
Fig. 12.
478. Why is it that in cloudy and showery days we see the sun's rays bursting through the clouds in different directions?
Because, in passing through clouds of different densities the rays are bent out of their course.
479. Why is the apparent depth of water always deceptive?
Because the light reflected from the objects at the bottom is refracted as it leaves the water.
480. How much deeper is water than it appears to be?
About one-third. A person bathing, and being unable to swim, should calculate before jumping into the water, that if it looks two feet deep, it is quite three feet.
481. Why can we seldom at the first attempt touch anything lying at the bottom of the water with a stick?
Because we do not allow for the different refractive powers of water and of air.
"I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth."
482. Why do we see the sun before sunrise, and after sunset?
Because of the refractive effects of the atmosphere. Rays of light, passing obliquely from the sun through the air to the earth, are refracted three or four times by the varying density of the medium. Each refraction bends the rays towards the perpendicular; and hence we see the sun before it rises and after it sets.
Fig. 13.—DIAGRAM EXHIBITING THE REFRACTION OF THE SUN'S RAYS IN PASSING THROUGH THE ATMOSPHERE.
483. Why do figures, viewed through the hot air proceeding from furnaces, and from lime-kilns, appear distorted and tremulous?
Because the ever varying density of the air which is flying away in hot currents, and succeeded by cold, constantly changes the refractive power of the medium through which the figures are viewed.
484. Why do the stars twinkle?
Because their light reaches us through variously heated and moving currents of air. In this case the earth is the kiln, and the stars the object that is viewed through the refractive medium.
485. Why does much twinkling of the stars foretell bad weather?
Because it denotes that there are various ærial currents of different temperatures and densities, producing atmospheric disturbance.
"And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud."—Genesis ix.
The refraction of the sun's rays by the falling rain.
487. Why does the rainbow exhibit various colours?
The colours belong to the elementary rays of light; and these rays having different degrees of refrangibility, some of them are bent more than others; they are therefore separated into distinct rays of different colours.
488. Why are there sometimes two rainbows?
Because the rays of refracted light, reflected upon other drops of rain, are again refracted, and then reflected again, forming a secondary bow.
489. Why are the colours of the secondary bow arrayed in the reverse order of the primary bow?
Because the secondary bow is a reflection of the primary bow, and, like all reflections, is reversed.
490. Why are reflections reversed?
Because those rays which first reach the reflecting surface are the first returned. If you hold your open hand towards the looking-glass, the light passing from the point of your finger will reach the reflector and be returned before the rays that pass from the back parts of the hand. Hence the image of the hand will present the reflection of the finger point towards the point of the finger.
491. Why are the colours of the secondary rainbow fainter than those of the primary?
Because they are derived from the refraction and reflection of rays which have already been refracted and reflected, and thereby their intensity has been diminished.
A lunar rainbow is caused by the light of the moon, in the same manner as the solar rainbow is caused by the light of the sun.
"I am come a light into the world, that whosoever believeth in me should not abide in darkness."—John xiii.
493. Why is the lunar rainbow fainter than a solar rainbow?
Because the light of the moon is the reflected light of the sun, and is therefore less intense.
A halo is a luminous ring, which forms between the eye of the observer and a luminous body.
Haloes may appear around the disc of the sun, moon, or stars. But in this country the lunar haloes are the most remarkable and frequent.
495. What is the cause of the luminous ring?
The refraction of light as it passes through an intervening cloud, or a stratum of moist and cold air.
496. Why are haloes sometimes large and at other times small?
Because they are sometimes formed very high in the atmosphere, at other times very low. Being high, and farther removed from the spectator, and nearer the source of light, they appear smaller; while the nearer they are, the larger they appear.
497. Why do haloes foretell wet weather?
Because they show that there is a great amount of atmospheric moisture, which will probably form rain.
498. Why do glass lustres and chandeliers exhibit "rainbow colours"?
Because they refract the rays of light in the same manner as the rain drops.
499. Why does a soap bubble show the prismatic colours?
Because, like a large rain drop, it refracts the rays of light, and shows the elementary rays.
500. What causes the rich tints displayed by "mother-of-pearl?"
The refraction of the light that falls upon the surface of the pearl.
"Light is sown for the righteous, and gladness for the upright in heart."—Psalm xcvii.
501. What causes the brilliant colours of the diamond?
The refraction of the rays of light by the various facets of the diamond.
The refraction of light, and the production of prismatic colours, surrounds us with most interesting phenomena. The laundress, whose active labours raise over the wash-tub a soapy froth, performs inadvertently one of the most delicate operations of chemistry—the chemistry of the imponderable agents—and the result of her manipulations manifests itself in the delicate colours that dance like a fairy light over the glassy films that follow the motion of her arms. The laughing child, throwing a bubble from the bowl of a tobacco pipe into the air, performs the same experiment, and produces a result such as that which filled the philosophic Newton with unbounded joy. The foam of the seashore, the plumage of birds, the various films that float upon the surface of waters, the delicate tints of flowers, and the rich hues of luscious fruits, all combine to remind us, that every ray of light comes like an angelic artist sent from heaven, bearing upon his palette the most celestial tints, with which to beautify the earth, and show the illimitable glory of God.
CHAPTER XXIV.
502. What is the difference between the refraction and the reflection of light?
Refraction is the deviation of rays of light from their course through the interference of a different medium; reflection is the return of rays of light which, having fallen upon a surface, are repelled by it.
503. What is the radiation of light?
The radiation of light is its emission in rays from the surface of a luminous body.
504. Do all bodies radiate light?
All bodies radiate light; but those that are not in themselves primary sources of light, are said to reflect it.
505. Do black bodies reflect any light?
Black bodies absorb the light that falls upon them. But they reflect a very small degree of light.
506. Why is glass transparent?
Because its atoms are so arranged that they allow the vibrations of light to continue through their substance.
"As in water face answereth to face, so the heart of man to man."—Proverbs xxvii.
507. Does glass obstruct the passage of any portion of light?
Glass reflects (sends back) a very small portion of light. This may be observed by holding a piece of paper, or a hand, a few inches from a window, when a faint reflection of it will be visible. Probably the small amount of light reflected by transparent glass, which gives a passage to the greater part of the rays, may serve to illustrate the small amount of light reflected from black surfaces, which absorbs the greater portion of light.
Instead of a piece of white paper, hold a piece of black cloth two or three inches from the window-pane, and you will have two reflections so weak that the image of the cloth will be almost lost. The first reflection is that of the very small amount of light from the black surface on to the glass, and the second reflection is that of the inconceivably small amount returned by the glass, and by which the faint image of the black cloth is produced. But put the black cloth outside of the window-pane, and then hold an object before them, and you will find that the two weak reflectors, acting together, produce an improved image, or reflection.
508. Why, if a book is held between a candle-light and the wall, does a shadow fall upon the wall?
Because the rays of light are intercepted by the book.
509. Why do the rays pass over the edges of the book in a direct line with the flame of the candle?
Because light always travels in straight lines.
510. Why is there some amount of light even where shadows fall?
Because, as all objects reflect light, some of them throw their light into the field of the shadow.
511. Why are some substances opaque to light?
Because the arrangement of their particles will not admit of the vibrations of the luminous ether passing through them.
Opaque—impervious to rays of light.
512. Why do we see our faces reflected in mirrors?
Because the rays of light from our faces are reflected by the surface of the quicksilver at the back of the glass.
"The day is thine, the night also is thine: thou hast prepared the light and the sun."—Psalm lxxiv.
513. Why does the quicksilver reflect the rays of light?
Because, being densely opaque to light, and presenting also a bright surface, it is a good reflector, and it throws back the whole of the rays.
514. What has the glass to do with the reflection?
The glass has nothing to do with the reflection, except that it affords a field upon which the reflecting surface of the quicksilver is spread; and it keeps the air and dirt from dulling the quicksilver.
The parts of a mirror from which the quicksilver is rubbed away give no reflection that could assist the reflecting power of the quicksilver. That the surface of the glass does not reflect the image, is shown by the fact, that if you put the point of any object against the glass, the thickness between the point and the place where the reflection of it begins, will show the exact thickness of the glass.
515. Why does a compound mirror (a multiplying mirror) exhibit a large number of images of one object.
Because all objects reflect rays of light in every direction, and therefore the different mirrors, being at various angles, receive each a reflection of the same object.
516. Why does a window-pane appear to be a better reflector by candle-light than by day-light?
The reflecting power of glass is precisely the same by night as by day, and is always very feeble. But it appears to be greater by night, because the surrounding darkness increases the apparent strength of the reflection.
517. How do we know that objects reflect light in every direction?
Because if we prick a hole in a card with a pin, and then look through that small hole upon a landscape, we can see some miles of country, and some thousands of objects; every part of every object throughout the whole scene, must have sent rays of light the small hole pricked in the card.
"Such knowledge is too wonderful for me; it is high, I cannot attain unto it."—Psalm cxxxix.
At one extremity of the landscape, viewed through the hole in the card, there may be a forest of trees; in the distance there may be hills bathed in golden light, and overhung with glittering clouds; in the mid-distance there may be a river winding its course along, as though it loved the earth through which it ran, and wished, by wandering to and fro, to refresh the thirsty soil; in the foreground may be a church, covered by a million ivy leaves; and grouping towards the sacred edifice may be hundreds of intending worshippers, old and young, rich and poor; flowers may adorn the path-ways, and butterflies spangle the air with their beauties; yet every one of those objects—the forest, the hills, the clouds, the river, the church, the ivy, the people, the flowers, the butterflies—must have sent rays of light, which found their way through the little hole in the card, and entered to paint the picture upon the curtain of the eye.
This is one of the most striking instances that can be afforded of the wonderful properties of light, and of the infinitude of those luminous rays that attend the majestic rising of the sun. Not only does light fly from the grand "ruler of the day" with a velocity which is a million and a half times greater than the speed of a cannon-ball, but it darts from every reflecting surface with a like velocity, and reaches the tender structure of the eye so gently that, as it falls upon the little curtain of nerves which is there spread to receive it, it imparts the most pleasing sensations, and tells its story of the outer world with a minuteness of detail, and a holiness of truth. Philosophers once sought to weigh the sunbeam; they constructed a most delicate balance, and suddenly let in upon it a beam of light; the lever of the balance was so delicately hung that the fluttering of a fly would have disturbed it. Everything prepared, the grave men took their places, and with keen eyes watched the result. The sunbeam that was to decide the experiment had left the sun eight minutes prior to pass the ordeal. It had flown through ninety-five millions of miles of space in that short measure of time, and it shot upon the balance with unabated velocity: but the lever moved not, and the philosophers were mute.
CHAPTER XXV.
518. Why, when we move before a mirror, does the image draw near to the reflecting surface as we draw near to it, and retire when we retire?
Because the lines and angles of reflection are always equal to the lines and angles of incidence.
519. What is the line of incidence?
If a person stands in a direct line before a mirror, the line through which the light travels from him to the mirror is the line of incidence.
Incidence—falling on.
"Blessed be the Lord, who daily loadeth us with benefits, even the God of our salvation."—Psalm lxviii.
520. What is the line of reflection?
The line of reflection is the line in which the rays of light are returned from the image formed in the glass to the eye of the observer.
Reflection—a turning back.
521. What is the angle of incidence?
The angle of incidence is the angle which rays of light, falling on a reflecting surface, make with a line perpendicular to that surface.
Fig. 14.—EXPLAINING THE LINES AND ANGLES OF INCIDENCE AND OF REFLECTION.
522. What is the angle of reflection?
The angle of reflection is the angle which is formed by the returning rays of light, and a line perpendicular to the reflecting surface. It is always equivalent to the angle of incidence.
Take a marble and roll it across the floor, so that it shall strike the wainscot obliquely. Let A in the diagram represent the point from which the marble is sent. The marble will not return to the hand, nor will it travel to the line B, but will bound off, or be reflected, to C. Now B is an imaginary line, perpendicular to the reflecting surface; and it will be found that the path described by the marble in rolling to the surface and rebounding from it, form, with the line B, two angles that are equal. These represent the angles of incidence and of reflection, and explain why the reflection of a person standing at A before a mirror, would be seen by another person standing at C. This simple law in optics explains a great many interesting phenomena, and therefore it should be clearly impressed upon the memory.
"And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."—Gen. i.
523. Why do windows reflect the sun in the evening?
Because the eye of the observer is in the line of the reflection.
524. Why do windows not reflect the sun at noon?
They do, but our eyes are not then in the line of the reflection.
Fig. 15.—SHOWING THE LINES OF INCIDENCE AND REFLECTION OF THE SUN'S RAYS AT NOON AND AT EVENING.
It is obvious from the foregoing diagram that the evening rays of reflection fall upon the eyes of spectators, while the reflections at noon are so perpendicular that they are lost.
525. Why do the sun and moon appear smaller when near the meridian, than when near the horizon?
Because, when near the horizon, they are brought into comparison with the sizes of terrestrial objects; but when near the meridian they occupy the centre of a vast field of sky, and as there are no objects of comparison surrounding them, they appear smaller.
This is one "Reason Why," assigned by some observers. But there is also another reason to be found in the fact that, when the sun or moon is near the horizon, we view it through a greater depth of atmosphere than we do when at the meridian. (See [Fig. 13].) A straight line passed upward through the air, would not be so long as that which passes to S. Consequently, as the air is generally impregnated with moisture, at the time when these effects are observed, the rays of light are caused to diverge more, and the disc of the sun or moon appears magnified. Probably both of these reasons contribute to the effect. This latter reason also explains why the disc of the sun or moon may sometimes appear oval in shape, the lower stratum of air being more loaded with moisture than that through which we view the upper part of the disc.
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"There is no darkness nor shadow of death, where the workers of iniquity may hide themselves."—Job xxxiv.
526. Why do our shadows lengthen as the sun goes down?
Because light travels only in straight lines, and as the sun descends, the direction of his rays becomes more oblique, thereby causing longer shadows.
527. What is the cause of the optical illusions frequently observed in nature?
There are various kinds of natural optical illusions:—
The mirage, in which landscapes are seen reflected in burning sands.
The fata morgana, in which two or three reflections of objects occur at the same time.
The ærial spectra, or ærial reflections, &c.
Fig. 16.—ILLUSTRATING THE APPEARANCE OF PHANTOM SHIPS.
The optical illusions above enumerated owe their origin to various atmospheric conditions, in which refractions and reflections are multiplied by the different densities of atmospheric layers. They chiefly occur in hot countries, where, from the varying effects of heat, the conditions of atmospheric refraction and reflection frequently prevail in their highest degree.
"In the morning ye say, it will be foul weather to-day, for the sky is red and lowering."—Matt. xvi.
528. Why do we have twilight mornings and evenings?
Because the coming and the departing rays of the sun are refracted and reflected by the upper portions of the atmosphere. (See [Fig. 13].)
529. How long before the sun appears above the horizon does the reflection of his light reach us?
The time varies with the refracting and reflecting power of the atmosphere, from twenty minutes to sixty minutes. But the sun's position is usually eighteen degrees below the horizon when twilight begins or ends.
The white light of the sun falls upon the earth without change; it is then reflected back by the earth, and as it passes through the atmosphere portions of it are again returned to us, and this double reflection produces a polarised condition of light which imparts to vision the sensation of a delicate blue. (See [549].)
531. Why do the clouds appear white?
Because they reflect back to us the solar beam unchanged.
532. Why does the sky appear red at sunset?
Because the light vapours of the air, which are condensed as the sun sets, refract the rays of light, and produce red rays. The refraction which produces red requires only a moderate degree of density.
533. Why do the clouds sometimes appear yellow?
Because there is a larger amount of vapour in the air, which produces a different degree of refraction, resulting in yellow.
534. Why does a yellow sunset foretell wet weather?
Because it shows that the air is heavy with vapours. The refraction that produces yellow requires a greater degree of density.
"When it is evening ye say it will be fair weather, for the sky is red."—Matt. xvi.
535. Why does a red sunset foretell fine weather?
Because the redness shows that the vapours in the air towards the West, or wet quarter, are light, as is evidenced by the degree of refraction of the sun's rays.
536. Why does a red sunrise foretell wet?
Because it shows that towards the East, or dry quarter, the air is charged with vapour, and therefore probably at other points the air has reached saturation.
537. Why does a grey sunrise foretell a dry day?
Because it shows that the vapours in the air are not very dense.
538. Why is "a rainbow in the morning the shepherd's warning?"
Because it shows that in the West, or wet quarter, the air is saturated to the rain point.
539. Why is "a rainbow at night the shepherd's delight?"
Because it shows that the rain is falling in the East, and as that is a dry quarter, it will soon be over. Rainbows are always seen in opposition to the sun.
CHAPTER XXVI.
540. What is the difference between light and heat?
The most obvious distinction is, that light acts upon vision, and heat upon sensation, or feeling.
Another distinction is, that heat expands all bodies, and alters their atomic condition; while light, though usually attended by heat, does not display the same expansive force, but produces various effects which are peculiar to itself.
"Ye are the light of the world. A city that is set on a hill cannot be hid."—Matthew v.
541. Are light and heat combined in the solar ray?
Yes. A ray of light, as well as containing elementary rays that produce colours under refraction, contains also chemical rays, and heat rays.
542. How do we know that light and heat are separate elements?
Because we have heat rays, as from dark hot iron, from various chemical actions, and from friction, which are unattended by the development of light. And we have light, or luminosity, such as that of phosophoresence, which is unaccompanied by any appreciable degree of heat.
But, besides this confirmation, further proof is afforded by the fact, that in passing rays of solar light through media that are transparent to heat, but not to light, the heat rays may be separated from the luminous rays, and vice versa.
Black glass, and black mica, which are nearly opaque to light, are transparent to heat to the extent of ninety degrees out of a hundred. While pale green glass, coloured by oxide of copper, and covered with a coating of water, or a thin coating of alum, will be perfectly transparent to light, but will be almost quite opaque to heat. These remarks apply, in a greater or less degree, to various other substances.
543. In what respects are light and heat similar?
Both heat and light have been referred to minute vibratory motions which occur, under exciting causes, in a very subtile elastic medium.
They are both united in the sun's rays.
They are both subject to laws of absorption, radiation, reflection, and refraction.
They are both essential to life, whether animal or vegetable.
Both may be developed in their greatest intensity by electricity.
They are both imponderable.
"When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained:"
544. In what respects are light and heat dissimilar?
Heat frequently exists without light.
Light is usually attended with heat.
Light may be instantly extinguished, but Heat can only be more gradually reduced, by diffusion.
The solar rays deliver heat to the earth by day, and the heat remains with the earth when the light has departed.
Heat diffuses itself in all directions.
Light travels only in straight lines.
The colours that absorb and radiate both light and heat do not act in the same degree upon them both. Black, which does not radiate light, is a good radiator of heat, &c., &c.
The oxy-hydrogen light emits a most intense heat, but glass which will transmit the rays of light, will afford no passage to the rays of the heat.
Heat is latent in all bodies, but no satisfactory proof has been found that light is latent in substances.
These are only a few of the analogies and distinctions that exist between the two mysterious agents, light and heat. But they are sufficient to supply the starting points of investigation.
The importance of the heat that attends the solar rays may be illustrated by the experiments performed a few years ago, by Mr. Baker, of Fleet-street, London, who made a large burning lens, three feet and a half in diameter, and employed another lens to reduce the rays of the first to a focus of half an inch in diameter. The heat produced was so great that iron plates, gold, and stones were instantly melted; and sulphur, pitch, and resinous bodies, were melted under water.
545. What is the point of heat at which bodies become luminous?
The point of heat at which the eye begins to discover luminosity has been estimated at 1,000 deg.
546. What is the velocity of artificial light?
The light of a fire, or of a candle, or gas, travels with the same velocity as the light of the sun,—a velocity which would convey light eight times round the world while a person could count "one."
547. At what rate of velocity does the light of the stars travel?
At the same velocity as all other light. And yet there are stars so distant that, although the light of the sun reaches the earth in eight minutes and a half, it requires hundreds of years to bring their light to us.
"What is man, that thou art mindful of him? and the son of man that thou visitest him?"—Psalm viii.
548. What is the relative intensity of primary and reflected light?
The intensity of a reflection depends upon the power of the reflecting surface. But, taking the sun and moon as the great examples of primary and reflected light, the intensity of the sun's light is 801,072 times greater than that of the moon.
Polarized light is light which has been subjected to compound refraction, and which, after polarization, exhibits a new series of phenomena, differing materially from those that pertain to the primary conditions of light.
550. What are the chief deductions from the phenomena observed under the polarization of light?
The polarization of light appears to confirm in a high degree the vibratory theory of light; and to show that the vibrations of light have two planes or directions of motion. The mast of a ship, for instance, has two motions: it progresses vertically as the ship is impelled forward, and it rolls laterally through the motion of the billows.
Something like this occurs in the vibrations of light, only the vertical vibration is the condition of one ray, and the lateral vibration is the condition of another ray, and the vibrations of these two rays intersect each other in the solar ray. When these vibrations occur together, the ray has certain properties and powers. But by polarization the rays may be separated, and the result is two distinct rays, having different vibrations.
It then appears that various bodies are transparent to these polarized rays only in certain directions. And this fact is supposed to show that bodies are made up of their atoms arranged in certain planes, through or between which the lateral or the vertical waves of light, together or singly, can or cannot pass; and that the transparency or the opacity of a body is determined by the relation of its atomic planes to the planes of the vibrations of light.
Ordinary light, passing through transparent media, produces no very remarkable effect in its course; but polarized light appears to illuminate every atom of the permeated substance, and by surrounding it with a prismatic clothing, to afford an illustration of its molecular arrangement.
"A man that is called Jesus made clay, and anointed mine eyes, and said unto me, Go to the pool of Siloam, and wash: and I went and washed, and I received sight."—John ix.
551. Why are two persons able to see each other?
Because rays of light flow from their bodies to each other's eyes, and convey an impression of their respective conditions.
In some popular works that have come under our notice, we find that the student is told that "we cannot absolutely see each other—we only see the rays of light reflected from each other." The statement is erroneous as expressed. We do not see the rays of light, for if we did so, the effect of vision would be destroyed, and all bodies would appear to be in a state of incandesence, or of phosphoresence. Rays of light, which are in themselves invisible, radiate from the objects we look upon, enter the pupil of the eye, and impress the seat of vision in a manner which conveys to the mind a knowledge of the form, colour, and relative size and position of the figure we look upon. If this is not seeing the object—what is? It would be just as reasonable to say, that we cannot hear a person speak—that we only hear the vibrations of the air. But as the vibrations are imparted to the air by the organs of voice of the speaker, as he sets the air in motion, and makes the air his messenger to us, we certainly hear him, and can dispense with any logical myths that confound the understanding, and contribute to no good result.
Actinism is the chemical property of light.
Actinism—ray power.
553. Why does silver tarnish when exposed to light?
Because of the actinic, or chemical power of the rays of the sun.
554. Why do some colours fade, and others darken, when exposed to the sun?
Because of the chemical power of the sun's rays.
555. Why can pictures be taken by the sun's rays?
Because of the actinic powers that accompany the solar light.
556. What is the particular chemical effect of light exhibited in the production of photographic pictures?
Simply the darkening of preparations of silver, by the actinic rays.
557. Why are photographic studios usually glazed with blue glass?
Because blue glass obstructs many of the luminous rays, but it is perfectly transparent to actinism.
"The hay appeareth, and the tender grass showeth itself, and herbs of the mountain are gathered."—Prov. xxvii.
558. Why do plants become scorched under the unclouded sun?
Because the heat rays are in excess. The clouds shut off the scorching light; but, like the blue glass of the photographer's studio, they transmit actinism.
559. What effect has actinism upon vegetation?
It quickens the germination of seeds; and assists in the formation of the colouring matter of leaves. Seeds and cuttings, which are required to germinate quickly, will do so under the effect of blue glass (which is equivalent to saying, the effect of an increased proportion of actinism), in half the time they would otherwise require.
560. In what season of the year is the actinic power of light the greatest?
In the spring, when the germination of plants demands its vitalising aid. In summer, when the maturing process advances, light and heat increase, and actinism relatively declines. In the autumn, when the ripening period arrives, light and actinism give way to a greater ratio of heat.
"But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him."—Corinth. Book i., ii.
We shall have frequently, in the progress of our lessons, to refer to light in its connection with the chemistry of nature, and with organic life. But let us now invite the student to pause, and for a moment contemplate the wonders of a sunbeam. How great is its velocity—how vast its power—how varied its parts—yet how ethereal! First, let us contemplate it as a simple beam in which light and heat are associated. How deep the darkness of the night, and how that darkness clings to the recesses of the earth. But the day beams, and darkness flies before it, until every atom that meets the face of day is lit up with radiance. That which before lay buried in the shade of night is itself now a radiator of the luminous fluid. Mark the genial warmth that comes as the sister of light; then stand by the side of the experimentalist and watch the point on which he directs the shining focus, and in an instant see iron melt and stones run like water, under the fervent heat! Now look upward to the heavens, where the falling drops of rain have formed a natural prism in the rainbow, and shown that the beam of pure whiteness, refracted into various rays, glows with all the tints that adorn the garden of nature. These are the visible effects of light. But follow it into the crust of the earth, where it is, by another power, which is neither light nor heat, quickening the seed into life; watch it as the germ springs up, and the plant puts forth its tender parts, touching them from day to day with deeper dyes, until the floral picture is complete. Follow it unto the sea, where it gives prismatic tints to the anemone, and imparts the richest colours to the various algae. Think of the millions of pictures that it paints daily upon the eyes of living things. Contemplate the people of a vast city when, attracted by some floating toy in the air, a million eyes look up to watch its progress. The sun paints a million images of the same object, and each observer has a perfect picture. It makes common to all mankind the beauties of nature, and paints as richly for the peasant as for the king. The Siamese twins were united by a living cord which joined their systems, and gave unity and sympathy to their sensations. In the great flood of light that daily bathes the world, we have a bond of union, giving the like pleasures and inspirations to millions of people at the same instant. And that which floods the world with beauty, should no less be a bond of unity and love.
CHAPTER XXVII.
Electricity is a property of force which resides in all matter, and which constantly seeks to establish an equilibrium.
562. Why is it called electricity?
Because it first revealed itself to human observation through a substance called, in the Greek language, electrum. This substance is known to us as amber.
563. In what way did electrum induce attention to this property of force in matter?
Thales, a Greek philosopher, observed that, by briskly rubbing electrum, it acquired the property of attracting light particles of matter, which moved towards the amber, and attached themselves to its surface, evidently under the influence of a force excited in the amber.
It is a resinous substance, hard, bitter, tasteless, and glossy. It has been variously supposed to be a vegetable gum, a fossil, and an animal product. It is probably formed by a species of ant that inhabit pine forests. The bodies of ants are frequently found in its substance.
"He made darkness his secret place: his pavilion round about him were dark waters and thick clouds of the skies."
565. Why does the rubbing of a stick of sealing-wax cause it to attract small particles of matter?
Because it excites in the sealing wax that force which was first observed in the amber. Sealing-wax, therefore, is called an electric (amber-like) body.
566. Why do we hear of the electric fluid?
Simply because the term fluid is the most convenient that can be found to express our ideas when speaking of the phenomena of electric force. But of the nature of electricity, except through its observed effects, nothing is known.
567. What substances are electric?
All substances in nature, from the metals to the gases. But they differ very widely in their electrical qualities.
568. What is positive electricity?
Electricity, when it exists, or is excited, in any body, to an amount which is in excess of the amount natural to that body, is called positive (called also vitreous).
569. What is negative electricity?
Electricity, when it exists, or is excited, in any body, in an amount which is less than is the amount natural to that body, is called negative (called also resinous).
570. Why is "positive" electricity called also "vitreous," and "negative" electricity called also "resinous"?
Because some philosophers believe that there is but one electricity, but that it is liable to variations of quantity or state, which they distinguish by positive and negative; while other philosophers believe that there are two electricities, which they name vitreous and resinous, because they may be induced respectively from vitreous and resinous substances, and they display forces of attraction and repulsion.
571. Upon what do the electrical phenomena of nature depend?
Upon the tendency of electricity to find an equilibrium between its positive and negative states (assuming there to be but one fluid); or upon the tendency of vitreous electricity to seek out and combine with resinous electricity (assuming that there are two fluids).
"The Lord also thundered in the heavens, and the Highest gave his voice; hailstones and coals of fire."
572. How does the equilibrium of electricity become disturbed?
By changes in the condition of matter. As electricity resides in all substances, and is, perhaps, an essential ingredient in their condition, so every change in the state of matter—whether from heat to cold, or from cold to heat; from a state of rest to that of motion; from the solid to the liquid, or the æriform condition, or vice versa; or whether substances combine chemically and produce new compounds—in every change the electrical equilibrium is disturbed; and, in proportion to the degree of disturbance, is the force exerted by electricity to resume its balance in the scale of nature.
573. How does electricity seek to regain equilibrium?
By passing through substances that are favourable to its diffusion; therefore they are called conducting or non-conducting bodies, according as they favour or oppose the transmission of the electrical current.
574. What substances are conductors of electricity?
Metals, charcoal, animal fluids, water, vegetable bodies, animal bodies, flame, smoke, vapour, &c.
575. What substances are non-conductors?
Rust, oils, phosphorous, lime, chalk, caoutchouc, gutta percha, camphor, marble, porcelain, dry gases and air, feathers, hair, wool, silk, glass, transparent stones, vitrefactions, wax, amber, &c. These bodies are also called insulators. Some of these substances, as chalk, feathers, hair, wool, silk, &c., though non-conductors when dry, become conductors when wetted.
Insulating—preventing from escaping.
576. Why are amber and wax classed among the non-conductors, when they have been pointed out as electrics, and used to illustrate electrical force?
It is because they are non-conductors that they have displayed, under excitement, the attractive force shown in respect to the particles of matter which were drawn towards their substances. If a bar of iron were excited, instead of a stick of wax, electricity would be equally developed; but the iron, being a good conductor, would pass the electricity to the hand of the operator as fast as it accumulated, and the equilibrium would be undisturbed.
"Yea, he sent out his arrows, and scattered them; and he shot out lightnings and discomfited them."—Psalm xviii.
577. What is the effect when electricity, in considerable force, seeks its equilibrium, but meets with insulating bodies?
The result is a violent action in which, intense heat and light are developed, and in the evolution of which the electric force becomes expended.
578. What is the cause of electric sparks?
The electric force, passing through a conducting body to find its equilibrium, is checked in its course by an insulator, and emits a spark.
579. What produces the electric light?
Currents of electricity pass towards each other along wires at the ends of which two charcoal points are placed. As long as the charcoal points remain in contact, the electric communication is complete, and no light is emitted, but, when they are drawn apart, intense heat and light are evolved.
Figs. 17 & 18.—SHOWING THE EFFECT OF THE UNION AND THE SEPARATION OF THE CHARCOAL POINTS.
580. What is the cause of lightning?
Lightning is the result of electrical discharges from the clouds.
581. What develops electricity in the clouds?
Evaporations from the surface of the earth; changes of temperature in the atmospheric vapour; chemical action upon the earth's surface; and the friction of volumes of air of different densities against each other.
"His lightnings enlightened the world: the earth saw and trembled."—Psalm xcvii.
582. Why do these phenomena produce electricity?
Because they disturb the equilibrium of the electric force, and produce positive and negative states of electricity.
583. When does lightning occur?
When clouds, charged with the opposite electricities approach, the forces rush to each other, and combine in a state of equilibrium.
584. Why does lightning attend this movement of the forces of electricity?
Because the atmosphere, being unable to convey the great charges of electricity as they rush towards each other, acts as an insulator, and lightning is caused by the violence of the electricity in forcing its passage.
585. Does lightning ever occur when the conducting power is equal to the force of the electricity?
No; electricity passes invisibly, noiselessly, and harmlessly, whenever it finds a sufficient source of conduction.
CHAPTER XXVIII.
586. Why does lightning sometimes travel through a "zigzag" course?
Because the electricity, being resisted in its progress by the air, flies from side to side, to find the readiest passage.
587. Why does lightning sometimes appear forked?
Because, being resisted in its progress by the air, the electricity divides into two or more points, and seeks a passage in different directions.
588. Why is lightning sometimes like a lurid sheet?
Because the flash is distant, and therefore we see only the reflection.
"He directeth it under the whole heavens, and his lightning unto the ends of the earth."
589. When is the flash of lightning straight?
When the distance between the clouds whose electricities are meeting, is small.
590. What is the cause of the aurora borealis?
The mingling of the electricities of the higher regions of the atmosphere.
591. When does the flash of lightning appear blue?
When the degree of electrical excitement is intense, and general throughout the atmosphere.
592. Why does lightning sometimes appear red, at others yellow, and at others white?
Because of the varying humidity, which affects the refracting power of the atmosphere.
593. Does lightning ever pass upwards from the earth to the clouds?
Yes; when the earth is charged with a different electricity to that which is in the clouds.
594. Does lightning ever pass directly from the clouds to the earth?
Yes; when the electricity of the clouds seeks to combine with the different electricity of the earth.
The mingling of the electricities of the earth and the air must be continually going on. But lightning does not attend the phenomena, because all natural bodies, vapours, trees, animals, mountains, houses, rocks, &c., &c., act more or less as conductors between the earth and the air. It is only when there is a great disturbance of the electrical forces, that terrestrial lightning is developed. When lightning strikes the earth with great force, it sometimes produces what are called fulgurites in sandy soils; these are hollow tubes, produced by the melting of the soil.
595. What is the extent of mechanical force of lightning?
Lightning has been proved, in one instance, to have struck a church with a force equal to more than 12,000 horse-power. A single horse-power, in mechanical calculations, is equivalent to raising a weight of 32,000 lbs. one foot in a minute. The force of lightning, therefore, has been proved to be equal to the raising of 384,000,000 lbs. one foot in a minute. This is equal to the united power of twelve of our largest steamers, having collectively 24 engines of 500 horse-power each. The velocity of electricity is so great that it would travel round the world eight times in a minute.
"After it a voice roareth: he thundereth with the voice of his excellency; and he will not stay them when his voice is heard."—Job xxxvii.
The church alluded to was St. George's church, Leicester, a new edifice, which was completely destroyed on the 1st of August, 1846, by a thunder-storm. The steeple was rent asunder, and massive stones were hurled to a distance of thirty feet. The vane rod and top part of the spire fell down perpendicularly and carried with it all the floors of the tower. A similar disaster occurred to St. Bride's church, Fleet-street, London, about 100 years ago. The lightning first struck upon the metal vane of the steeple, and then ran down the rod and attacked the iron cramps, smashing the large stones that lay between them. The church was nearly destroyed. By the same wonderful force, ships have been disabled, trees split asunder, houses thrown down, and animals struck dead.
596. Why is it dangerous to stand near a tree during an electric storm?
Because the tree is a better conductor than air, and electricity would probably strike the tree, and then pass to the person standing near.
597. If trees are good conductors, why do they not convey the electricity to the ground?
Trees are only indifferent conductors, and the electricity would quit the tree to pass through any better conductor.
598. Why is it dangerous to sit near a fire during an electric storm?
Because the chimney, being a tall object, and smoke a good conductor, would probably attract the electricity, and convey it to the body of a person sitting near the fire.
599. Why is it dangerous to be near water during an electric storm?
Because water is a good conductor, and the vapour arising from it might attract the electricity. Man, being elevated over the water, might form the first point attacked by the electricity.
600. Are iron houses dangerous during an electric storm?
No; they are very safe, because their entire surface is a good conductor, and would convey the electricity harmlessly to the earth.
"To him that rideth upon the heavens of heavens, which were of old; lo, he doth send out his voice, and that a mighty voice."—Psalm lxviii.
601. Why does electricity seize upon bell wires and iron fastenings?
Because copper wires are the very best conductors of electricity; and iron articles are also good conductors.
602. Supposing electricity to attack a bell wire, where would the point of danger exist?
At the extremities of the wire, where the conducting power of the wire would cease, and the electricity would seek to find another conductor.
603. Are umbrellas, with steel frames, dangerous in an electric storm?
They are dangerous in some degree, because they might convey electricity to the hand, and then transfer it to the body. But, generally speaking, when it rains, the rain itself, being a good conductor, relieves the disturbance of electricity by conveying it to the ground.
604. Are iron bedsteads dangerous in electric storms?
No, they are safe, because the iron frame, completely surrounding the body, and having a great capacity for conduction, would keep the electricity away from the body.
605. Why is it safe to be in bed during an electric storm?
Because feathers, hair, wool, cotton, &c., especially when dry, are good insulators or non-conductors.
606. What is the safest situation to be in during an electric storm?
In the centre of a room, isolated as far as possible from surrounding objects; sitting on a chair, and avoiding handling any of the conducting substances. The windows and doors should be closed, to prevent drafts of air.
607. In the open air, what is the safest situation?
To keep aloof, as far as possible, from elevated structures; and regard the rain, though it might saturate our clothes, as a protection against the lightning stroke, for wet clothes would supply so good a conductor, that a large amount of electricity would pass over man's body, through wet garments, and he would be quite unconscious of it.
"God thundereth marvellously with his voice: great things doeth he, which we cannot comprehend."—Job xxxvi.
During a violent electric storm in the Shetland Islands, a fishing boat was attacked by the electric fluid, which tore the mast to shivers. A fisherman was sitting by the side of the mast at the time, but he felt no shock. Upon taking out his watch, however, he found that the electric current had actually fused his watch into a mass. In this case, it is more than probable that the man was saved through the saturation of his clothes with rain.
608. Do lightning conductors "attract" electricity?
Not unless the electric current lies in their vicinity.
609. Why have lightning conductors sometimes been found ineffective?
Because they have been unskilfully constructed; have been too small in their dimensions, and have not been properly laid to convey the electricity harmlessly away.
610. What is the best metal for a lightning conductor?
Copper, the conducting power of which is five times greater than that of iron.
611. Why should a large building have several conductors?
Because the influence of a conductor over the electricity of the surrounding air does not extend to more than a radius of double the height of the conductor above the building: for instance, a conductor rising ten feet high above the building would influence the electricity twenty feet all round the conductor.
612. Why should conductors have at their base several branches penetrating the earth?
To facilitate the discharge of the accumulated electricity into the earth.
613. Why does electricity affect the shapes of clouds?
Because electricity does not penetrate the masses of bodies, but affects generally their surfaces. Hence electricity exists in the surfaces of clouds, and in its efforts to find an equilibrium it causes the clouds to roll in heavy masses, having dark outlines.
"All ye inhabitants of the world, and dwellers on the earth, see ye, when he lifteth up an ensign on the mountains; and when he bloweth a trumpet, hear ye."—Isaiah xviii.
The fact that electricity resides in, and is conducted by, the surfaces of bodies, is well established, and should receive due attention in the protective measures adopted to secure life and property against the effects of lightning. A practical suggestion that arises out of this fact is, that tubes of copper would form far more efficient conductors than bars of the same metal. A copper tube, of half an inch diameter, would conduct nearly double the amount of electricity which could be conveyed away by a bar of copper of the same diameter. The upper extremity of the tube should be open obliquely, that the electric current might be induced to pass over both the inner and outer surfaces.
CHAPTER XXIX.
Thunder is the noise which succeeds the rush of the electrical fluid through the air.
615. Why does noise follow the commotion caused by electricity?
Because, by the violence of the electric force, vast fields of air are divided; great volumes of air are rarefied; and vapours are condensed, and thrown down as rain. Thunder is therefore caused by the vibrations of the air, as it collapses, and seeks to restore its own equilibrium.
616. Why is the thunder-peal sometimes loud and continuous?
Because the electrical discharge takes place near the hearer, and therefore the vibrations of the air are heard in their full power.
617. Why is the thunder-peal sometimes broken and unequal?
Because the electrical discharge takes place at a considerable distance, and the vibrations are affected in their course by mountains and valleys. Because, also, the forked arms of the lightning strike out in different directions, causing the sounds of thunder to reach us from varying distances.
"Lo, these are parts of his ways; but how little a portion is heard of him? but the thunder of his power who can understand?"—Job xxv.
618. Why has the thunder-peal sometimes a low grumbling noise?
Because the electrical discharges, though violent, take place far away, and the vibrations of the air become subdued.
619. Why does the thunder-peal sometimes follow immediately after the flash of lightning?
Because the discharge of electricity takes place near the hearer.
620. Why does the thunder-peal sometimes occur several seconds after the flash?
Because the discharge takes place far away, and light travels with a much greater velocity than sound.
621. Through what distance will the sound of thunder travel?
Some twenty or thirty miles, according to the direction of the wind, and the violence of the peal.
622. Through what distance will the light of lightning travel?
The light of lightning, and its reflections, will penetrate through a distance of from a hundred and fifty to two hundred miles.
623. How may we calculate the distance at which the electric discharge takes place?
Sound travels at the rate of a quarter of a mile in a second. If, therefore, the peal of thunder is heard four seconds after the flash of lightning, the discharge took place about a mile off. The pulse of an adult person beats about once in a second; therefore, guided by the pulse, any person may calculate the probable distance of the storm:—
2 beats, ½ a mile.
3 beats, ¾ of a mile.
4 beats, 1 mile.
5 beats, 1¼ miles.
6 beats, 1½ miles.
7 beats, 1¾ miles.
8 beats, 2 miles, &c.
Attention should be paid to the direction and speed of the wind, and some modifications of the calculation be made accordingly. Persons between 20 and 40 years of age should count five beats of the pulse to a mile; under 20, six beats.
"The clouds poured out water; the skies sent out a sound; thine arrows also went abroad."
624. Why are electric storms more frequent in hot than in cold weather?
Because of the greater evaporation, as the effect of heat; and also of the effect of heat upon the particles of all bodies.
625. Why do electric storms frequently occur after a duration of dry weather?
Because dry air, being a bad conductor, prevents the opposite electricities from finding their equilibrium.
626. Why is a flash of lightning generally succeeded by heavy rain?
Because the electrical discharge destroys the vescicles of the vapours. If a number of small soap-bubbles floating in the air were suddenly broken by a violent commotion of the atmosphere, the thin films of the bubbles would form drops of water, and fall like rain.
627. Why is an electrical discharge usually followed by a gust of wind?
Because the equilibrium of the atmosphere is disturbed by the heat and velocity of lightning, and the condensation of vapour. Air, therefore, rushes towards those parts where a degree of vacuity or rarefaction has been produced.
The name thunderbolt is applied to an electrical discharge, when the lightning appears to be developed with the greatest intensity around a nucleus, or centre, as though it contained a burning body. But there is, in reality, no such thing as a thunderbolt.
"The voice of the Lord is upon the waters: the God of glory thundereth; the Lord is upon many waters."—Psalm xxix.
629. Why do electric storms purify the air?
Because they restore the equilibrium of electricity which is essential to the salubrity of the atmosphere; they intermix the gases of the atmosphere, by agitation; they precipitate the vapours of the atmosphere, and with the precipitation of vapours, noxious exhalations are taken to the earth, where they become absorbed; they also contribute largely to the formation of ozone, which imparts to the air corrective and restorative properties.
Ozone is an atmospheric element recently discovered, and respecting which differences of opinion prevail. It is generally supposed to be oxygen in a state of great strength, constituting a variety of form or condition.
631. Why do we know that electricity contributes to the formation of ozone?
Because careful observations have established the fact that the proportion of ozone in the atmosphere is relative to the amount of electricity.
632. What are the properties of ozone?
It displays an extraordinary power in the neutralisation of putrefactions, rapidly and thoroughly counteracting noxious exhalations; it is the most powerful of all disinfectants.
Schonbien, the discoverer of ozone, inclines to the opinion that it is a new chemical element. Whatever it may be, there can be no doubt that it plays an important part in the economy of nature. Its absence has been marked by pestilential ravages, as in the cholera visitations; and to its excess are attributed epidemics, such as influenza. It was found, during the last visitation of cholera, that the fumigation of houses with sulphur had a remarkable efficacy in preventing the spread of the contagion. The combustion of sulphur ozonised the atmosphere; the same result occurs through the emission of phosphoric vapours; ozone is also developed by the electricity evolved by the electrical machine, and in the greater electrical phenomena of nature. The smell imparted to the air during an electric storm is identical with that which occurs in the vicinity of an electrical apparatus—it is a fresh and sulphurous odour. The opinion is gaining ground that the respiration of animals and the combustion of matter are sources of ozone, and that plants produce it when under the influence of the direct rays of the sun. It is also believed to be produced by water, when the sun's rays fall upon it. The most recent opinion respecting ozone is, that it is electrized oxygen. The subject is of vast importance, and opens another field of discovery to the pioneers of scientific truth.
"The voice of thy thunder was in the heaven: the lightnings lightened the world, the earth trembled and shook."—Psalm lxvii.
Magnetism is the electricity of the earth, and is characterised by the circulation of currents of electricity passing through the earth's surface.
634. What are magnetic bodies?
Magnetic bodies are those that exhibit phenomena which show that they are under the influence of terrestrial electricity, and which indicate the direction of the poles, or extreme points, of magnetic force.
Galvanism is the action of electricity upon animal bodies, and is so called from the name of its first discoverer, Galvani.
636. What is Voltaic electricity?
Voltaic electricity is the electricity that is developed during chemical changes, and is so called after Volta, who enlarged upon the theory of Galvani.
637. What are the differences between mechanical, or frictional electricity, Voltaic electricity, Galvanism, and magnetism?
Frictional electricity is electricity suddenly liberated under the effects of the motion, or the mechanical disturbance of bodies.
Voltaic electricity is a steady flow of an electric current, arising from the gradual changes of chemical phenomena.
Galvanism and Voltaism are almost identical, since the latter is founded upon, and is a development of, the former. But the term Galvanism is frequently used when speaking of the development of electricity in animal bodies.
Magnetism is the electricity of the earth, and is understood to imply the fixed electricity of terrestrial bodies.
"And I heard as it were the voice of a great multitude, and as the voice of many waters, and as the voice of mighty thunderings, saying Alleluia: for the Lord God omnipotent reigneth."—Rev. xix.
Man knows not what electricity is; yet, by an attentive observance of its effects, he avails himself of the power existing in an unknown source, and produces marvellous results. When the Grecian philosopher, Thales, sat rubbing a piece of amber, and watching the attraction of small particles of matter to its surface, he little knew of the mighty power that was then whispering to him its offer to serve mankind. And when Franklin, with the aid of a boy's plaything, drew down an electric current from the clouds, and caught a spark upon the knuckles of his hand, even he little conjectured that the time was so near when that strange element, which sent its messenger to him along the string of a kite, would become one of man's most submissive servants.
So many great results have sprung from the careful observation of the simplest phenomena, that we should never pass over inattentively the most trifling thing that offers itself to our examination. Nature, in her revelations, never seeks to startle mankind. The formation of a rock, and the elaboration of a truth, are alike the work of ages. It was the simple blackening of silver by the sun's rays which led to the discovery of the chemical agency of light. It was the falling of an apple which pointed Newton to the discovery of the laws of gravitation. It was the force of steam, observed as it issued from beneath the lid of a kettle, that led to the invention of the steam-engine. And it is said of Jacquard, that he invented the loom which so materially aided the commerce of nations, while watching the motions of his wife's fingers, as she plied her knitting. As great discoveries spring from such small beginnings, who among us may not be the herald of some great truth—the founder of some world-wide benefaction?
That the area of discovery has not perceptibly narrowed its limits, is evident from the fact that the greatest elements in nature are still mysteries to man. And though it may not be within the power of a finite being to unravel the chain of wonders that enfold the works of an infinite God,—still it is evident, from the progress which discovery has made, and from the good which discovery has done, that God does invite and encourage the human mind to contemplate the workings of Divine power, and to pursue its manifestations in every element, and in every direction.
The wonderful force of electricity astonishes us all the more when we view it in contrast with that equally wonderful element, light. We have seen that light travels with a velocity of 192,000 miles in a second, but that it falls upon a delicate balance so gently, that it produces no perceptible effect. As far as we know the nature of electricity, it is even more ethereal than light; yet, while the ether of light falls harmlessly and imperceptibly—even with the momentum of a flight of ninety-five millions of miles, the ether of electricity, bursting from a cloud only five hundred yards distant, will split massive stones, level tall towers with the dust, strike majestic trees to the ground, and instantly extinguish the life of man! Why does the one ether come divested of all mechanical force, while that which seems to be even more ethereal than it, is capable of exerting the mightiest force over material things? Does it not appear that the Creator of the universe has established these paradoxes of power to testify his Omnipotence—to show to man that with Him all things are possible; and that, in the grand cosmicism of the universe, every attribute of Omnipotence has been fulfilled?
"And the seventh angel poured out his vial into the air; and there came a great voice out of the temple of heaven, from the throne, saying, It is done."—Rev. xvi.
Let us now consider man's relation to this Omnipotence. He sees that electricity smites the tall edifice, and observes that in doing so it displays a choice of a certain substance through which it passes harmlessly, and that its violence is manifested only when its path is interrupted. Man, taking advantage of this preference of electricity for a particular conductor, stretches out an arm of that substance, and points it upwards to the clouds; electricity accepts the invitation, and passes harmlessly to the earth. But this not all: man learns by observation that electricity resides in all matter; that it may be collected or dispersed; that it travels along a good conductor at the rate of half-a-million of miles in a second of time; he constructs a battery, a kind of scientific fortress, in which he encamps the great warrior of nature; and then, laying down a conducting wire, he liberates the mighty force: but its flight must be on the path which man has defined, and its journey must cease at the terminus which man has decreed, where, by a simple contrivance of his ingenuity (the movements of a magnetic needle), the electric current is made to deliver whatever message of importance he desires to convey. Thus, the element which in an instant might deprive man of life, is subdued by him, and made the obedient messenger of his will.
CHAPTER XXX.
The atmosphere is the transparent and elastic body of mixed gases and vapours which envelopes our globe, and which derives its name from Greek words, signifying sphere of vapour.
639. To what height does the atmosphere extend?
It is estimated to extend to from forty to fifty miles above the surface of the earth.
640. Why is it supposed that the atmosphere does not extend beyond that height?
Because it is found, by experiment and observation, that the air becomes less dense in proportion to its altitude from the earth's surface. The gradual decrease of atmospheric density observed in ascending a mountain, or in a balloon, supplies sufficient data to enable us to calculate the height at which the atmosphere would probably altogether cease.
At an altitude of 18,000 feet the air is indicated by the barometer to be only half as dense as at the surface of the earth. And as the densities of the atmosphere decrease in a geometrical progression, the density will be reduced to one-fourth at the height of 36,000 feet; and to one-eighth at 54,000 feet. The effects of the decreasing density of the atmosphere are, that the intensity of light and sound are diminished, and the temperature is lowered. Persons who have reached a very high elevation, state that the sky above them began to assume the appearance of darkness; and there can be no doubt that, if it were possible to reach an altitude of some fifty to sixty miles, there would be perfect blackness although the sun's rays might be pouring through the darkened space, to illuminate the atmosphere. Upon the summit of Mont Blanc, the report of a pistol at a short distance can scarcely be heard. When Gay Lussac reached the height of 23,000 feet, he breathed with great pain and difficulty, and felt distressing sensations in his ears, as though they were about to burst. Upon the high table-lands of Peru, the lips of Dr. Ischudi cracked and burst; and blood flowed from his eyelids.
"For he looketh to the ends of the earth, and seeth under the whole heaven; To make the weight for the winds."—Job xxviii.
641. What is the amount of atmospheric pressure at the earth's surface?
The pressure of the atmosphere at the earth's surface is fifteen pounds to every square inch of surface. That is to say, that the column of air, extending fifty miles over a square inch of the earth, presses upon that square inch with a weight equal to fifteen pounds.
642. Is that the weight of dry or moist air?
That is the weight of air at what is called the point of saturation, when it is fully charged with watery vapour.
643. What is the proportion of watery vapour in the atmosphere?
The proportion constantly varies. Evaporation is not a result of accident; it seems an established law that the air shall constantly absorb vapour until it has reached the maximum that it can hold. Experiments have been tried, in which dry air has been pressed upon the surface of water with great force, but no degree of pressure could prevent the formation of vapour. (See [431].)
644. What is the total amount of atmospheric pressure on the earth's surface?
The total amount of atmospheric pressure on the earth's surface, at 15 lbs. to the square inch, amounts to 12,042,604,800,000,000,000 lbs. This pressure is equal to that of a globe of lead of sixty miles in diameter.
645. What is the pressure of the atmosphere upon the human body?
Estimating the surface of man's body to be equal to fifteen square feet, he sustains an atmospheric pressure of 32,400 lbs., or nearly fourteen tons and a-half. The mere variation of weight, arising out of the changes in the state of the atmosphere, may amount to as much as a ton and a-half.
"I therefore so run, not as uncertainly; so fight I, not as one that beateth the air."—Corinth. ix.
646. Why does not man feel this pressure?
Because the diffusion of air which, surrounding him in every direction, and acting upon the internal as well as the external surfaces of his body, and probably surrounding every atom of his frame, establishes an equilibrium, in which every degree of pressure counteracts and sustains itself.
647. What is the weight of air relative to that of water?
A cubic foot of air weighs only 523 grains, a little more than an ounce; a cubic foot of water weighs one thousand ounces.
648. What is the greatest height in the atmosphere which any human being has ever reached?
M. Gay Lussac, in the year 1804, ascended to the height of 23,000 feet.
A vacuum is a space devoid of matter. The term is generally applied to those instances in which air is drawn from within an air-tight vessel.
650. Is it possible to form a perfect vacuum?
It is probably impossible to do so, even with the most powerful instruments—some portion of air would remain, but in so thin a form that it would be imperceptible.
651. Why does the depression of a pump-handle cause the water to flow?
Because the putting down of the handle lifts up the piston with its valve closed, thereby tending to produce a vacuum; but the pressure of the air upon the water not contained in the pump, forces more water up into the part where a vacuum would otherwise be formed. Then, when the handle is raised, and the piston forced downwards, the valve opens, and the water rushes through.
There is a second valve, below the piston, which closes with the downward movement, to prevent the water from rushing back again.
"The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit."—John ii., iii.
652. How high will atmospheric pressure raise water in the bore of a pump?
It will raise water to an elevation of thirty feet above its level.
653. Why will it raise water to an elevation of thirty-feet?
Because a column of water of thirty feet high, nearly balances the weight of a column of air of equal surface, extending to the whole height of the atmosphere. When, therefore, water is elevated to the height of thirty feet, the power of the pump is enfeebled, as the air and the water balance each other.
654. How is water raised to a greater elevation when it is required?
By mechanical contrivances, by which the water is forced to a greater elevation.
655. Why does water run through the bent tube called a syphon?
Because the atmospheric pressure upon the water on the outside of the syphon forces it into the tube as fast as the syphon empties itself through its longer arm.
656. Why does water run through the longer arm of the syphon?
Because the weight of the water in the longer arm of the syphon is greater than that in the shorter; therefore it runs out by its own gravity. And, as in running out, it creates a tendency towards a vacuum, the pressure of the outer air comes into operation, and forces the water through the tube.
657. Why does water issue from the earth in springs?
Some springs are caused by natural syphons formed in the fissures of rocks, which, communicating with bodies of water, are continually filled by atmospheric pressure, and therefore convey streams of water to the point where they are set free.
"Ascribe ye strength unto God: his excellency is over Israel, and his strength is in the clouds."—Psalm lviii.
658. Why, if a wine glass is filled with water, and a card laid upon it, and the whole inverted, will the water remain in the glass?
Because the pressure of the atmosphere upon the surface of the card counteracts the weight of the water.
659. What has the card to do with the experiment?
It forms a base upon which the water may rest, while the glass is being inverted; and it prevents the air from acting upon the fluidity of the water, and forcing it out of the glass.
660. Why will not beer run out of the tap of a cask until a spile has been driven in at the top?
Because the pressure of the air upon the opening of the tap counteracts the weight of the beer. But when the spile is driven in, the air enters at the top, and counteracts its own pressure at the bottom.
661. Why does a cup in a pie become filled with juice?
Because the heat expands the air, and drives nearly all of it out of the cup. When the pie is taken out of the oven, and begins to cool, air cannot get into the cup again, because its edges are surrounded by juice. A partial vacuum, therefore, exists within the cup, and the pressure of the external air forces the juice into it.
662. Does the cup prevent the juice from boiling over?
No. So long as the heat exists, the cup remains empty; and as it occupies space, the air is driven out of it, into the pie, it rather tends to force the juice over the sides of the dish. It is only when cooling that the juice enters the cup.
663. Why can flies walk on the ceiling?
Because their feet are so formed that they can form a vacuum, under them; their bodies are therefore sustained in opposition to gravitation by atmospheric pressure.
664. How did Mr. Sands perform the feat of walking across the ceiling?
By having large discs of wet leather attached to his feet, so that when they were placed upon a smooth surface, the air was excluded, and when he allowed his weight to act upon one of the discs, it formed a hollow cup and a vacuum. By forming a vacuum of only twelve square inches he gained a pressure of 180 lbs.; this being more than his weight he could accomplish the feat with no other difficulty than that of remaining in an inverted position. The air was admitted underneath the discs by valves, which were closed by springs, which being pressed by the heels of the performer, let in the air, and set the feet free.
"And God made a wind to pass over the earth."—Genesis viii.
665. Why is it difficult to strike limpets from rocks?
Because they have the means of forming a vacuum under their shells, and are pressed on to the rocks by the weight of the atmosphere.
666. Why can snails move over plants in an inverted position?
Because they form a vacuum with the smooth and moist surfaces of their bodies, and are supported by atmospheric pressure.
CHAPTER XXXI.
Wind is air in motion. (See [234].)
667. What are the velocities of winds?
A breeze travels ten feet in a second; a light gale, sixteen feet in a second; a stiff gale, twenty-four feet in a second; a violent squall, thirty-five feet in a second; storm wind, from forty-three to fifty-four in a second; hurricane of the temperate zone, sixty feet in a second; hurricane of the torrid zone, one hundred and twenty to three hundred feet in a second. When wind flies at one mile an hour, it is scarcely perceptible. When its velocity is one hundred miles an hour, it tears up trees, and devastates its track.
Trade winds are vast currents of air, which sweep round the globe over a belt of some 12,000 miles in width.
"They shall be as the morning cloud, and as the early dew that passeth away, as the chaff that is driven with the whirlwind out of the floor, and as the smoke out of the chimney."—Hosea xiii.
669. What is the cause of trade winds?
The air over the tropical regions becomes heated and ascends; it then diverges in two high currents, one towards the north, and the other towards the south pole, where, being cooled, it again descends, and returns towards the equator to replace the air as it ascends therefrom. There is, therefore, a constant revolution of vast currents of air between the tropics and the poles, producing north and south winds.
670. Why do the trade winds blow from east to west, though, in their origin, their direction is from north to south and from south to north?
Because, as the north and south winds blow towards the equator, they are affected by the revolution of the earth from west to east. As the two winds from the poles approach the equator, they are gradually diverted from their northerly and southerly course, to an easterly direction, by the revolution of the earth.
671. Why is there a prevalence of calms at the equator?
Because, as the north and the south winds move towards the equator, they drive before them volumes of atmosphere, which, meeting in opposite directions, resist and counterpoise each other, and abide in a state of stillness between the north and south-easterly winds, one on the north and the other on the south of the equator.
Monsoons are periodical winds which blow at a given period of the year from one quarter of the compass, and in another period of the year from the opposite quarter of the compass.
673. What is the cause of monsoons?
Monsoons are caused by changes in the position of the sun. When the sun is in the southern hemisphere, it produces a north-east wind, and when it is in the northern hemisphere, a north-west wind. The north-east monsoon blows from November to March, and the south-west monsoon from the end of April to the middle of October. The region of monsoons lies a little to the north of the northern border of the trade wind, and they blow with the greatest force, and with most regularity, between the eastern coast of Africa and Hindustan.
"He shall blow upon them and they shall wither, and the whirlwind shall take them away as stubble."—Isaiah xl.
674. What determines the character of winds?
The character of winds is influenced by the condition of the surfaces over which they blow. Winds blowing over dry and arid plains and deserts are dry and hot. Winds blowing across snow-capped mountains and regions of ice are cold. Winds that cross oceans are wet; and those that cross extensive continents are dry.
675. What winds are most prevalent in England?
In England out of a thousand days, north winds prevail in 82; north-east, 111; east, 99; south-east, 81; south, 111; south-west, 225; west, 171; north-west, 120.
676. What is the cause of storms?
Storms result from violent commotions of the atmosphere, and are chiefly the result of extreme changes of temperature.
The magnetic state of the earth, and the electrical state of the atmosphere, also materially influence the phenomena of storms.
By some persons the theory is entertained that storms result from various winds rushing into a centre in which the atmosphere has become extremely condensed. According to this theory, a storm is a mighty whirlwind.
A most violent hurricane occurred in 1780, which destroyed Lord Rodney's fleet, and a vast number of merchant ships. It is said to have killed 9,000 persons in Martinique alone, and 6,000 in St. Lucia. The town of St. Pierre in Martinique was totally destroyed; and only fourteen houses in the town of Kingston, in St. Vincent, were left uninjured.
677. Why do the most violent storms occur in and near the tropics?
Because there the temperature is very high, and the cold currents of air rushing towards the equator from the poles, causes great atmospheric disturbance.
Whirlwinds are produced by violent and contrary currents meeting and striking upon each other, producing a circular motion. They generally occur after long calms, attended by much heat.
Whirlwinds occurring at sea, or over the surface of water, sometimes put the water in motion, and as the wind rises upwards it lifts with it a whirling mass of water, producing a water spout.
"Out of the south cometh the whirlwind; and cold out of the north."—Job xxxvii.
Fig. 19.—A WATER SPOUT.
679. Why does the chimney smoke when the fire is first lighted?
Because the air in the chimney is of the same temperature as that in the room, and therefore will not ascend.
680. Why does the smoking (into the room) cease, after the fire has been lighted a little while?
Because the air in the chimney, being warmed by the fire beneath, becomes lighter and ascends rapidly.
681. Why does a long chimney create a greater draught than a short one?
Because the short chimney contains less air than the long one; there is, consequently, less difference of weight between the warm air of the short chimney and the external air; it therefore has not so great an ascensive power.
"And, lo, the smoke of the country went up as the smoke of a furnace."—Gen. xix.
682. Why does smoke issue in folds and curls?
Because it is pressed upon by the cold air which always rushes towards a rarer atmosphere. It thus illustrates the development of storms.
683. Why do some chimneys smoke when the doors and windows are closed?
Because the draught of air is not sufficient to supply the wants of the fire, and enable it to create an upward current.
684. What is the best method of conveying air to fires?
Tubes built in the walls, communicating with the outer air, and terminating underneath the grates.
685. Why is this the best method of ventilation?
Because doors and windows may then be made air-tight, and draughts across rooms be prevented.
686. Why do chimneys that stand under elevated objects, such as hills, trees, and high buildings, smoke?
Because the wind, striking against the elevated object, flies back, and a part of it rushes downward.
687. Why do sooty chimneys smoke?
Because the accumulation of the soot diminishes the size of the flue, and lessens the ascensive power of the draught, by reducing the quantity of warm air. It also obstructs the motion of the air, by the roughness of its surface.
688. Why do chimneys smoke in damp and gusty weather?
Because the ascending air is suddenly chilled by gusts of damp and cold air, and driven down the chimney.
"Remember that thou magnify his work, which men behold. Every man may see it; man may behold it afar off."—Job xxxvi.
689. Why does smoke ascend in a straight line in mild and fine weather?
Because the air is still, and being dry and warm it does not chill the smoke, nor drive it out of its course.
690. Why do the wings of wind-mills turn round?
Because the wind, striking at an angle upon the wings, forces them aside; and as there are four wings all upon the same angle, and fixed upon the same centre, the oblique pressure of the wind causes the centre to rotate.
There is a world of miniature phenomena which has never been fully recognised, in which we may see the mightier works of nature pleasingly and truthfully illustrated.
When the wind blows into the corner of a street, and whirling around, catches straw, dust, and feathers in its arms, and then wheels away, flinging the troubled atoms in all directions,—it is a miniature of the mightier whirlwind, which wrecks ships, uproots trees, and levels houses with the earth.
When a cloud of dust, on a hot summer's day, rises and flies along the thirsty road, making the passenger close his eyelids, and dusting the leaves of wayside vegetation,—it is a miniature of the terrible simoom, which blows from the desert sands, scattering death and devastation in its track.
When steam issues from the tea-urn, and becomes condensed in minute drops upon the window-pane,—the miniature is of the earth's heat, evaporating the waters, and the cold air of night condensing the vapours into dew.
When grass and corn bend before the wind, and are beaten down by its force; when the pond forgets its calm, and rises in troubled waves, casting the flotilla of natural boats that move upon its surface, in rude disorder upon its windward shore,—the little storm is but a miniature of those great hurricanes which wrecked a fleet in the Black Sea, and levelled the encampments of a mighty army.
When the snow that has gathered upon the house-top, warming beneath the smiles of the sun, slips from its bed, and drops in accumulated heaps from the roof,—it is a miniature of those terrible avalanches which in the Pyrenees bury villages in their icy pall, and doom man and beast to death.
When the rivulet hurries on its course, and meeting with obstructions, leaps over them in mimic wrath, overturning some little raft upon which, perchance, a weary fly has alighted,—it is a miniature of those rapids on whose banks the hippopotamus and the alligator yet live; and where, though rarely, man may be seen directing his raft over the troubled current, amid the rush of debris from forests unexplored.
And when, in a basin of the rivulet, two opposing currents meet, and form a little vortex into which insect life and vegetable fragments coming within the sphere of its influence are drawn,—it is a miniature of the roaring whirlpool, or the wilder maelstrom of the Norwegian seas.
Nature rehearses all her parts in mild whispers; and for every picture that she paints, she places a first study upon the canvas. Man need not go into the heart of her terrors to understand their laws. Many an unknown Humboldt, sitting by the river's side, may rejoice in the "aspects of nature," and share the bliss of knowledge with the great philosopher.
"Can any understand the spreadings of the clouds, or the noise of his tabernacle?"—Job xxxvi.
CHAPTER XXXII.
A barometer is an instrument which indicates the pressure of the atmosphere, and which takes its name from two Greek words signifying measurer of weight.
692. Why does a barometer indicate the pressure of the atmosphere?
Because it consists of a tube containing quicksilver, closed at one end and open at the other, so that the pressure of the air upon the open end balances the weight of the column of mercury (quicksilver), and when the pressure of the air upon the open surface of the mercury increases or decreases, the mercury rises or falls in response thereto.
693. Why is a barometer called also a "weather-glass"?
Because changes in the weather are generally preceded by alterations in the atmospheric pressure. But we cannot perceive those changes as they gradually occur; the alteration in the height of the column of mercury, therefore, enables us to know that atmospheric changes are taking place, and, by observation, we are enabled to determine certain rules by which the state of the weather may be foretold with considerable probability.
694. Why are barometers constructed with circular dials, and an index to denote changes?
Because that is a convenient mechanical arrangement, by which the alterations of the relative pressures of the air and the mercury are more clearly denoted than by an inspection of the mercury itself.
"Fair weather cometh out of the north: with God is terrible majesty."—Job xxxvii.
Fig. 20.—BAROMETER.
Fig. 21.—TUBE OF BAROMETER, WHEEL, AND PULLEY.
695. Why does the hand of the weather dial change its position when the column of mercury rises or falls?
Because a weight, which floats upon the open surface of the mercury, is attached to a string, having a nearly equal weight at the other extremity; the string is laid over a revolving pivot to which the hand is fixed, and the friction of the string turns the hand, as the mercury rises or falls.
"Thou visitest the earth, and waterest it: thou greatly enrichest it with the river of God, which is full of water: thou preparest them corn, when thou hast so provided for it."—Psalm lxv.
696. Why does tapping the face of the barometer sometimes cause the hand to move?
Because the weight on the surface of the mercury frequently leans against the sides of the tube, and does not move freely. And, also, the mercury clings to the sides of the tube by capillary attraction; therefore, tapping on the face of the barometer sets the weight free, and overcomes the attraction which impedes the rise or fall of the mercury.
[Fig. 21] illustrates the mechanism at the back of the barometer. A is a glass tube; between A and E there exists a vacuum, caused by the weight of the mercury pressing downwards. This space being a vacuum, makes the barometrical column more sensitive, as there is no internal force to resist or modify the effects of the external pressure. E represents the height of the column of mercury; C the open end of the tube; F the weight resting on the surface of the mercury; P the pivot over which the string passes, and upon which the hand turns; W the weight which forms the pulley with the weight F.
![[Fig. 21]](#i-158b.jpg){kind=link}
697. Which is the heavier, dry or vaporised air?
Dry air is heavier than air impregnated with vapours.
698. Why is dry air heavier than moist air?
Because of the extreme tenuity of watery vapours, the density of which is less than that of atmospheric air.
699. Why does the fall of the barometer denote the approach of rain?
Because it shows that as the air cannot support the full weight of the column of mercury, the atmosphere must be thin with watery vapours.
The fall of the mercury in the long arm of the tube would cause the weight F to be pressed upwards. This would release the string to which the weight W is attached; it would, therefore, fall, and turn the hand down to Rain or Much Rain.
700. Why does the rise of the barometer denote the approach of fine weather?
Because the external air becoming dense, and free from highly elastic vapours, presses with increased force upon the mercury upon which the weight F floats; that weight, therefore, sinks in the short tube as the mercury rises in the long one, and in sinking turns the hand to Change, Fair, &c.
"He caused an east wind to blow in the heaven; and by his power he brought in the south wind."—Psalm lxxviii.
701. Why does the barometer enable us to calculate the height of mountains?
Because, as the barometer is carried up a mountain, there is a less depth of atmosphere above to press upon the mercury; it therefore falls, and by comparing various observations, it has been found practicable to calculate the height of mountains by the fall of the mercury in a barometer.
702. To what extent of variation is the weight of the atmosphere liable?
It may vary as much as a pound and a half to the square inch at the level of the sea.
703. When does the barometer stand highest?
When there is a duration of frost, or when north-easterly winds prevail.
704. Why does the barometer stand highest at these times?
Because the atmosphere is exceedingly dry and dense, and fully balances the weight of the column of mercury.
705. When does the barometer stand lowest?
When a thaw follows a long frost; or when south-west winds prevail.
706. Why does the barometer stand lowest at those times?
Because much moisture exists in the air, by which it is rendered less dense and heavy.
707. What effect has heat upon the barometer?
It causes the mercury to fall, by evaporating moisture into the air.
708. What effect has cold upon the barometer?
It causes the mercury to rise, by checking evaporation, and increasing the density of the air.
"For so the Lord said unto me, I will take my rest, and I will consider in my dwelling place like a clear heat upon herbs, and like a cloud of dew in the heat of harvest."—Isaiah xviii.
In noting barometrical indications, more attention should be paid to the tendency of the mercury at the time of the observation, than to the actual state of the column, whether it stands high or low. The following rules of barometric reading are given as generally accurate, but liable to exceptions:—
Fair weather indicated by the rise of the mercury.
Foul weather by the fall of the mercury.
Thunder, indicated by the fall of the mercury in sultry weather.
Cold, indicated by the rise of the mercury in spring, autumn, and winter.
Heat, by the fall of the mercury in summer and autumn.
Frost, indicated by the rise of the mercury in winter.
Thaw, by the fall of the mercury during a frost.
Continued bad weather, when the fall of the mercury has been gradual through several fine days.
Continued fine weather, when the rise of the mercury has been gradual through several foul days.
Bad weather of short duration, when it sets in quickly.
Fine weather of short duration, when it sets in quickly.
Changeable weather, when an extreme change has suddenly set in.
Wind, indicated by a rapid rise or fall unattended by a change of temperature.
The mercury rising, and the air becoming cooler, promises fine weather; but the mercury rising, and the air becoming warmer, the weather will be changeable.
If the top of the column of mercury appears convex, or curved upwards, it is an additional proof that the mercury is rising. Expect fine weather.
If the top of the column is concave, or curved downwards, it is an additional proof that the mercury is falling. Expect bad weather.
CHAPTER XXXIII.
The thermometer is an instrument in which mercury is employed to indicate degrees of heat. Its name is derived from two Greek words, meaning heat measurer.
710. Why does mercury indicate degrees of heat?
Because it expands readily with heat, and contracts with cold; and as it passes freely through small tubes, it is the most convenient medium for indicating changes of temperature.
"When ye see a cloud rise out of the west straightway ye say, There cometh a shower; and so it is. And when ye see the south wind blow, ye say there will be heat; and it cometh to pass."—Luke xiii.
711. Why are there Reaumur's Thermometers and Fahrenheit's Thermometers?
Because their inventors, after whom they are named, adopted a different system of notation, or thermometrical marks; and as their thermometers have been adopted by various countries and authors, it is now difficult to dispense with either of them.
Fig. 22.—THE THERMOMETERS OF REAUMUR AND FAHRENHEIT COMPARED.
We have combined the two (see [fig. 22].) The diagram will, we have no doubt, prove exceedingly useful to scientific readers and experimentalists. There is also another system of notation, adopted by the French, called the centigrade, but it is not much referred to in Great Britain. In the centigrade thermometer 0 zero is the freezing point, and 100 the boiling point. Fahrenheit's scale is generally preferred. Reaumur's is mostly used in Germany. Of Fahrenheit's scale 32 is the freezing point, 55 is moderate heat, 76 summer heat in Great Britain, 98 is blood heat, and 212 is the boiling point. Mr. Wedgwood has invented a thermometer for testing high temperatures, each degree of which answers to l30 degrees of Fahrenheit. According to his scale cast iron melts at 2,786 deg.; fine gold at 2,016 deg.; fine silver 1,873 deg.; brass melts at 1,869 deg.; red heat is visible by day at 980 deg.; lead melts 612 deg.; bismuth melts 476 deg.; tin melts 412 deg.; and there is a curious fact with regard to the three metals, lead, bismuth, and tin, that if they are mixed in the proportions of 5, 8, and 3 parts respectively, the mixture (after previous fusion) will melt at a heat below that of boiling water.
![[fig. 22]](#i-162.jpg){kind=link}
712. What is the difference between the thermometer and the barometer?
In the thermometer the column of mercury is much smaller than in the barometer, and is sealed from the air; while in the barometer the column of mercury is open at one end to atmospheric influence.
713. Why does the mercury in the thermometer, being sealed up, indicate the external temperature?
Because the heat passes through the glass, in which the mercury is enclosed, and expanding or contracting the metal within the bulb, causes the small column above it to rise or fall.
"Blessed is the people that know the joyful sound: they shall walk, O Lord, in the light of thy countenance."—Psalm lxxxix.
714. When does the thermometer vary most in its indication of natural temperature?
It varies more in the winter than in the summer season.
715. Why does it vary more in the winter than in the summer?
Because the temperature of our climate differs more from the temperature of the torrid zones in the winter than it does in the summer, and the inequalities of temperature cause frequent changes in the degree of prevailing heat.
The same remarks (714, 715,) apply to the barometer.
CHAPTER XXXIV.
Sound is an impression produced upon the ear by vibrations of the air.
717. What causes the air to vibrate and produce sounds?
The atoms of elastic bodies being caused to vibrate by the application of some kind of force, the vibrations of those atoms are imparted to the air, and sound is produced.
718. How do we know that sounds are produced by the vibrations of the air, induced by the vibrations of the atoms of bodies?
If we take a tuning fork, and hold it to the ear, we hear no sound. If we move it rapidly through the air, or if we blow upon it, it produces no sound; but if we strike it, a sound immediately occurs; the vibration of the fork may be seen, and felt by the hand that holds it; and as those vibrations cease, the sound dies away.
719. How do we know that without air there would be no sound?
Because if a tuning fork were to be struck in a vacuum (as under the receiver of an air pump) no sound would be heard, although the vibrations of the fork could be distinctly seen.
"And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped."—Corinth. xiv.
720. How are the vibrations of sonorous bodies imparted to the air?
When a bell is struck, the force of the blow gives an instant agitation to all its particles. The air around the bell is driven back by the impulse of the force, and thus a vibration of compression is imparted to the air; but the air returns to the bell, by its own natural elasticity, thus producing a vibration of expansion—when it is again struck, and thus successive vibrations of compression and expansion are transmitted through the air.
721. How rapidly are these vibrations transmitted through the air?
They travel at a rate of rather more than a quarter of a mile in a second, or twelve miles and three-fourths in a minute.
722. Do all sounds travel at the same rate?
All sounds, whether strong or weak, high or low, musical or discordant, travel with the same velocity.
723. Why are bells and glasses stopped from ringing by touching them with the finger?
Because the contact of the finger stops the vibration of the atoms of the metal and glass, which therefore cease to impart vibrations to the air.
724. Why does a cracked bell give discordant sounds?
Because the connection between the atoms of the bell being broken, their vibrations are not uniform: some of the atoms vibrate more intensely than the others; the vibrations imparted to the air are therefore jarring and discordant.
725. Why, when we see a gun fired at a distance, do we see the flash and smoke, before we hear the report?
Because light, which enables us to see, travels at the velocity of 192,000 miles in a second; while sound, by which we hear, travels only at the rate of a quarter of a mile in a second.
"My heart maketh a noise in me: I cannot hold my peace, because thou hast heard, O my soul, the sound of the trumpet, the alarm of war."—Jer. iv.
726. Why does the tread of soldiers, when marching in long ranks, appear to be irregular?
Because the sounds proceeding from different distances, reach our ears in varying periods of time.
727. What are the numbers of vibrations in a second that produce the various musical sounds?
C or Do, 480 vibrations in a second; B or Si, 450 vibrations; A or La, 400 vibrations; G or Sol, 360 vibrations; F or Fa, 320 vibrations; E or Mi, 300 vibrations; D or Re, 270 vibrations; C or Do, 240 vibrations. It is thus seen that the more rapid the vibrations, the higher the note, and vice versa.
728. Why does the length of a wire or string determine the sound that it produces?
Because the shorter the string the more rapid are its vibrations when struck.
729. Why does the tension of a wire or string affect its vibrations?
Because when the string or wire is tight, a touch communicates vibrations to all its particles; but when it is loose the vibrations are imperfectly communicated.
730. Why are some notes low and solemn, and others high and quick?
Because the vibrations of musical strings vary from 32 vibrations in a second, which produces a soft and deep bass, to 15,000 vibrations in a second, which produces the sharpest treble note.
731. Why can our voices be heard at a greater distance when we speak through tubes?
Because the vibrations are confined to the air within the tube, and are not interfered with by other vibrations or movements in the air; the tube itself is also a good conductor of sound.
"And I will cause the noise of thy songs to cease; and the sound of thy harps shall no more be heard."—Ezekiel xxvi.
732. Is air a good conductor of sound?
Air is a good conductor, but water is a better conductor than air; wood, metals, the earth, &c., are also good conductors.
733. Why can we hear sounds at a greater distance on water than on land?
For various reasons: because the smooth surface of water is a good conductor; because there are fewer noises, or counter vibrations, to interfere with the transmission of sound; and because there are no elevated objects to impede the progress of the vibrations.
734. Why do sea-shells give a murmuring noise when held to the ear?
Because what may be called expended vibrations always exist in air where various sounds are occurring. These tremblings of the air are received upon the thin covering of the shell, and thus being collected into a focus, are transmitted to the ear.
735. Why can people in the arctic regions converse when more than a mile apart?
Because there the air, being cold and dense, is a very good conductor; and the smooth surface of the ice also favours the transmission of sound.
736. Why do savages lay their heads upon the earth to hear the sounds of wild beasts, &c.?
Because the earth is a good conductor of sound. For this reason, also, persons working under ground in mines can hear each other digging at considerable distances.
737. Why can church clocks be heard striking much more clearly at some times than at others?
Because the density of dry air improves the sound-conducting power of the atmosphere. The transmission of sounds is also assisted by the direction of the winds.
"The morning is come unto thee, O thou that dwellest in the land: the time is come, the day of trouble is near, and not the sounding again of the mountains."—Ezekiel vii.
738. Why may the scratching of a pin at one extremity of a long pole be heard by applying the ear to the opposite extremity?
Because wood is a good conductor of sound, and its atoms are susceptible of considerable vibration. It is, therefore, chosen in numerous instances for the construction of musical instruments.
Deaf persons have been known to derive pleasure from music by placing their hands upon the wood-work of musical instruments while being played upon.
739. Why is the hearing of deaf persons assisted by ear-trumpets?
Because ear-trumpets collect the vibrations of the air into a focus, and make the sounds produced thereby more intense.
740. Why are sounding-hoards used to improve the hearing of congregations?
Because, being suspended over, and a little behind, the speaker, they collect the vibrations of the air, and reflect them towards the congregation.
Echoes are sounds reflected by the objects on which they strike.
742. Why do some echoes occur immediately after a sound?
Because the reflecting surface is very near; therefore the sound returns immediately.
743. Why do some echoes occur a considerable time after a sound?
Because they are at a considerable distance, and the sound takes time to travel to it, and an equal time to return.
744. Why do some echoes change the tone and quality of sound?
Because the reflecting surface, having vibratory qualities of its own, mingles its own vibrations with that of the sound.
745. Why are there sometimes several echoes to one sound?
Because there are various reflecting surfaces, at different distances, each of which returns an echo.
"And God said, Let the waters under the heaven be gathered together onto one place, and let the dry land appear: and it was so."—Gen. i.
746. Are sounds reflected only by distant objects?
Sounds are doubtless reflected by walls and ceilings around us. But we do not perceive the echoes, because they are so near that they occur at the same moment with the sound. In lofty buildings, however, there is frequently a double sound, making the utterance of a speaker indistinct. This arises from the echo following very closely upon the sound.
747. Why, when we are walking under an arch-way or a tunnel, do our voices appear louder?
Because the sounds of our voices are immediately reflected. And as a gas reflector increases the intensity of light, so a sound reflector will increase the apparent strength of our voices.
There are many places where remarkable echoes occur. On the banks of the Rhine, at Lurley, if the weather be favourable, the report of a rifle, or the sound of a trumpet, will be repeated at different periods, and with various degrees of strength, from crag to crag, on opposite sides of the river alternately. A similar effect is heard in the neighbourhood of some of the Lochs in Scotland. There is a place at Woodstock, in Gloucestershire, which is said to echo a sound fifty times. Near Rosneath, a few miles from Glasgow, there is a spot where, if a person plays a bar of music upon a bugle, the notes will be repeated by an echo, but a third lower; after a short pause, another echo is heard, again in a lower tone; then follows another pause, and a third repetition follows in a still lower key. The effect is very enchanting. The whispering galleries of St. Paul's, of the cathedral church of Gloucester, and of the Observatory of Paris, owe their curious effects to those laws of the reflection of sound, by which echoes are produced; but in these cases the effect is assisted by the elliptical form of the edifice, each person being in the focus of an ellipse.
CHAPTER XXXV.
Water is a fluid composed of two volumes of hydrogen to one of oxygen, or eight parts by weight of oxygen to one of hydrogen. It is nearly colourless and transparent.
749. Why, if a saucer of water be exposed to the air, will it gradually disappear?
Because water is highly expansive, and rises in thin vapour, when in contact with warm and dry air.
"Behold there ariseth a little cloud from the sea, of the bigness of a man's hand. And it came to pass in the meantime, that the heaven was black with clouds and wind, and there was a great rain."—1 Kings xviii.
750. Why does steam issue from the spout of a kettle?
Because the heat of the fire passes into the water, and drives its atoms apart, making those of them that rise quickly to the surface lighter than the air, upon which they consequently rise.
751. Why does water become solid when it freezes?
Because the latent heat of the water passes away from between its atoms into the air; the atoms, therefore, draw closer together.
752. Why, if the atoms of water draw closer together when freezing, does ice expand, and occupy greater space than water?
Because, when the atoms of water are congealing, they do not form a compact mass, but arrange themselves in groups of crystal points, which occupy greater space. Water contracts when freezing until it sinks to 40 deg., and then it expands as ice is formed.
32 deg. is said to be the freezing point, but it should be called the frozen point.
Because heat, entering into the lower portions of the water, expands it; the heated portions are then specifically lighter than those that are cooler; the hot water therefore rises upward, and forces the cooler water down.
754. What proportion of the earth's surface is covered with water?
There are about one hundred and forty seven millions of square miles of water, to forty-nine and a half millions of square miles of land.
755. What is the amount of water pressure?
The pressure of the sea, at the depth of 1,100 yards, is equal to 15,000 lbs. to the square inch.
"But the land, whither ye go to possess it, is a land of hills and valleys, and drinketh water of the rain of heaven."—Deut. xi.
756. What element is the most abundant in nature?
Oxygen, which forms so large a part of water. Of animal substances, oxygen forms three-fourths; of vegetable substances it forms four-fifths; of mineral substances it forms one-half; it forms eight-ninths of the waters and one-fifth of the atmosphere; and aggregating the whole creation, from one-half to two-thirds consists of oxygen.
757. In what ways does man use oxygen?
Man eats, drinks, breathes, and burns it, in various proportions and combinations. It is estimated that the human race consume in those various ways 1,000,000,000 lbs. daily; that the lower animals consume double that amount; and that, in the varied works of nature, no less than 8,000,000,000 lbs. of oxygen are used daily.
758. Why does water dissolve various substances?
Because the atoms of water are very minute; they therefore permeate the pores, or spaces, between the atoms of those bodies, and overcoming their attraction for each other, cause them to separate.
759. Why does hot water dissolve substances more readily than cold?
Because the heat assists to repel the particles of the substance undergoing solution, and gives the water a freer passage between the atoms.
760. Why is pump water sometimes hard?
Because, in passing through the earth, it has become impregnated with mineral matters, usually the sulphate and carbonate of lime.
Because it is derived from vapours which, in ascending to the clouds, could not bear up the mineral waters with them. It therefore became purified or distilled.
762. Why do kettles become encrusted with stony deposits?
Because that portion of the water which is driven off in steam leaves the mineral matters behind; they therefore form a crust around the sides of the kettle.
It is said that if a child's marble be placed in a kettle, it will attract the earthy particles, and prevent the encrusting of the sides of the vessel.
"He gathereth the waters of the sea together as an heap; he layeth up the depth in storehouses."—Psalm xxxiii.
763. Why is it difficult to wash in hard water?
Because the soap unites with the mineral matters in the water, and being neutralised thereby, cannot dissolve the dirt which we desire to cleanse away.
Because salt is a mineral which prevails largely in the earth, and which, being very soluble in water, is taken up by the ocean.
Lakes and rivers, also, even those that are considered fresh, hold in solution some degree of saline matters, which they contribute to the ocean.
As, in the evaporations from the sea, the salt remains in it, while the vapours fall as rain, and again wash the earth and carry some of its mineral properties to the ocean, the greater saltness of the sea, as compared with rivers, is accounted for.
By some persons the opinion is entertained that the sea has been gradually getting salter ever since the creation of the world. This, they say, arises from the evaporation of water free from salt, and the returns of the water to the sea, taking with it salt from the land.
765. What is the estimated amount of salt in the sea?
The amount of common salt in the various oceans is estimated at 3,051,342 cubic geographical miles, or about five times more than the mass of the mountains of the Alps.
766. What is the depth of the sea?
The extreme depth has not, probably, been ascertained. But Sir James Ross took soundings about 900 miles west of St. Helena, whence he found the sea to be nearly six miles in depth. Now, if we take the height of the highest mountain to be five miles, the distance from that extreme rise of the earth, to the known depth of the sea, will be no less than eleven miles.
767. Why are the waters of some springs impregnated with mineral matters?
Because the water passes through beds of soda, lime, magnesia, carbonic acid, oxides of iron, sulphate of iron, &c., &c., and takes up in some slight degree the particles of those minerals, according to the proportions in which they abound.
"Who hath measured the waters in the hollow of his hand, and meted out heaven with the span, and comprehended the dust of the earth in a measure and weighed the mountains in scales, and the hills in a balance?"—Isaiah xl.
768. Why does iron rust rapidly when wetted?
Because the water contains a large proportion of oxygen, some of which combines with the iron and forms an oxide of iron, which is rust.
769. Why does stagnant water become putrid?
Because the large amount of oxygen which it contains accelerates the decomposition of dead animal and vegetable substances that accumulate in it.
770. Is there danger in drinking water on account of the living animalcules which it contains?
No danger arises from the living creatures in water; but putrefactive matters may produce serious diseases.
771. What is the best method of guarding against impurities?
By obtaining water from the purest sources, and by filtering it before drinking, by which nearly all extraneous matters would be separated from it.
CHAPTER XXXVI.
Attraction is the tendency of bodies to draw near to each other. It is called attraction, from two Latin words signifying drawing towards.
773. How many kinds of attraction are there?
There are five principal kinds of attraction:—
1. The attraction of gravitation.
2. The attraction of cohesion.
3. The attraction of chemical affinity.
4. The attraction of electricity.
5. And capillary attraction.
"Behold, the nations are as a drop of a bucket, and are counted as the small dust of the balance: behold, he taketh up the isles as a very little thing."—Isaiah xl.
774. Why do all bodies heavier than the air fall to the earth?
Because they are influenced by the attraction of gravitation, by which all bodies are drawn towards the centre of the earth.
775. Why do bodies lighter than the air ascend?
Because the air, being a denser body, obeys the law of attraction, and in doing so displaces lighter bodies that interfere with its gravitation.
776. Why do fragments of tea, and bubbles floating upon the surface of tea, draw towards each other, and attach themselves to the sides of the cup?
Because they are influenced by the attraction of cohesion.
Cohesion.—The act of sticking together.
777. Why will a drop of water upon the blade of a knife leave a dark spot?
Because the iron of the knife attracts the oxygen of the water, by chemical affinity; and the two substances form a thin coating of oxide of iron.
Affinity.—Attraction between dissimilar particles through which they form new compounds.
778. Why do clouds sometimes move towards each other from opposite directions? and
779. Why do light particles of matter attach themselves to sealing wax, excited by friction?
Because they are moved by the attraction of electricity.
780. Why will a towel, the corner of which is dipped in water, become wet far above the water?
Because the water is conveyed up through the towel, by capillary attraction. The atoms of the water are attracted by the threads of the towel, and drawn up into the small spaces between the threads.
Capillary.—Resembling a hair, small in diameter.
"He stretcheth out the north over the empty place, and hangeth the earth upon nothing."—Job xxvi.
781. Why do small bodies floating upon water move towards larger ones?
Because the attractive power of a large body is greater than that of a small one. As each atom of matter has inherent power of attraction, it follows that a large aggregation of particles must attract in proportion to the number of those particles.
782. Why do clouds gather around mountain tops?
Because they are attracted by the mountains.
783. Why would a piece of lead tied to a string, and let down from a church steeple, incline a little from the perpendicular towards the church?
Because the masses of stone of which the church is built would attract the lead.
784. How can man weigh the earth?
By observing what is called the deflection of small bodies when brought within given distances of larger bodies, the degree of attraction exercised by the large body upon the smaller one becomes known. This attraction of the large body exercised over the smaller body is an opposing influence, acting against the earth's attraction of the small body, which is drawn out of its course: it constitutes a natural balance between the influence of the earth and another body, acting in opposition to it. Founded upon these, and some other data, man can weigh the earth, and give a morally certain result!
Deflection.—The act of turning aside.
785. How can man weigh the planets?
The planets exercise as certain an influence upon each other as do two pieces of wood floating upon a basin of water. As the planetary bodies fly through their prescribed orbits, and approach nearer to, or travel further from, each other, they are observed to deviate from that course which they must have pursued but for the increase or the decrease of some influence of attraction. By making observations at various times, and by comparing a number of results, it is possible to weigh any planetary body, however vast, or however distant.
"Is not God in the height of the heaven? and behold the height of the stars, how high they are?"—Job xii.
786. How can man measure the distances of the planets?
By making observations at different seasons of the year, when the earth is in opposite positions in her orbit; and by recording, by instruments constructed with the greatest nicety, the angle of sight, at which the planetary body is viewed; by noticing, also, the various eclipses, and estimating how long the first light after an eclipse has ceased reaches the earth, it is possible to estimate the distances of heavenly bodies, no matter how far in the depths of the universe those orbs may be.
787. What are the opinions founded upon estimates respecting the magnitude of the sun?
The diameter of the sun is 770,800 geographical miles, or 112 times greater than the diameter of the earth; its volume is 1,407,124 times that of the earth, and 600 times greater than all the planets together; its mass is 359,551 times greater than the earth; and 738 times greater than that of all the planets. A single spot seen upon its surface has been estimated to extend over 77,000 miles in diameter, and a cluster of spots have been estimated to include an area of 3,780,000 miles.
788. What is the weight of the earth?
The earth has a circumference of 25,000 miles, and is estimated to weigh 1,256,195,670,000,000,000,000,000 tons.
789. What is the specific gravity of a body?
It is its weight estimated relatively to the weights of other bodies.
790. What determines the force with which bodies fall to the earth?
Generally speaking, their specific gravity, which is proportionate to the density, or compactness of the atoms of which they are composed.
791. Why does a feather fall to the earth more gradually than a shilling?
Because the specific gravity of the feather and of the shilling is relative to that of the air, the medium through which the feather and the shilling pass. If there were no air, a shilling and a feather dropped at the same time from a height of forty miles, would reach the earth at the same moment.
CHAPTER XXXVII.
"Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding."
Repulsion is that property in matter by which it repels or recedes from, those bodies for which it has no attraction or affinity.
793. Why does dew form into round drops upon the leaves of plants?
Because it repels the air, and the substances of the leaves upon which it rests. Because, also, its own particles cohere.
794. Why do drops of water roll over dusty surfaces?
Because they repel the particles of dust; and also because their own particles have a stronger attraction for each other than for the particles of dust.
795. Why does a needle float when carefully laid upon the surface of water?
Because the needle and the water mutually repel each other.
796. Why does water, when dropped upon hot iron, move about in agitated globules?
Because the caloric repels the particles of the water.
797. Why does oil float upon the surface of water?
Because, besides being specially lighter than water, the particles of the oil and the water mutually repel each other.
Carbonic acid is a mixture of carbon and oxygen, in the proportion of 3 lbs. of carbon to 8 lbs. of oxygen.
"Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?"
799. Where does carbonic acid chiefly exist?
It exists in various natural bodies in which carbon and oxygen are combined; it is evolved by the decomposition of numerous bodies called carbonates, in which carbon is united with a particular base, such as the carbonate of lime, the carbonate of iron, the carbonate of copper, &c. It is also evolved by the processes of fermentation, by the breathing of animals, the combustion of fuel, and the functions of plants. Carbonic acid also exists in various waters.
Carbonic acid is found most largely in solid combinations with other bodies: it forms 44-100ths of all limestones and marbles, and it exists in smaller quantity, combined with other earths, and with metallic oxides.
800. What are the states in which pure carbonic acid exists?
Pure carbonic acid may exist in the solid, the liquid, or the æriform state. In the solid state it is produced only by artificial means, and it is then a white crystallised body, in appearance like snow; in the liquid state it is a heavy colourless fluid; in the æriform state it is a pungent, heavy, colourless gas, and is known as carbonic acid gas.
801. Why does bottled porter produce large volumes of froth, much more than the bottle could contain?
Because, by the fermentive process, carbonic acid has been developed in the porter, and is held in liquid solution; but it always has a strong tendency to escape, and directly the pressure is removed, it evolves into gas, by which it occupies much greater space, and forces the porter in millions of small bubbles out of the bottle.
802. Why does soda-water effervesce?
Because carbonic acid gas is forced into the water by pressure. Pressure alters the gas into a liquid, and directly the pressure ceases, the liquid again evolves into gas.
803. Why does spring water taste fresh and invigorating?
Because it contains carbonic acid.
"Whereupon are the foundations thereof fastened? or who laid the cornerstone thereof."—Job xxxviii.
804. Why does boiled water taste flat and insipid?
Because the carbonic acid has been driven off by boiling.
805. Why does beer which has been standing in a glass taste flat?
Because its carbonic acid has escaped as carbonic acid gas.
806. Why, when we look into a glass of champagne, do we see bubbles spontaneously appear at the bottom, and then rise to the top?
Because, in the places where the bubbles are formed, the liquid carbonic acid is evolving into carbonic acid gas.
807. Why do the bubbles arise from two or three points in columns, rapidly succeeding each other?
Because, when the formation of gas once begins, and bubbles ascend, there is less pressure in the line of the column of bubbles; the carbonic acid, therefore, draws towards those points as the easiest channel of escape.
These explanations equally apply to the "working" of beer, by which yeast is produced; to the effervescence of various waters, acidulated drinks, ginger beer, &c., and also to the "sponging" of bread, &c.
808. Why does gunpowder explode?
Gunpowder is made of a very intimate mechanical mixture of nitrate of potash, charcoal, and sulphur. When these substances are heated to a certain degree, the nitrate of potash is decomposed, and its oxygen combines with the charcoal and sulphur, instantaneously forming large volumes of carbonic acid gas and nitrogen, which, seeking an escape, produce an explosion.
"Thus saith the Lord, Let not the wise man glory in his wisdom, neither let the mighty man glory in his might, let not the rich man glory in his riches."—Jeremiah ix.
809. Why does charcoal act as a powerful disinfectant?
Because the carbon readily absorbs, and combines with various gases, neutralising their offensive odours, and destroying their unhealthy properties.
Let us now pause for a few moments to consider the importance of those two great divisions of nature, Air and Water, and to reflect upon the wisdom of some of those laws which are connected with the phenomena thereof, and which have not yet been sufficiently explained.
We have seen that the air is a thin elastic body surrounding the globe; that it consists of certain gases essential to the life of animals, and to the growth of plants; and that it takes part in most of those chemical changes, which mark the transformations of the inorganic creation. Whether it be the burning of a piece of wood, the evaporation of a drop of water, the breathing of an animal, the respiration of a plant, or the fermentation of bodies, the air in almost every instance gives or receives—and in most of the operations in which it engages, it does both.
But there is one point of view, which we must add to those which have already been considered: the order of nature consists of generation, life, and death. Every beat of the watch signals the birth of millions of living things, and the same beat proclaims that as many living organisms have yielded up their vital spark, and that forthwith the elements of which they are composed must be dissolved, and restored to the great laboratory of nature.
The air is the vast receptacle of those organic matters which are undergoing dissolution. The body of the shipwrecked mariner, cast upon the shore of a desolate island, blackens in the sun, and the full round form gradually dwindles to skin and bone, until at last the few atoms that remain crumble into dust, and are scattered to the wind. The same process occurs, with some modifications, whether bodies are buried in the earth, or dissolve upon its surface. The leaves of forests fall and accumulate in heaps, where they ferment and dissolve, leaving only their more earthy particles behind.
The amount of matter which day by day passes from the state of the living to that of the dead, must be enormous; but from the difficulties of acquiring data, beyond the possibility of calculation. Such statistics as we have, however, enable us to form conclusions as to the mighty agencies in which the air is constantly engaged. There are on the earth 1,000,000,000 inhabitants of whom nearly 35,000,000 die every year, 91,824 every day, 3,730 every hour, and 60 every minute. But even the living die daily, and undergo an invisible change of substance, as we shall hereafter explain.
The bodies of those many millions are dissolved in the air, in vapours and gases which, before the dissolution of each corporeal organism is complete, begin to live again in the various forms of vegetable and animal life.
Of the number of animals living and dying upon the face of the earth, we can form no adequate estimate. Of mammals there are about 2,000 ascertained species; of birds 8,000 species; of reptiles 2,000 species; of fishes some 8,000 or 10,000 species; of molluscs some 15,000 species; of shell fish 8,000 species; of insects 70,000 species. And, including others not specified here, the total number of species of animals probably amounts to no less than 250,000,—each species consisting of many millions of living creatures.
In the area of London alone, no less than 200,000 tons of fuel are annually cast into the air in the form of smoke. And if we take into account the vast operations of nature in evaporation, fermentation, and putrefactive decomposition, we may be enabled to form a conception of the mighty part which that thin air, of which we think so little, plays in the grand alchemy of nature.
"I will praise thee; for I am fearfully and wonderfully made; marvellous are thy works; and that my soul knoweth right well."—Psalm cxxxix.
In addition, also, to the facts already communicated, respecting the sound-bearing and light-refracting properties of air, it must be remarked, that but for the atmosphere, and the general refraction of light by its particles—each atom as it were catching a fairy taper, and dancing with it before our view—the condition of vision would be widely opposite to that which exists, and totally unsuited to our wants. The various objects upon which the illuminating rays of the sun fell, would be lighted up with an intense glare, but all around would be darkness, just as when a single ray of light is passed into a dark chamber, and directed upon a solitary object. The air, without becoming itself visible, diffuses luminous rays, in modified intensity, in every direction. If the air reflected so much light as to render itself visible, it would appear like the glittering surface of the water reflecting the solar rays, and we should then be unable to see the various objects which surround us.
Of the importance of Water in the scheme of creation, man generally entertains an imperfect conception. It is simply supposed to afford moisture to plants, drink to animals, and to promote salubrity by its cleansing properties. Let us, however, contemplate man as he stands before us, noble in form, erect in position, full of strength, joy, ambition. How much of that noble form is composed of water? Suppose that it could all be instantaneously withdrawn—not the oxygen and the hydrogen, which might combine to form water—but the fluid that exists in his body as water, unchanged—except by mechanical admixture with the secretions of the body—Why then that beautiful temple would collapse and become a mere shred, so thin, that it would seem but a shadow of the body as it existed before, and the beholder might doubt whether life ever inhabited a frame whose structure was so frail. It is said that three-fourths by weight of the human body consist of water. Thus, if man weighs 120lbs., 90lbs. consist of water, and this subtracted, only 30lbs. of solid matter remain. This statement is rather under than over the fact.
The assertion is startling, but so true that it can be verified by simple experiment. A piece of lean flesh—say of beef—cut an inch thick, and placed in a slow oven, and allowed to remain until all its water was driven off in vapour, would become as thin as a wafer, and as light as a cork. With a more scientific arrangement, it would be possible to collect the water, and the weights of the condensed vapour, and of the solid residue, would together make up the weight of the beef: if the piece weighed sixteen ounces, the weight of the water would be about 14 ounces, and the solid matter about two ounces.
Water holds a similar proportion in the bodies of all animals, and of vegetables. It is evident, therefore, that it occupies a more important place in the scale of creation than is generally accorded to it by the unobservant mind. We are indebted to it for those atmospheric changes which constitute the peculiar feature of our varying climate. Rising in invisible vapours, it builds palaces of glory in the skies, and often presents to the view of man the imagery of heaven. Persons who have ascended above the altitude of the clouds, have described the scene upon looking down towards them as the most celestial that the mind can conceive. Fields of fleecy radiance, majestically rolling like a sea of gold, occupied the whole range of vision, and seemed to embellish an eternity of space. Those golden clouds that at one time are decked in the richest splendour, and occupy the upper chambers of the Court of Nature, become grave councillors when the earth grows thirsty, and the plant droops with languor. They roll their heavy brows together, as in consultation upon some grave necessity: down come the refreshing showers, the mighty tongue of thunder rocks the air, the earth is drenched, and becomes fresh with the salubrity of her toilette; obnoxious substances, with their offensive exhalations, are swept away: living things rejoice, and beautiful flowers throw their incense in thanksgiving into the air; the broad blue heavens for a time look down and smile upon the blessed work; and then the clouds again gather in a golden train, and one by one fill the high arches of the atmosphere, until the earth once more grows thirsty, and the flower supplicates for drink.
"How mighty are his wonders! his kingdom is an everlasting kingdom, and his dominion is from generation to generation."—Daniel iv.
With reference to Light, its wonders, and the curious but imperfect theories respecting it, we have little to add, except with regard to its physiological action upon the eyes of man and of animals, which will be given in another place. But of its sister, Darkness—for it would not do now to call darkness the antagonist of light, since it will be seen that they work harmoniously for good—we have to say, that recent discoveries indicate that darkness is as necessary to the health of nature as light. Not only is it necessary to compose man and animals to sleep, to give rest to the over-wrought nerves of the industrious—but light is the quickening power of vegetation, and although plants grow by night, they grow, as man does, when stretched upon his bed—but some of their functions, which are actively excited in the presence of light, are at rest in darkness. Nor is this all: there is not an atom upon the face of the earth which is not affected by the rays of the sun, their light, their heat, their actinism. Colours change: some are bleached, others are darkened. All bodies are expanded. The hardest rock sustains an effect from the sun's rays; and an unceasing sun, shining upon the hardest granite, would in time produce such a disturbance of its atomic condition, that adamant would crumble away to dust.
The going down of the sun, therefore, marks the period when not only does the bird fly to her resting-place, and man turn to his couch; but when every atom of a vast hemisphere subsides into a state of quietude, and when homogeneous particles of matter return to their mutual rest.
In a few succeeding lessons, we intend to point out some of the scientific truths that are illustrated in the use of toys. We think we shall be able to show to our young readers, that even the hours of play may be made the periods of delightful instruction; and that there is no"reason why" the acquirement of knowledge should not sweetly accord with the occasional pursuit of those pastimes by which health of body and vigour of mind are induced.
But before we commence the discharge of that pleasant duty, let us say a few words respecting Carbon, that important agent in the world's history. It is, doubtless, perplexing to the minds of many persons, to understand how the diamond can be pure carbon; how charcoal can be carbon a little less pure than the diamond; and how coal and sugar can also be carbon, less pure than the charcoal. The statement that in the diamond carbon exists in a different atomic condition, is almost as instructive to the inquiring mind, as to say, "It is so, because it is."
Diamonds are expensive things, and so difficult to experiment upon, even if they were not expensive, that the doors of inquiry seem locked. To turn diamonds into charcoal, or into carbonic acid gas, is a very costly formula of experiment. Charcoal fires, thus sustained, would soon burn a man out of his house; and soda water, impregnated with carbonic acid gas, produced from diamonds, would be a very expensive beverage. If we could only turn charcoal into diamonds, and carbonic acid gas into brilliants, that would be quite another affair. A new Eldorado would be discovered, and there would be so many experimenters that, when they all succeeded, they would find that diamonds had lost their value. However, as a fact for the encouragement of those who would like to be early in the race, we may state that the atoms of charcoal which are repulsed from the charcoal points, during the electric agitation which produces the electric light, acquire a hardness and a sharpness almost equal to that of the diamond—only there is still the awkward obstacle in the way, that they happen to be black.
"He delivereth and rescueth, and he worketh signs and wonders in heaven and in earth."—Daniel vii.
We must see, therefore, whether there is anything in nature that we can experiment upon, theoretically or practically, to give us a clearer conception of this difficult matter. There is a large dew-drop resting upon a luxuriant cabbage leaf—one of those great leaves that have flourished in defiance of the snail, and now spreads out like the gigantic frond of the Victoria Regina. That dew-drop is one of the beautiful diamonds which Nature sprinkles about on cloudless nights, as if to show the stars, in answer to their twinkling, that we have something that will glisten and twinkle too.
The dew-drop is a very good imitation of a diamond, and to the lover of God's works, quite as precious as the stone set in gold. It does not consist of carbon—it probably may have a mite of carbonic acid in its embrace—but that is not necessary to our purpose: all we want to know is, the different atomic conditions of which bodies are susceptible, and the very dissimilar appearances they exhibit under the variations of atomic states. It doesn't glisten so much as the diamond, because it is round—if we could cut it into a number of facets, it would refract light almost as perfectly as the diamond. It is not solid—but we can freeze it, and we shall at once exhibit two different atomic conditions, that will represent nearly enough the diamond, and the liquid carbonic acid. Then, if we evaporate the dew-drop, we shall produce a volume of vapour nearly two thousand times as large as the dew-drop. The steam will be white; but we have only to imagine it black, and then we get an analogy of the differences of the atomic conditions that prevail in the diamond, carbonic acid, and charcoal, tinder, lamp-black, or any light form of carbon. Of course we have been illustrating atomic conditions only, and not chemical composition.
There are a few other facts connected with carbon that merit consideration. Carbonic acid gas, entering the lungs, is a deadly poison; but entering the stomach, which lies close under the lungs, and is over-lapped by them, it is a refreshing beverage. Although charcoal, when burnt, gives off the most poisonous gas, it seems to be very jealous of other gaseous poisons; for if it be powdered, and set about in pans where there is a poisonous atmosphere, it will seize hold of poisonous gases, and, by absorbing, imprison them. Even in a drop of toast and water, the charred bread seizes hold of whatever impurities exist in the water; and water passed through beds of charcoal, becomes filtered, and made beautifully pure, being compelled to give up to the charcoal whatever is obnoxious. If a piece of meat that has already commenced putrifying, be sprinkled with charcoal, it will not only object to the meat putrifying any further, but it will sweeten that which has already undergone putrefaction. Although, in the form of gas, it will poison the blood, and cause speedy stupefaction and death; if it be powdered, and stitched into a piece of silk, and worn before the mouth as a respirator, it will say to all poisonous gases that come to the mouth with the air, "I have taken this post to defend the lungs, and I arrest you, on a charge of murderous intention." Such are the various facts connected with carbon; and they forcibly indicate that those who understand Nature's works, are likely to receive her best protection.
"The father of the righteous shall greatly rejoice; and he that begetteth a wise child shall have joy of him."—Proverbs xxiii.
CHAPTER XXXVIII.
810. Why does a humming-top make a humming noise?
Because the hollow wood of the top vibrates, and the edges of the hole in its sides strike against the air as it spins; the air is thereby set in vibration.
811. Why does a peg-top hum less than a humming-top?
Because, being a solid body of wood, and having no hole in its sides, its particles are not so easily thrown into vibration; consequently it does not so readily impart vibrations to the air.
812. Why does a peg-top sometimes hum, and at other times not?
Because, if it is spun with great force, and its peg is struck sharply against the pavement, the wood is set in vibration, and the surface of the top, repelling the air by its rapid motion, causes vibratory waves. But if it be spun with insufficient force, the wood is not set in vibration.
Fig. 23.—HUMMING-TOP BEFORE SPINNING.
Fig. 24.—HUMMING-TOP SPINNING.
813. Why do we see the figures painted upon the humming-top, before it spins, but not while it is spinning?
Because the rapid whirling of the top brings the images of its different parts so quickly in succession upon the retina of the eye, that they deface each other, and impart an impression of coloured rings, instead of definite objects.
"Train up a child in the way he should go; and when he is old, he will not depart from it."—Proverbs xxii.
814. Why does a top stand erect when it spins, but fall when it stops?
Because the top is under the influence of, and is balanced between opposing forces. The rapid rotation of the top gives to all its particles a tendency to fly from the centre. If the atoms of the wood were not held together by the attraction of cohesion, they would fly away in a circle outward from the top, just as drops of water fly off from a mop, while it is being twirled. If you take a spoonful of sand, salt, or dust, and drop it upon the top, it will be scattered in a circle, just as the atoms of the top would be, if they were free to separate, but not with the same force, because the atoms of the salt, &c., not being in an active state of rotation, would only be influenced by momentary contact with the rotating body. This tendency of the particles of a rotating body to fly outward from the centre, is called the centrifugal force.
Centrifugal.—From two Latin words meaning receding from the centre.
The other force influencing the top is the attraction of gravitation: the attraction which, were the top not spinning, would draw it towards the earth. The "spill" projecting from the bottom of the top stands in the line in which the top is drawn towards the earth and keeps it from obeying the law of gravitation. Therefore the rotatory motion given to the top, by the rapid unwinding of the string, and the tendency of its atoms to fly outward, balance the top upon the line in which it is drawn to the earth, and which is occupied by the spill, which prevents it falling to the ground.
815. Why does a top first reel around upon the spill, then become upright, and "sleep," and then reel again, and fall?
Fig. 25.—PEG-TOP "REELING."
Because, in being thrown from the hand, the top is delivered a little out of the perpendicular, but the spill is rounded off at the point, and when the top is rotating rapidly, the gravitative force which attracts the top to the ground continually acting upon it, draws the weight of the top on to the extreme centre of the round point. When the rotation subsides, and the centrifugal force is weakened, then the top is no longer balanced upon the extreme point of the spill, but falls upon its sides, until the force of gravitation is exerted beyond the line of the spill, upon the body of the top, and then it falls to the ground.
"Even a child is known by his doings, whether his work be pure, and whether it be right."—Proverbs xx.
Because at that period of its spinning, which is called "sleeping," the centrifugal and the gravitative forces acting upon the top, are nearly balanced; and the top, obeying chiefly the rotatory force, appears to be in a state of comparative rest.
817. Why does the top cease to spin?
Because the friction of the air against its sides, and the friction of the spill against the ground, act in opposition to the rotatory force, which is a temporary impulse applied by external means—the hand of the person who spins it—and as soon as this applied force is expended, the top yields to the law of gravitation, which is a permanent and ever-prevailing force.
818. Why does a marble revolve, as it is propelled along the ground?
Because, in propelling the marble, the thumb impels the upper surface forward, and the finger draws the under surface backward. This gives a tendency to the upper and lower hemispheres of the marble to separate, which they would do, but for the cohesion of the atoms of the marble. The upper part of the marble, therefore, rolls forward, drawing after it the under part, which acquires a forward motion by the force with which it is drawn upward, and in this way the opposite portions of the marble act upon each other in the successive revolutions.
When the marble strikes upon the earth, a new influence is exerted upon it, which is the friction of the earth upon the surface that comes in contact with it; but the upper part of the marble, being free, overcomes the friction acting upon the lower part, and thus the marble continues to progress, until the applied force which projected it is expended.
"Better is a poor and a wise child, than an old and foolish king who will no more be admonished."—Ecclesiastes iv.
819. Why does a striped marble appear to have a greater number of stripes when rolling, than when at rest?
Because the stripes are presented in rapid succession to the eye; and as the eye receives fresh impressions of stripes before the previous impressions have passed away, the stripes appear multiplied.
Fig. 26.—MARBLE AT REST.
Fig. 27.—MARBLE ROLLING.
820. Why does a marble rebound when dropped upon the pavement?
Because the force of its fall to the earth compresses the atoms of which the marble is composed; and the atoms then exert the force of elasticity to restore themselves to their former condition; and by the exercise of this force the marble is repelled, or thrown upward from the pavement. Although a marble may be made of very hard stone, yet that stone may be elastic, and possess, though in a much less degree, the same kind of elasticity which causes the India-rubber ball to rebound from the earth.
821. Why does a marble, assuming it to be impelled with equal force, roll further on ice than on pavement, and further on pavement than on a pebble walk?
Because the friction is greater upon pavement than upon ice, and greater upon a pebble walk than upon pavement.
822. How many forces contribute to stay the progress of a rolling marble?
The friction of the air, the friction of the earth, and the attraction of gravitation, which tends to bring all bodies to a state of rest.
"He shall turn the heart of the fathers towards the children, and the heart of the children to their fathers."—Malachi iv.
823. Why do the stripes upon a marble disappear when it is spun with great velocity?
Fig. 28—MARBLE SPINNING RAPIDLY.
Because, as in the case of the humming-top, the different parts of the surface are brought so rapidly in succession to the sight, that they deface or confuse the impressions upon the retina.
824. Why are rings most perceptible at the opposite points, or poles, of the marble?
Because the point, or pole, upon which the marble spins, and that which corresponds to it, on the upper surface, travel less rapidly than the central portions, which being of a larger circumference, pass through a greater amount of space, in the same period of time. The stripes at the poles of the marble, are, therefore visible, while those at its equator are imperceptible. (See [522].)
CHAPTER XXXIX.
825. Why are soap-bubbles round?
Because they are equally pressed upon all parts of their surface by the atmosphere.
826. Why are bubbles elongated when being blown?
Because the unequal pressure of the current of breath by which they are being filled, alters the relative pressure upon the outer surfaces.
827. Why does the bubble close, and become a perfect sphere, when shaken from the pipe?
Because the attraction of cohesion draws the particles of soap together, directly the bubble is set free from the bowl.
"Children's children are the crown of old men; and the glory of children are their fathers."—Proverbs xvii.
Fig. 29.—BLOWING SOAP BUBBLES.
828. Why do bubbles, blown in the sunshine, change their colours?
Because the films of the bubbles constantly change in thickness, through the atoms from the upper part descending towards the bottom, and therefore the varying thickness of film refracts, in different degrees, the rays of light.
Because the atoms that compose their films fall towards the earth by gravitation; the upper portion of the bubbles then becomes very thin, and as the denser air of the atmosphere presses towards the warm breath within the bubble, it bursts the film.
See [236], [237], etc., [501], etc.
830. Why do balloons ascend in air?
Because the air or gas which they contain is specifically lighter than the atmosphere; the atmosphere, therefore, forces itself underneath the balloon, by its own tendency towards the earth, and the balloon is thereby raised upwards. A balloon is but a larger kind of bubble, made of stronger materials.
831. Why does an air-balloon become inflated when the spirit set upon the sponge is lit?
Because the heat of the flame, and the burning of the spirit, A, create a volume of rarefied, or thin air, which inflates the balloon, and makes it specifically lighter than the surrounding medium.
"A wise son heareth his father's instruction."—Proverbs xiii.
832. Why do balloons sometimes burst when they ascend very high?
Because, as they get into the thinner air, which exists at high altitudes, the gas within them expands, and the coating of the balloon is burst asunder.
Fig. 30.—AIR-BALLOON.
Fig. 31.—PAPER PARACHUTE.
833. Why does the gas of balloons expand in thin air?
Because the air exerts a less amount of pressure upon the air or gas contained in the balloons.
834. Why do parachutes fall very gradually to the ground?
Because the air, coming in contact with the under surface of the expanded head of the parachute resists its downward progress.
835. Why does a shuttlecock travel slowly through the air?
Because the air acts upon the feathers of the shuttlecock, in the same manner as it does upon the parachute—it strikes against their expanded surface, and resists their progress through the air.
836. Why does the shuttlecock spin in the air?
Because the surfaces of the feathers fall upon the air obliquely, or slantingly, and therefore, as the shuttlecock descends, it turns in the air.
"Come ye children, hearken unto me, I will teach you the fear of the Lord."—Psalm xxxv.
Fig. 32.—BATTLEDORE AND SHUTTLECOCK.
837. Why do we hear a noise when we strike the shuttlecock with the battledore?
Because the percussion of the shuttlecock upon the parchment of the battledore causes it to vibrate, and the vibrations are imparted to the air.
838. Why is the sound a dull and short one?
Because the vibrations of the parchment are not very rapid, therefore there is little intensity in the vibrations of the air.
839. Why does the exercise, afforded by playing battledore and shuttlecock, make us feel warm?
Because it makes us breathe more freely, and causes the blood to flow faster; we, therefore, inhale more oxygen, which produces heat by combining with the carbon of our blood.
840. Why does a kite rise in the air?
A kite rises in the air by the force of the wind, which strikes obliquely upon its under surface. The string is attached to the "belly-band" in such a manner that it is nearer the top than the bottom of the band: this causes the bottom of the kite, when its surface is met by the wind, to recede in the direction of the wind: the top is accordingly thrown forward, and the kite is made to lie obliquely upon the current of air moving against it. The kite then being drawn by the string in one direction, and pressed by the air in another direction, moves in a line which describes a medium between the two forces acting upon it.
"Be ye therefore followers of God, as dear children; and walk in love, as Christ also hath loved us."—Ephesians v.
Fig. 33.—DIAGRAM EXPLAINING THE FLIGHT OF A KITE.
841. Why does the kite-string feel hot when running through the hand?
Because the rapid friction sets free the latent heat of the string, attracts the heat of the hand to the spot where the friction occurs, and sets free the latent heat of the air, which follows the string through the hand, and is compressed by the friction.
842. Why does running with the kite cause it to rise higher?
Because it increases the force with which the wind strikes upon the surface of the kite. If a person were to run with a kite at the rate of five miles an hour, through a still air, the effect would be equal to a wind flying at the rate of five miles an hour against a kite held by a stationary string.
843. Why does the flying-top rise in the air?
Because its wings meet the air obliquely, just as the surface of the kite does. And the twirling of the top, causing the oblique surfaces of its wings to strike the air, produces the equivalent effect of a wind from the earth blowing the top upwards.
"Children obey your parents in the Lord: for this is right."
844. Why does the flying-top return to the earth when its rotations are expended?
Because the reaction produced by its wings striking upon the air, is insufficient to counteract the attraction of gravitation.
Fig. 34.—FLYING-TOP.
Fig. 35.—PEA AND PIPE.
845. Why does a pea, into which a pin has been stuck, dance in suspension upon a jet of air blown through a pipe?
Because the jet of air, being slightly compressed under the convex form of the pea, by the weight of the pin, forms a concave cup of air, in which the pea rests.
In the case put, it is supposed that the pin is passed through the pea until its head comes in contact with it. The pin is dropped into the hole of the pipe, and the breath is then applied, the pipe being held upright. The pea will rise in the air, and be suspended upon the jet, while the point of the pin will rotate around the stem of the pipe. There are other methods of fixing the pin which alter the result, and require a different explanation to that given above.