ON
MOLECULAR and MICROSCOPIC
SCIENCE
VOLUME THE FIRST

By the same Author.

ON the CONNECTION of the PHYSICAL SCIENCES. 9th Edition. Portrait. Post 8vo. 9s.

PHYSICAL GEOGRAPHY. New Edition, thoroughly revised. Portrait. Post 8vo. [In the press.

Fig. 89, p. 20.

EUCYRTIDIUM CRANOIDES.

[Frontispiece to Vol. I.

ON

MOLECULAR

AND

MICROSCOPIC SCIENCE

BY MARY SOMERVILLE

AUTHOR OF ‘THE MECHANISM OF THE HEAVENS’ ‘PHYSICAL GEOGRAPHY’

‘CONNECTION OF THE PHYSICAL SCIENCES’ ETC.

Deus magnus in magnis, maximus in minimis—St. Augustine

In Two Volumes—Vol. I.

WITH ILLUSTRATIONS

LONDON

JOHN MURRAY, ALBEMARLE STREET

1869

The right of translation is reserved

LONDON: PRINTED BY

SPOTTISWOODE AND CO., NEW-STREET SQUARE

AND PARLIAMENT STREET

PREFACE.

Microscopic investigation of organic and inorganic matter is so peculiarly characteristic of the actual state of science, that the Author has ventured to give a sketch of some of the most prominent discoveries in the life and structure of the lower vegetable and marine animals in addition to a few of those regarding inert matter.

The Author feels bound to return her best thanks to kind friends—Sir John Herschel, Mr. Huggins, Mr. Gwyn Jeffrey, Prof. Tyndall, and Mr. T. Moore of Chelsea, who have aided in revising some of the sheets for press, and have thus counteracted the disadvantage under which she labours, of a residence abroad, and at a distance from libraries of reference.

CONTENTS
OF
THE FIRST VOLUME.

ILLUSTRATIONS
TO
THE FIRST VOLUME.

(The Author is indebted to the works of Dr. Carpenter, Rev. M. J. Berkeley, Mr.

Gosse, and Mr. Darwin, for the larger part of these Illustrations.)

Errata.

Page 3, line 5 from bottom, insert the before earth

59, line 19, dele the

59, line 21, dele the

59, lines 20 and 22, for part read parts

100, In the Table of Atomic Weights, read Copper 32; Zinc 32·5; Rubidium 86; Cæsium 133

101, lines 10 and 33, for 32 read 32·5

104, line 12 from bottom, for 29 read 32

MOLECULAR AND MICROSCOPIC

SCIENCE.

PART I.
ATOMS AND MOLECULES OF MATTER.

SECTION I.
ELEMENTARY CONSTITUTION OF MATTER.

The investigations which have revealed the most refined and wonderful relations between light, heat, electricity, and highly elastic media; the relation of these powers to the particles of solid and liquid matter, new methods of analysis, and the microscopic examination of that marvellous creation, animal and vegetable, which is invisible to the unaided eye of man, have brought a new accession to the indefinitely small within the limits of modern science.

Wherever the astronomer has penetrated into the depths of space, luminous points are visible; and since light merely consists in the undulations of the ethereal medium, matter must exist in every part of the universe of which man is cognizant, for although the luminiferous ether is so attenuated that its very existence is almost an hypothesis, its atoms are not more inconceivably small than those of highly elastic ponderable matter on earth. Atoms are the ultimate constituents of homogeneous simple substances; molecules, or groups of heterogeneous atoms united in definite proportions, constitute such as are compound. High pressure steam is invisible as it issues from the boiler, yet each of its molecules contains two atoms of hydrogen and one of oxygen. The perfume of a flower is a compound invisible substance formed of molecules.

We know nothing of the forms either of atoms or of those groups of atoms which we call molecules; but we cannot suppose them otherwise than as excessively hard, since conceive them how we will, we are sure that an atom, whatever be its form or nature, is ever the same. It never wears, it never changes, though it may have formed part of thousands of bodies and entered into thousands of combinations, organic and inorganic; when set free by their dissolution, it is ready to enter into a new series; it is indestructible even by fire, the same now as when created. Nor has the quantity of matter in our terrestrial abode ever been increased or diminished; liable to perpetual change of place and combination, the amount remains the same: the bed of the seas may be changed to dry land, and the ocean may again cover the lofty mountains, but the absolute quantity of matter changes not.

All substances, whether solid, liquid, or aëriform, are supposed to consist of hard separate atoms or particles, and in conformity with that supposition to be surrounded by the ethereal medium, otherwise they could not transmit light and heat, which are merely vibrations of that medium. Even the hardest and most compact substances are capable of compression, and have been compressed to an enormous degree by the hydraulic press; but it probably transcends mechanical force to bring their atoms into contact: in fact, no known substance is impervious to both light and heat, however thin.

By far the greater number of terrestrial substances consist of heterogeneous atoms chemically combined into atomic systems or molecules; but there are sixty-four which have never yielded to chemical analysis, and are therefore believed to be respectively formed of only one kind of atoms. Thirty-five of these are metals found either pure or as ores, and sixteen are metals existing naturally in chemical combination with alkalies, alkaline earths, or earthy bases, that is as salts, from which they have been obtained by the analytical power of electricity or other means. The thirteen remaining simple substances are non-metallic: some are aëriform, some solid, one liquid.

The alkaline metals are sodium, potassium, lithium, cæsium, rubidium, and thallium. They are distinguished by their energetic affinities for, and the simplicity of their compounds with, non-metallic elements. They are never met with native, and are amongst the most difficult metals to reduce from their ores, and their spectra are remarkable for simplicity. Sodium and potassium—which have been such important agents in spectrum science—were reduced from their alkalies of soda and potash by Sir Humphry Davy by means of the voltaic battery, a discovery which led the way to the reduction of many of the others. Lithium is a white metal which burns brilliantly in air and oxygen; it swims in naphtha, and is the lightest solid body known. Cæsium is the most energetic of all metals in its chemical affinities.

The metals of the alkaline earths are barium, strontium, calcium, and magnesium. They possess, like the preceding, energetic affinities for the non-metallic elements, and are reduced with difficulty from their ores. Barium is obtained from earth baryta: it is powerfully alkaline, and its salts are colourless and poisonous. Calcium is obtained from limestone, chalk, marble, and gypsum, which are amongst the most abundant constituents in the crust of the earth; it is a bright ductile metal of a bronze colour. Magnesium, which is a brilliant silver-white hard brittle metal, is obtained from magnesium limestone or dolomite. Although the ores of calcium and magnesium cover vast areas of the globe, the metals form a very small comparative proportion of them.

The metals derived from non-alkaline earths are glucinum, yttrium, thorinum, zirconium, and aluminium, which is the only one of any interest: it is now becoming a very useful metal. It combines readily with oxygen to form clay. The ruby, sapphire, and oriental topaz are merely coloured varieties of corundum, which is nothing but crystallised clay. Rubidium, cæsium, and thallium were discovered by spectrum analysis.

The avidity of some of these metals for oxygen is quite remarkable: potassium and rubidium inflame when they touch ice or cold water; they decompose the water and combine with its oxygen. Calcium becomes luminous in warm water, and burns with intense light when heated to redness; but a magnesium wire burns with such intense brilliancy that it has been employed for photography, and will probably become useful for household purposes, as two ounces and a half of magnesium wire when burnt give a light equal to that of twenty pounds’ weight of stearine candles.

The metals whose oxides are not reducible by heat without the aid of some form of carbon include nearly all the useful metals. They are all polyatomic, that is, they combine with other elements in the number of atoms varying from two to eight, and are divided into seven groups in regard to this property. For instance, zinc, copper, and cadmium are diatomic. Zinc is invaluable as a source of electric light and heat in the voltaic battery, and its vapour burns brilliantly. Copper is one of the most useful of metals, while cadmium is of no value at all. Nickel, cobalt, and uranium form the triatomic group; they are remarkable for their complex spectra. Nickel is usually an ingredient in meteorites; cobalt is employed in pigments and in sympathetic inks; and the oxide of uranium is used to stain glass, and gives it some very peculiar properties, as will be shown. The precious metals have a feeble affinity for oxygen at any temperature, and their oxides are decomposed by heat alone, and sometimes even by the undulations of light.

Metals are excellent conductors of heat, but they vary exceedingly in that respect; both theory and experiment prove that the best conductors are invariably the worst radiators. In fact those atoms which transfer the greatest amount of motion to the ethereal medium, that is, which radiate most powerfully, are the least competent to communicate motion to each other, that is, to conduct with facility. Silver and copper are the best conductors of heat, but the worst radiators. These two metals are the best conductors of electricity, but it is influenced by temperature; for MM. Matthiessen and Von Bose’s experiments have proved that all pure metals in a solid state vary in conducting power to the same extent between zero and 100° Cent., and that the alkaline metals conduct electricity better when heated than when cold.

All metals are capable of being vaporized, but at very different degrees of temperature. Platinum requires the heat of the oxy-hydrogen blowpipe, which by estimation amounts to 8801° Cent. This property makes it valuable for terminal points to the conducting wires of the voltaic battery and magneto-electric induction machine where great heat can be employed without fusing the platinum terminals. Copper is always employed for the conducting wire on account of its superior conductive power. The coil of wire in the magneto-electric machine, which is often miles long, is insulated by a coating generally of green silk thread. But in experiments of extreme delicacy where magnetism might vitiate the results, perfectly pure copper wire which is diamagnetic is used for the conducting wires in the thermo-electric pile of the goniometer, and the wires are coated with white silk thread, since it was discovered that the green dye contains some magnetic metal.

The mass of the metals however constitutes comparatively but a small part of the terrestrial globe, which is formed of chemical combinations of only thirteen simple elementary substances,—a wonderful manifestation of creative power that could form a world of such variety and beauty by means of atoms so little diversified; still more wonderful is it that four simple elements alone constitute the basis of nearly the whole organic fabric. The air we breathe, water, the bodies of men and living creatures, and the vegetation that adorns the earth, are chiefly combinations of three invisible gases, oxygen, hydrogen and nitrogen, with carbon, the purest amorphous form of coal.

Oxygen gas forms three-fourths of the superficial crust of the terrestrial globe, its productions and its inhabitants. At least a third part of the solid crust of the earth is oxygen in combination; it constitutes eight parts out of nine in water, and water covers three-fourths of the surface of the globe; it forms more than twenty parts out of a hundred of atmospheric air, and in the organic kingdom it is an essential constituent. Except in the atmosphere, oxygen is never uncombined, but may be obtained by distilling chlorate of potash, by the decomposition of water by voltaic electricity, and by other means. When pure it is a colourless, tasteless, inodorous, invisible gas; it is incombustible at ordinary temperatures, yet absolutely essential to combustion; no animal can live long in it, and none can exist without it. In the atmosphere oxygen is highly magnetic; its magnetism increases with cold and decreases with heat; hence its intensity varies with night and day, winter and summer, but its magnetic property vanishes when it enters into composition.

Oxygen is perfectly quiescent and passive as a gas in the atmosphere, and as a constituent of water and solid bodies, yet that inactivity conceals the most intense energy, which only requires to be called into action. Thus combustion of extreme intensity takes place when ignited sulphur is put into a vessel containing oxygen gas; the metal potassium is instantly inflamed by it on touching water; some of its combinations with chlorine are highly explosive, and phosphorus burns in it with dazzling splendour. Thus a stupendous amount of energy is latent in oxygen under the most tranquil appearance.

M. Schönbein of Basle discovered that oxygen exists in another state, which has neither the extreme quiescence on the one hand, nor the intense violence on the other, of its ordinary form; and to express that intermediate condition, in which its activity is less in amount and different in quality, it has been called by another name, viz. ozone, from the following peculiarity.

It had long been observed that there is a peculiar smell when an electric machine is in activity, and when objects are struck by lightning; that smell Professor Schönbein ascertained to arise from the change of oxygen into ozone, and actually produced ozone by passing electric sparks through that gas. Ozone differs from oxygen in having a strong smell and powerful bleaching property; it purifies tainted air, changes vegetable colours, and stains starch prepared by iodide of potassium blue, which thus becomes a test of its presence; yet it certainly is oxygen in an allotropic or changed state, for it readily oxidizes or rusts silver and other metals, and when ozonized gas is sent through a red-hot tube, it comes out pure oxygen. According to the experiments of Messrs. Tait and Andrews, oxygen gas loses six eighths of its volume, and becomes four times more dense by the change; it contracts more readily with obscure electricity than with the spark. The experiments of Professor Tyndall on the absorption of radiant heat by gases give reason to believe that ozone is produced by the packing of the atoms of elementary oxygen into oscillating groups, and that heating dissolves the bond of union and restores the ozone to the form of oxygen. Ozone chiefly exists in air that has passed over a great expanse of sea, and the quantity is increased during the aurora, which alone might lead to a surmise of that phenomenon being electric.

The change of oxygen into ozone is not the only instance of Allotropism,—that is to say, the existence of the same substance in two states differing from each other in every respect,—for ozone itself is allotropic. Professor Schönbein has discovered that there are two kinds of ozone standing to one another in the relation of positively and negatively active oxygen; namely ozone and antozone, which neutralize each other into common oxygen when brought into contact. In this respect they are analogous to electricity, and, like electricity too, one kind cannot be produced without a simultaneous development of the other.

When a metal, such as silver for example, is oxidized or rusts, it gives polarity to the atoms of oxygen in the atmosphere and divides them into the opposite states of ozone and antozone; the ozone combines with the silver and rusts or oxidizes it, at the same time that the antozone is dissolved in the moisture or aqueous vapour in the air and forms peroxide of hydrogen. The oxidized or rusted silver, as well as every other oxidized substance, is an ozonide, while the peroxide of hydrogen is an antozonide.

Since both kinds of ozone are produced during the decomposition of water by electricity, and as sea air is always found to contain more or less free ozone, the ocean is probably an antozonide, for all the antozone formed by electricity during thunderstorms must be either dissolved in the sea-water, or carried into it in the form of peroxide of hydrogen by the rain. Ozone must be exceedingly abundant in the zone of calms and light breezes near the equator known as the variables, which is subject to heavy rains and violent thunderstorms, and also in the regions of the monsoons. On land one of the benefits arising from these formidable phenomena is the production of ozone, which oxidizes decomposing organic matter and hastens its decay, while the antozone, which is dissolved in the atmospheric vapour, forms the peroxide of hydrogen and frees the air from the antagonist principle.

The peroxide of hydrogen thus produced is a transparent colourless inodorous liquid with a metallic taste, and contains one equivalent of hydrogen and two of oxygen. It retains its liquid state under a great degree of cold, and mixes with water in any proportion. It has a strong bleaching property, instantly destroying vegetable colour. If exposed suddenly to a temperature of boiling water it is decomposed with violent explosion, and readily gives off oxygen at 59° Fahr. The mere touch of an oxidized metal, as the oxide of silver, completely and instantaneously decomposes it, and oxygen gas is evolved by the union of the ozone and antozone so rapidly as to produce a kind of explosion attended by an intense evolution of heat.

During the combustion of phosphorus in the atmosphere both kinds of ozone appear, and Professor Schönbein considers the slow combustion of that substance, which unites with the ozone and sets the antozone free, as the type of all the slow oxidations which organic and inorganic bodies undergo in moist atmospheric air; that true oxidation is always preceded by the appearance of the peroxide of hydrogen, and that this compound acts an important part in slow oxidations, and is deeply concerned in animal respiration, and in many other chemical actions going on in nature.

In confirmation of these views, it is certain that ozone is a powerful minister in the work of decay. If wood be made explosive like gun-cotton by a similar process, it becomes pulverulent after a time, and burns without exploding, though it still retains its shape. In the natural state of the wood the oxygen is passive and quiescent, for oxygen is a constituent of wood; in its second state it is explosive, and after a time that is succeeded by the semi-active state of ozone, which by a slow imperceptible combustion causes the wood to decay. Mr. Faraday observes that the force which would have been explosive had it been concentrated into one effort, expends itself in a long continued progressive change.

‘The majestic phenomena of combustion bespeak our admiration and rivet our attention because of their imposing grandeur; yet these are but spasmodic efforts in the grand economy of the material world, occurrences of now and then. The slower but continuous progress of the elements to their appointed resting-place, the silent, tranquil, ever progressing metamorphic changes involved in the phenomena of decomposition and decay, these we count for nothing and pass unheeded by. Yet with all their majesty, with all their brilliancy, all their development of tremendous energy, what are the phenomena of combustion in the grand scheme of the universe compared with these? When the loud crash of the thunder or the lightning’s flash awakens us from our thoughtless abstractions or our reveries, our feelings become impressed with the grandeur of Omnipotence and the might of the elements he wields, yet the whole fury of the thunderstorm—what is that in comparison with the electric energies which silently and continually exert themselves in every chemical change? Why, the electric force in a single drop of water, and disturbed when that water is decomposed, is of itself greater than in the electricity of a whole thunderstorm. Those of us who limit our appreciation of the powers of oxygen to the energies displayed by this element in its feebly active state, form but a very inadequate idea of the aggregate results accomplished by it in the economy of the world.’ Oxygen is the only known gas that is allotropic, and is the only known substance that is doubly allotropic, that is existing in three different states similar to oxygen, ozone, and antozone.

Hydrogen when pure is an invisible gas without smell or taste; it is a constituent of various acids and alkalies, but is itself neither acid nor alkaline. It is highly inflammable, burning with a pale light, and, as already mentioned, a combined jet of oxygen and hydrogen produces heat of 8801°, which is so intense that nothing can withstand it. It is the lightest substance known. A balloon having the form of a globe ten feet in diameter, would hold 32¹⁄₂ pounds weight of common air, while two pounds weight of hydrogen gas would fill it. Associated with this small quantity of ponderable matter, hydrogen has an enormous power of combination, but its activity is only called forth by some exterior and exciting cause. A mixture of two measures of hydrogen and one of oxygen gas would remain inert for ever, but the instant an electric spark is sent through it, a bright flash and an explosion takes place, and the result is water: thus a tremendous force lies quiescent in that bland element.

Hydrogen gas is introduced into the atmosphere by imperfect combustion, but it is instantly diffused and becomes harmless, for aëriform fluids are capable of rapid and perfect diffusion through one another, each having a capacity peculiar to itself, which under the same circumstances is greater as its density is less; therefore hydrogen the lightest of gases not only rises in the air on account of its levity, but is more quickly and completely diffused than oxygen which is the support of life. Though hydrogen is inferior in density to every other gas, it surpasses them all in conducting electricity, just as silver and copper conduct electricity better than platinum, though far less dense. The great refrigerating power of hydrogen is owing to its extreme mobility and consequent rapid convection of heat, in which it surpasses all other gases. It is as permeable to radiant heat as atmospheric air, has a very high refractive power, a specific heat of 3·2936, and may be substituted in many chemical formulæ for a metal, without altering their character: hence it is sometimes called a metalloid.

The quantity of nitrogen gas or azote that exists in nature is enormous. It constitutes four-fifths of the atmosphere, whence it may be had in a pure state, as well as by chemical means. Like oxygen, this gas is permanently elastic, without smell, taste, or colour; it is neither acid nor alkaline, it does not change vegetable colours, it neither burns nor supports combustion, and is incapable when breathed of supporting animal life. It abounds in organic bodies, in all parts of the animal texture, in the blood, muscles, nerves, even in the brain; and is either a highly nutritious or poisonous principle in the vegetable kingdom.

Nitrogen gas is altogether passive; it has no affinity for the metals, and cannot be liberated from any of its compounds even by electricity. Excepting boron and titanium, it will not combine directly or spontaneously with any simple element, even under the highest temperature, but its indirect combinations are numerous and violent: those with hydrogen are either noxious or poisonous, those with oxygen are all deadly poisonous. Had nitrogen combined spontaneously with either of these gases, especially with oxygen, life would have been impossible as the organized creation is constituted; its inertness renders its mixture with oxygen in atmospheric air innocuous. However, combinations of nitrogen and hydrogen, forming nitrate of ammonia, have been discovered in the atmosphere by Professor Schönbein, the union of evaporation, heat and air being the cause; and as evaporation is continually going on, he concludes that nitrate of ammonia, nitrates and other salts are generated in the moist air, and are speedily washed down in our rainy climates into the springs and rivers. He considers the formation of nitrates out of water as highly important for vegetation, because each plant becomes a generator of a portion at least of its azotized food, while the rain furnishes the ground on which it stands with a supply of the same.

In the atmosphere, nitrogen has all the mechanical properties of common air, but with a greater refractive power, and its specific gravity is nearly the same with that of oxygen. Since the atmospheric gases are the most permeable to radiant heat, the earth is in the most favourable circumstances for being warmed by the solar rays, and thus the properties of the elementary gases are admirably adapted for our comfort, nourishment, safety, and pleasure.

Carbon, which combined with the three elementary gases forms the basis of the organic creation, is widely distributed throughout the globe, in enormous coal formations, the vegetation of former ages. Diamond is its purest crystalline form; and charcoal, which is wood whence the volatile matters have been driven off by heat, is its purest amorphous state. To this simple substance and to hydrogen, we are indebted for terrestrial light and heat, whether our fuel be coal or wood, our light a candle or a lamp. The products of combustion are carbonic acid gas, whether pure or mixed with smoke, for ashes are the incombustible earthy matter mixed with coal or wood, and smoke is unconsumed carbon arising from the bad construction of our chimneys; so that the waste is enormous in a great city like London where coal is the only fuel. Light is given out by incandescent solid particles, which become luminous sooner than gas, for all gases have a feeble illuminating power, and heat results from the chemical combination of the carbon with oxygen, a process in which the chemical force merges into its correlative heat. Mr. Faraday observes, that had the result of the combination of carbon and oxygen been a gas only, we should have had very little light, and had it been a permanent solid, the world would have been buried in its own ashes.

Diamond and pure carbon leave no residuum when consumed; they combine with the oxygen of our atmosphere into carbonic acid gas, which is invisible, poisonous, and so heavy, that it may be poured from one vessel to another like water, thereby showing how much carbon it contains in an invisible state. The quantity of carbonic acid gas thrown into the atmosphere in this invisible yet ponderous state is immense, since six tons weight of atmospheric air rushes hourly through an average size blast furnace, carrying with it more than half a ton of carbon in the form of that gas, whose constitution and properties are always the same, whether it arises from combustion, fermentation, or respiration, which latter may be regarded as a slow combustion, consuming us to the bones if not supplied with carbon by means of food. It has been computed that two thousand million pounds weight of oxygen gas is daily converted into carbonic acid gas by these operations, and given into the atmosphere, which would soon be contaminated by its poison and suffocating quality, were it not for vegetables which decompose it, assimilate the carbon and set the oxygen free to mingle with the air and make it again fit for respiration. Carbon has a greater power of combination than any other simple substance except hydrogen.

Mr. Faraday compressed carbonic acid gas into a liquid by the pressure of its own elasticity when disengaged from combination in close vessels, a force equal to the weight of thirty-five times that of our atmosphere; and the liquid was reduced to a solid by M. Thilorier by rapid evaporation, during which the heat was given out so quickly by one part of the liquid, that the remainder was condensed into a substance like snow, which could be touched with impunity, but when mixed with sulphuric ether its temperature was reduced to 166° below zero of Fahrenheit’s thermometer.

Carbon appears naturally under a great variety of forms, and exhibits one of the most striking instances of allotropism, the same substance showing the greatest contrast in appearance and physical properties. The diamond, the most resplendent, transparent, and hardest of gems, is identical with carbon, which is black, dull, opaque, and brittle. Both are combustible; carbon is easily ignited, but it requires a heat of 1860° to consume the diamond.

However numerous the crystalline forms assumed by substances either naturally or artificially may be, they are all capable of being grouped into geometrical systems; each system possessing its own allied and derivative forms capable of mutual variations among themselves, but the forms of one system never assuming those of the other. With that law, however, carbon and a few other substances are completely at variance. The diamond crystallizes in octohedrons, while graphite, which is also carbon, crystallizes in six-sided plates,—two forms that belong to different systems quite irreconcilable with one another: and thus carbon possesses the property of being dimorphous.

Sulphur is a simple inflammable mineral abounding in volcanic countries, either in a crystalline or amorphous state, and forming a constituent in organic substances, animal and vegetable. It is readily dissolved by bisulphide of carbon, by benzine, and by a moderate heat; and copper filings exposed to its vapour spontaneously take fire, the chemical force of combination merging into light and heat. Sulphuretted hydrogen gas, a combination of sulphur and hydrogen, forms naturally during the putrefaction of organic matter, and Mr. Faraday observes with regard to the affinities of sulphur, ‘so numerous are its relations, so extensive its range of combinations, that we must consider it to be the very foundation on which chemical manufacture is built up.’

Though a simple substance, sulphur exhibits the two remarkable phenomena of dimorphism and the allotropic property. When reduced by heat to vapour and cooled slowly, it crystallizes in rhombic octohedrons; when merely melted and allowed to cool slowly, it takes the form of oblique rhombic prisms. Here the same atoms when in vapour and in a liquid state are acted upon by different forces; but however that may be, sulphur is another singular exception to the law of the immutability of the crystalline systems.

Sulphur becomes allotropic by the continued application of heat; that is to say, it entirely changes its appearance and character, though it remains chemically the same. Naturally it is yellow and brittle, but when fused, it is a colourless pellucid fluid which by continued heat is changed into a black tenacious substance that becomes like India rubber or gutta percha when thrown into water. In this allotropic state it is endowed with properties more powerful, energetic, and exalted; its tendencies to act chemically being increased like those of ozone. That this black tenacious substance is chemically the same with common sulphur there can be no doubt, for when it is exposed to greater heat, it again becomes a colourless pellucid fluid, which thrown into water resumes the form of brittle yellow sulphur.

These new arrangements among atoms of the same kind show that the immutability of matter is not without exceptions.

The animal kingdom is the great reservoir of phosphorus, a simple substance that is never found uncombined. It is sparingly met with in the vegetable kingdom, and still less in the mineral, but may be procured abundantly from calcined bones. When pure it is colourless, transparent, solid, extremely poisonous, and so inflammable that it must be kept in water. In air it is in continual combustion with oxygen, during which ozone is produced. When burnt in a current of air phosphorus leaves a residuum consisting of two substances, of which one is an acid, the other is red allotropic phosphorus, which has been extensively used in the manufacture of lucifer matches, because its fumes are not deleterious, and because it inflames less easily than common phosphorus, to which it is reduced by heat or friction, which generates heat.

Silicon is a simple substance, never found alone, but when forty-eight parts of it are combined with fifty-two parts of oxygen gas it forms rock crystal, the purest form of silica or quartz. Silica is so abundant that it may be said to constitute the basis of the mineral world. The sand on the sea-shore, which is the debris of quartz rocks, shows how universally it prevails. It is even abundant in the vegetable kingdom, giving strength to the stalks and leaves of the grasses, and may be felt in the harshness of the beards of wheat and barley. Silicon exists in three different states—the amorphous, which has no form; the graphic, which takes the form of small hexagonal plates; and that of octohedral silicon: hence this substance is dimorphous.

A singular analogy obtains between silicon and carbon: the amorphous form of silicon corresponds to charcoal, the graphic form of silicon corresponds to the graphic form of carbon, and the octohedral form of silicon to the diamond; yet the chemical relations between the two substances are very small.

Silica has hitherto been considered to be insoluble in pure water; at least M. Bischoff states that only one part of silica dissolves in 769,230 parts of water; but by a method hereafter to be explained, Professor Graham has actually obtained a limpid solution of silica in pure water.

Boron is a constituent of boracic acid, a natural production in Thibet and Monte Corbalo in Tuscany. It is a greenish-brown solid, insoluble in water, but when heated to about 600° it burns in open air with a vivid flame.

Fluorine is a constituent of a very beautiful mineral, well known as fluor spar, which is found in cubic crystals of a green, yellow, or purple colour. Hydrofluoric acid obtained chemically from the mineral is highly volatile and extremely corrosive.

Three of the non-metallic simple substances, chlorine, bromine, and iodine, are connected by the most remarkable analogies. They are marine productions, for chlorine is obtained from common sea-salt and in greater purity from rock-salt, both of which are compounds of chlorine and the metal sodium. When sea-water is evaporated, salt and a substance called bittern remain, which contains a salt whence bromine is separated.

Again, when kelp, the ashes of burnt seaweeds, is purified from the carbonate of soda and the chloride of potassium, a salt is left which is the iodide of potassium, whence iodine is obtained. Iodine is also found in sponges, oysters, and other low sea animals, as well as in certain mineral springs, and sometimes in combination with silver. These three elemental bodies have little affinity for one another, but they combine powerfully with other substances.

Chlorine is a yellowish-green gas, twice as heavy as atmospheric air, with a noxious suffocating smell and astringent taste. It has a powerful bleaching property, and when combined with water, which absorbs twice its volume of the gas, it is used for bleaching linen, in calico-printing, and other arts. The clear solution of chloride of lime is still more in use for the same purpose, as well as for an antidote against contagion and unwholesome smells. Carbon does not burn in chlorine gas, yet it is capable of supporting combustion, for oil of turpentine, phosphorus, thin leaves of tin and copper, and powdered antimony, take fire spontaneously in it. This gas shows its power by the development of intense heat, but not by brilliant light, because the results of its combustion are mostly vapours, or such gases as have a feeble illuminating power; so chlorine differs materially from oxygen in the phenomena of combustion. Mr. Faraday observes, however, that the bleaching powder is analogous to ozone in being an intermediate state, for chlorine is pernicious and violently destructive as a gas, perfectly innocuous and quiescent in common salt and in its other natural combinations, while in the bleaching substances its energy is subdued by art, so as to make it an important agent in various manufactures.

Providentially, chlorine is never found free; but in a combined state it exists in enormous quantities in the salt of the ocean, in salt lakes, brine springs, and in extensive deposits of rock-salt, as well as in organic liquids. It has a strong affinity for hydrogen, and forms muriatic acid. A mixture of these two gases remains inactive in the dark, but explodes in sunshine.

By chemical means chlorine is made to combine with oxygen so as to produce four substances, two of which are gases of such unstable equilibrium and weak affinity that the slightest cause makes them detonate violently; the other two are more stable, though they contain a greater quantity of oxygen. The only combination of chlorine with nitrogen is the most powerful and dangerous explosive compound known. Chlorine combines naturally with sulphur, and with the metals so as to form ores.

Common salt affords a remarkable instance of change of volume by chemical combination. Twenty-four parts in bulk of salt contain 20·7 parts of sodium and 23·3 parts of liquid chlorine; hence by chemical combination a bulk of 44 is compressed into a bulk of 24, yet that great compression is consistent with perfect transparency, crystallized salt being perfectly transparent to light, and more so as regards radiant heat than any other substance. Thus chemical affinity does what no mechanical power could accomplish.

At an ordinary temperature and barometric pressure, bromine is an orange red, extremely volatile fluid, which congeals and becomes brittle at a temperature a little below the zero of Fahrenheit’s thermometer, and if combined with water at that degree of cold it crystallizes in octohedral crystals which are permanent even at 50° Fahr. Bromine is very poisonous, corrodes the skin, has a disagreeable taste, and a smell similar to that of chlorine, but more pungent and hurtful. It possesses a powerful bleaching property, does not conduct electricity, and like chlorine a taper will not burn in its gas, though it spontaneously sets fire to phosphorus, and some of the metals. Reasoning from analogy Professor Schönbein believes that chlorine and bromine are not simple substances; he considers them to be ozonides analogous to the peroxides of manganese, lead, &c. He believes chlorine to be the peroxide of murium, and bromine to be the peroxide of bromium. Professor Tyndall’s experiments on the absorption and radiation of gases show that the action of these two substances is very different from that of the simple gases.

Iodine is a dark purple solid, crystallized in scales or elongated octohedral plates. It slowly evaporates at ordinary temperatures, and at that of 350° Fahr. it is volatilized into a beautiful violet coloured gas which changes starch into a bright blue, and for that reason a little starch will detect the millionth of a grain of iodine in composition. Iodine is slightly soluble in water, has a hot acrid taste, and although used in medicine it is poisonous when taken in large doses. Its bleaching properties are inferior to those of its congeners, but its chemical combinations are the same. With hydrogen it forms a highly explosive compound, which detonates with the slightest pressure.

These three simple substances are analogous in almost every respect. They all possess a bleaching property, many of their compounds are exceedingly explosive, combustible substances do not burn in their gases, while their gases set fire spontaneously to substances generally reckoned incombustible. Hence, though not combustible, they support combustion, but in a very different manner from oxygen. Chlorine and the gases of bromine and iodine diluted with common air, do not transmit blue and violet light; that is to say, the spectrum of a sunbeam transmitted through them is deprived of its most refrangible coloured rays, and that which remains is crossed by more than a hundred equidistant dark lines; their spectral properties however will be given hereafter. They resemble oxygen in one respect—that when a current of electricity is passed continuously through a glass tube filled with any of these three gases, much attenuated, they slowly combine with the platinum wire of the negative pole of the battery inserted in the tube. The electricity by degrees passes in diminished quantity, and at last ceases altogether, showing that matter, however attenuated, is requisite to conduct it.

According to the experiments of M. Dumas, the volatility of a compound is in the inverse ratio of the condensation of the substances composing it, and simple bodies come under the same law. For example, chlorine is more volatile than bromine, and bromine is more volatile than iodine; hence according to that law, chlorine is the least dense of the three, bromine is intermediate, and iodine is the most dense, which is actually the case: for chlorine is a gas, bromine a liquid, and iodine a solid at ordinary temperatures, which proves that there is a sequence in the intensity of the cohesive forces in this triad.

SECTION II.
ON FORCE, AND THE RELATIONS BETWEEN FORCE AND MATTER.

Force is only known to us as a manifestation of divine power which can neither be created nor destroyed. The store of force or energy in nature is ever changing its form of action, its amount never. It may be dispersed in various directions, and subdivided so as to become evanescent to our perceptions; it may be balanced so as to be in abeyance, or it may become potential as in static electricity; but the instant the impediment is removed the power is manifested by motion. Whatever form force may assume it has invariably a compensation or equivalent, whether in the heavens or on the earth. The total sum of the living forces, vis viva, or actual energy of the planets is the same every time they return to the same relative positions with regard to one another, to their orbits and to space, whatever may have been their velocities or mutual disturbances. In the ocean, the energy by which 25,000 cubic miles of water flow over a quarter of the globe in six hours, is exactly equal to the force or energy that makes it ebb during the succeeding six hours. A body acquires heat in the exact proportion that the adjacent substances become cold; and when heat is absorbed by a body, it becomes an expansive energy at the expense of those around it, which contract. Chemical action many miles distant from the electro-magnet, as in telegraphs, is perfectly equivalent to the dominant chemical action in the battery. The two electricities, positive and negative, are developed in equal proportions, which may be combined so as to produce many changes in their respective relations, yet the sum of the energy of the one kind can never be made in the smallest degree either to exceed or to come short of the sum of the other.

The mechanical energy of machinery or working power is exhausted by the very act of working, and cannot be restored except by the action of other forces. In clockwork, the weight must sink to move the wheel, and when the weight is down, the store of energy is gone, and can only be restored by raising the weight through the expenditure of energy in the human arm, and the expenditure of human energy must be restored by food and rest. The heat given off from the bodies of men and animals is restored by the combustion of the oxygen inhaled during respiration and the carbon of the food, and the light and heat given out by the combustion of fuel, whether in the form of coal or wood, is compensated by the light and heat of the sun stored up in living vegetables. It is this equivalent for force or energy which prevails in every department of nature that constitutes the universal and invariable law of the Conservation of Energy, ‘a principle in physics as large and sure as that of the indestructibility of matter or the invariability of gravity. No hypothesis should be admitted nor any assertion of a fact credited, that denies this principle. No view should be inconsistent or incompatible with it. Many of our hypotheses in the present state of science may not comprehend it, and may be unable to suggest its consequences, but none should oppose or contradict it.’[^[1]] Thus, ‘there is a definite store of energy in the universe, and every natural change or technical work is produced by a part only of this store, the store itself being eternal and unchangeable.’[^[2]]

Cohesion is a force which acting at inappreciably small distances unites atoms and molecules of the same kind into solids, liquids, and aëriform fluids, exactly according to the law of the conservation of energy; for it requires the very same amount of force to dissolve their union as to form it. Cohesion varies with temperature both in simple and compound bodies, for metals can be fused and vaporized by artificial heat, and ice becomes water and aqueous vapour as the seasons change from winter to summer.

In solids the force of cohesion is so strong, that their atoms and molecules always retain their respective places; that power is so weak in liquids, that their atoms and molecules are capable of motion among themselves, and in gases and the ethereal medium the atoms are free and have no cohesion whatever. The resistance offered by substances to compression is an equal and contrary force.

The reciprocal attraction between solids and liquids in capillary tubes is a case of cohesion. If a glass tube of extremely fine bore be plunged into a glass of water or alcohol, the liquid will immediately rise in the tube above the level of that in the cup, and the surface of the little suspended column will be a hollow hemisphere. If on the contrary mercury be the liquid, it will not rise so high in the glass tube, and the surface of the little column will be a convex hemisphere. There is a reciprocal attraction between the glass tube and the liquid, and another between the particles of the liquid itself; and the effect is produced by the difference between the two. In the first case the attraction of the glass is greater than that of the liquid, and in the second it is less; hence the water rises higher in the tube than the mercury, and its surface is concave, while that of the mercury is convex. The elevation or depression of the same liquid in different tubes of the same matter is in the inverse ratio of their internal diameters, and altogether independent of their thickness; whence it follows that molecular action is insensible at sensible distances, for when tubes of the same bore are wetted throughout their whole extent with water, mercury will rise to the same height in all of them whatever be their thickness or density, the film of water being sufficient to intercept the molecular action, and to supply the place of a tube by its own capillary attraction. The action of this force is daily seen in the absorption of water by sponges, sugar, salt and other porous bodies, and it is a most important agent in the circulation of fluids in animals and vegetables.

Every atom of matter is subject to the force of gravitation, but each substance has its own peculiar weight of specific gravity, that is to say, the same bulk of different substances contains different quantities of matter. Since nothing is known of absolute weight it is necessary to have some standard of comparison, and for that purpose pure water at the temperature 39° Fahr. (that of its maximum density) is chosen for solids and liquids; while for gases and vapours atmospheric air at the temperature of sixty degrees of Fahrenheit’s thermometer, and a barometric pressure of thirty inches, is assumed as the unit of specific gravity.

The foot-pound, which is the unit of mechanical force as established by Mr. Joule, is the force that would raise one pound of matter to the height of one foot; or it is the impetus or force generated by a body of one pound weight falling by its gravitation through the height of one foot. Now impetus or vis viva is equal to the mass of a body multiplied by the square of the velocity with which it is moving: it is the true measure of work or mechanical labour. For if a weight be raised ten feet, it will require four times the labour to raise an equal weight to forty feet. If both these weights be allowed to fall freely by their gravitation, at the end of their descent, their velocities will be as one to two, that is as the square roots of their heights, but the effect produced will be as their masses multiplied by one and four; but these are the squares of their velocities. Hence impetus or vis viva is equal to the mass multiplied by the square of the velocity. Thus impetus is the true measure of the labour employed to raise the weights, and of the effect of their descent, and is entirely independent of time.

It is well known that iron becomes red-hot by percussion or impetus. The atoms of the iron are thrown into vibration, and these minute motions communicated to the nerves produce the sensation of heat. Now the mechanical labour required to raise the hammer to any number of feet is equal to the weight of the hammer multiplied by that number of feet; but the impetus or mechanical effect of the fall of the hammer is equal to its mass multiplied by the square of the velocity, that is to the vis viva: hence the quantity of heat generated is proportional to the vis viva. The circumstances being the same, if the mass be doubled the amount of heat is doubled; and if the velocity be doubled the amount of heat is quadrupled. If the weight and the perpendicular height through which a body has fallen be known, the quantity of heat generated may be determined. The same amount of heat is generated by the same amount of force, whatever that force may be, whether impetus, friction, or any other.

Dr. Thomson has put in a strong point of view the quantity of heat that might be generated by percussion or impetus. He computed that if by any sudden shock the earth were arrested in its orbit, the heat generated by the impulse would be equal to 11,200 degrees of the centigrade thermometer, even if the capacity of our planet for heat were as low as that of water; it would therefore be mostly reduced to vapour, and should the earth then fall to the sun as it certainly would do, the quantity of heat developed by striking on the sun would be 400 times greater. It is even supposed that the light and heat of the sun are owing to showers of bodies falling on the surface with impetus proportionate to his attraction, for had he been in combustion he would have been burnt out ages ago. The masses of meteoric iron and stone that occasionally fall on the earth show that matter may be wandering in space; the vast zone of smaller bodies that in their annual revolutions round the sun come within the earth’s attraction in August and November, when thousands of them take fire and are consumed on entering our atmosphere, show that a great amount of matter of small dimensions exists within our own system. Much may be beyond it which drawn by the sun’s attraction may fall on his surface.

When a body is heated, it absorbs one part of the heat; the other part raises its temperature. The part absorbed increases the bulk or volume of the body, the expansion being the exact measure, or mechanical equivalent of the heat absorbed. In fact the coefficient of expansion is the fractional part of the expansion in length, surface, or volume of the body when its temperature is raised one degree. When the body is cooled, its volume is diminished, and then the contraction is an exact measure, or mechanical equivalent of the heat given out, and thus expansion and contraction are correlatives with and represent heat and cold.

Specific heat is the quantity of heat required to raise a given bulk or a given weight of a body a given number of degrees. In the one case it is distinguished as the specific heat for a constant volume, in the other for a constant weight.

Although the specific heat of a substance remains the same, its sensible and absorbed heat may vary reciprocally to a great extent.

As there can be no direct measurement of heat independent of matter, its mutations and action on matter are the sole means we have of forming our judgment concerning its agency in the material world.

Mr. Joule has proved that the quantity of heat requisite to raise the temperature of a pound of water one degree of the centigrade thermometer is equivalent to the mechanical work or force that would raise the same mass of water to the height of 1,389 feet. This is the unit, or mechanical equivalent of heat.

In fact, for every unit of force expended in percussion, friction, or raising a weight, a definite quantity of heat is generated; and conversely, when work is performed by the consumption of heat, for each unit of force gained, a unit of heat disappears. For since heat is a dynamical force of mechanical effect, there must be an equivalence between mechanical work and heat as between cause and effect. That equivalence is a law of nature. The mechanical force exerted by the steam engine is exactly in proportion to the consumption of heat, neither more nor less; for if we could produce a greater quantity than its equivalent we should have perpetual motion, which is impossible. When steam is employed to perform any work, the temperature of the steam is lowered; the heat that disappears is transformed into the force that performs the work, and is exactly proportional to the work done, and vice versâ.

The heat which is the motive force in the steam engine is due to the chemical combination of the carbon of the fuel with the oxygen of the atmosphere. A pound weight of coal when consumed in one of our best steam engines produces an effect equal to raising a weight of a million of pounds a foot high, yet marvellous as that is, the investigations of recent years have demonstrated the fact, that the mechanical energy resident in a pound of coal and liberated by its combustion is capable of raising to the same height ten times that weight.[^[3]] The quantity of coal existing in the whole globe is believed to be inexhaustible, hence the energy in abeyance is incalculable. The chemical energy continually and actually exerted in the great laboratory of nature is greater than that which maintains the planets in their orbits.

The act of the combination of the atoms of carbon and oxygen in combustion is ‘now regarded exactly as we regard the clashing of a falling weight against the earth, and the heat produced in both cases is referable to the same cause;’[^[4]] so chemical combination in combustion is only a particular case of falling bodies. Drummond’s light, the most brilliant of artificial illuminations, is produced by a simultaneous shower of the atoms of oxygen and hydrogen gas upon lime; and platinum, the least fusible of metals, is vaporized by a similar shower from the oxy-hydrogen blowpipe, and thus impetus generates both light and heat, for although the atoms are too small to admit of an estimation of their individual vis viva, there can be no doubt that like causes produce like effects.

In what manner or under what form magnetism and electricity exist when quiescent in matter we know not, but the compass needles show that numerous lines of magnetic force, subject to periodic and secular variations, perpetually traverse the earth and the ocean; and that waves of magnetic force occasionally sweep rapidly over great tracts of the globe. These phenomena would seem to stand in some periodic connection with the solar spots. Professor Lamont of Munich has discovered that a permanent and regular current of electricity is propagated parallel to the equator all over the earth, and another similar to it in the atmosphere. Besides these, there are currents of electricity in the surface of the earth, sometimes in one direction and sometimes in another, which decrease with the depth; and M. Lamont conceives that this electric system is the cause of terrestrial magnetism. Electricity of intense power and inappreciable quantity certainly exists in abeyance in the atmosphere and in all terrestrial matter till the equilibrium between the antagonist forces be disturbed, and then it bursts forth with terrific violence in the lightning flash and stunning crash of thunder. Since it requires electricity equivalent to that in activity during a thunderstorm to form one drop of water, what must that power have been which the Omnipotent wielded when he created that deep over the face of which ‘darkness brooded.’

Electricity, though the most formidable power in nature, is made available to man by the voltaic battery, and by the electro-magnetic induction apparatus, in the battery of which it is generated by the chemical action of dilute sulphuric acid on zinc. The positive and negative electricities thus produced pass in opposite directions through the two conducting wires of the machine by a continuous transmission of force or vibration from atom to atom, a circulation that is accompanied by a continual development of heat in overcoming the resistance it meets with in the wires. The electricity decreases as the heat increases, and vice versâ; the action is reciprocal. Thus electricity is merely a transmission of force. Mr. Joule has proved that the quantity of heat produced in a unit of time is proportional to the strength of the current, whatever may be its direction, and that its power to overcome resistance is as the square of the force of the current. The force is exactly in proportion to the chemical action which produces it, and that is measured by the quantity of zinc consumed in the battery. Thus chemical action produces electricity, and conversely electricity is a powerful agent in the chemical composition and decomposition of matter.

The light and heat of the electric spark are intense though instantaneous; but a powerful induction apparatus like Ruhmkorff’s gives so rapid a succession of sparks that the light and heat are sensibly continuous and of great intensity. The light and heat, powerful as lightning itself, are produced by the combined currents of two batteries, each consisting of fifty Bunsen elements of moderate size. This formidable united current passes through a circuit of thick copper wire coated with silk thread, with an intensity of perpetually renewed heat that no substance can resist. When the copper conducting wires are fitted with charcoal terminals and brought near to one another, the dazzling lights emanating from each pole combine in one blaze of insupportable brilliancy. The most refractory substances, silica, alumina, iron and platinum, when placed between the poles, immediately melt like wax, and volatilize. Charcoal is so good a conductor of electricity that when the terminals are in contact they complete the circuit, and neither light nor heat appear. Air and glass are non-conductors, yet the spark has passed through several inches of air and perforated a mass of glass two inches thick. A long electric spark combines or decomposes a greater quantity of gas or vapour than a short one, and for a given induction apparatus and induction current, M. Perrot has shown that there exists a length of spark corresponding to a maximum chemical action.

Professor Seebeck of Berlin discovered that electric currents are produced by the partial application of heat to a circuit formed of two solid conducting substances as antimony and bismuth soldered together,—another proof of the correlation of heat and electricity.

There cannot be a doubt that the atoms of a conducting wire are in motion, and that they successively take definite and momentary positions during the passage of an electric current, after which they return successively to their normal state. When electricity is invariably sent from the same pole of an inductive apparatus through the wire of a telegraph, in a very short time the wire is torn or divided into small sections, which destroy its continuity; but when the electricity is sent from each pole alternately, the conducting wire is not injured. As each atom of the wire has its own electricity, this seems to indicate that during the successive transits of the same kind of electricity, the pole of each individual atom is attracted more and more in the same direction, till at last they no longer return to their normal state, the cohesive force is overcome, and a rupture takes place, the more readily if there be any imperfection in the wire. Since the electricity from the other pole of the machine would have the same effect, but in the contrary direction, an alternate motion in the atoms must maintain the continuity of the wire.

A closed current of electricity or magnetism is accompanied by a simultaneous current of the opposite force in the tangential direction equal in quality and intensity. Thus the electric and magnetic currents, which are merely transmissions of energy, differ by moving at right angles to one another; their effects are alike, yet they are not identical.

The amount of the chemical action of light has been determined by Professor Roscoe to be directly proportional to the intensity of the light; and when the light is constant the amount of action is exactly proportional to the time of exposure. It appears that equal volumes of chlorine and hydrogen explode in sunshine, but combine slowly in shade; and as the combined gases are absorbed by water as soon as combined, the gradual diminution of the volume of the mixed gases during the time of absorption is a measure of the amount of action exerted by the light.

Professor Wm. Thomson has computed, by the aid of Poullet’s data of solar radiations and Mr. Joule’s mechanical equivalent of heat, that the mechanical value of the whole energy, active and potential, of the disturbances kept up on the ethereal medium by the vibrations of the solar light in a cubic mile of our atmosphere, is equal to 12,050 times the unit of mechanical force: that is to say, twelve thousand and fifty times the force that would raise a pound weight of matter to the height of one foot. The sensible height of the atmosphere is about forty miles, whence some idea may be formed of the vast amount of force exerted by the sun’s light within the limits of the terrestrial atmosphere. The green mantle which clothes the earth proves under a beautiful form the influence of light on the organic world.

It has been proved that at any given fixed temperature the amount of light and heat absorbed and that which is emitted remains constant for all bodies. The greater the amount absorbed, the greater the amount radiated. The molecules or atoms of the bodies in consequence of the law of resonance emit those ethereal undulatory motions which have been previously impressed upon them, as a musical instrument resounds in answer to the note impressed upon it. The whole is referable to molecular or atomic motion, for in absorption the vibrations of the ether are communicated to the atoms, and in radiation, the vibrations are returned again to the ether. This principle is known as the law of exchange.[^[5]]

Matter has a decomposing and an elective power with regard to both radiant light and heat; most coloured bodies, such as flowers, green leaves, dyed cloth, &c., though seen by reflection, owe their colour to absorption. The light by which they are seen is reflected, but it is not in reflection that the selection of the rays is made which causes the objects to appear coloured. When light falls upon red cloth, a small portion is reflected at the outer surfaces of the fibres, and this portion, if it could be observed alone, would be found to be colourless. The greater portion of the light penetrates into the fibres, when it immediately begins to suffer absorption on the part of the colouring matter. On arriving at the second surface of the fibre, a portion is reflected and a portion passes on, to be afterwards reflected from, or absorbed by, fibres lying more deeply. At each reflection the various kinds of light are reflected in as nearly as possible the same proportion, but in passing across the fibres while going and returning they suffer very unequal absorption on the part of the colouring matter; so that in the aggregation of the light perceived the different components of white light are present in proportions widely different from those they bear to each other in white light itself, and the result is a vivid colouring.

In certain substances however, as gold and copper, the different components of white light are reflected with different degrees of intensity, and the light becomes coloured by these reflections. Gold is yellow by reflection; red cloth is red by absorption. In the same sense, physically speaking, in which the red cloth is red, gold is not yellow but blue or green; such is in fact the colour of gold by transmission through gold leaf, and therefore gold is greenish blue by absorption. In this case we see that while the substance copiously reflects and intensely absorbs rays of all kinds, it more copiously reflects the less refrangible rays with respect to which it is more intensely opaque. In general absorption and radiation are independent of colour.

There is a vast diversity in the property which substances possess with regard to the transmission of radiant light and heat; glass, for instance, transmits light abundantly, but is impervious to heat from non-luminous sources; while other substances, which are altogether opaque to light, transmit heat copiously, as the bisulphide of carbon, which of all liquids is the most diathermic, while water in all its forms is almost impervious to heat.

Sir William Herschel discovered that invisible rays of high heating power exist beyond the red end of the solar spectrum, and Mr. Tyndall has shown that the reason of a substance being impervious to the light of the most brilliant flame and at the same time pervious to these extra red rays is, that the intercepted rays of light are those whose periods of recurrence coincide with the periods of oscillation possible to the atoms of the substance in question. The elastic forces which separate these atoms are such as to compel them to vibrate in definite periods, and when their periods synchronize with those of the ethereal waves, the latter are absorbed. Thus transparency in liquids as well as in gases is synonymous with discord, while opacity is synonymous with accord between the periods of the waves of ether and those of the body on which they impinge. All ordinary transparent and colourless substances owe their transparency to the discord which exists between the oscillating periods of their molecules and those of the waves of the whole visible spectrum. The general discord of the vibrating periods of the molecules of compound bodies with the light-giving waves of the spectrum may be inferred from the prevalence of the property of transparency in compounds, while their greater harmony with the extra red periods is to be inferred from their opacity to the extra red rays. Water illustrates this transparency and opacity in the most striking manner. It is highly transparent to the luminous rays, which demonstrates the incapacity of its molecules to oscillate in periods which excite vision. It is as highly opaque to the extra red oscillations, which proves the synchronism of its periods with more of the longer waves. If, then, to the radiation from any source water shows itself to be eminently or perfectly opaque, it is a proof that the molecules whence the radiation emanates must oscillate in extra red periods.

It has been already mentioned that many substances which transmit radiant heat freely radiate badly, and vice versâ. Rock-salt is extremely permeable to radiant heat but radiates feebly; the reason according to Mr. Tyndall is, that the motion of the molecules of the salt, instead of being expended on the ether between them and then communicated to the ether external to the mass, is transmitted freely from molecule to molecule.

Alum is exactly the reverse. Mr. Balfour Stewart proved that alum is an excellent radiator, and Mr. Tyndall proved it to be a very bad conductor, imparting freely and with ease the motion of its molecules to the external ether, and ‘for that very reason it finds difficulty in transferring the motion from molecule to molecule. The molecules are so constituted that when one of them approaches its neighbour, a swell is produced in the intervening ether; this motion is immediately communicated to the ether outside, and is thus lost for the purposes of conduction.’[^[6]]

Melloni had investigated the laws of the radiation and absorption of radiant heat in solid and liquid matter; but its radiation and absorption by gases and vapours was unknown previous to the experiments of Mr. Tyndall.

The apparatus employed was a horizontal brass tube four feet long, between two and three inches in diameter, polished inside, and closed air-tight at each end by a plate of rock-salt, which transmits more heat than any other substance. The air could be pumped out of the tube by one pipe, and the gas or vapour for the experiment introduced by another. Close to one end of the brass tube there was a thermo-electric pile connected with its goniometer. On each side of this arrangement there was a vessel of water kept at the boiling point. These two vessels were so placed that when the rays of heat from one of them passed through the exhausted tube, and fell upon one face of the thermo-electric pile, their effect was so neutralized or balanced by the rays of heat falling on the opposite face of the pile from the other, thus the needle of the goniometer was steadily maintained at zero, and its deflection instantly showed the absorbent effect produced by any gas or vapour that was admitted into the exhausted tube.

Since aqueous vapour has a very exalted absorbent power, a gas or vapour was rendered perfectly dry before its absorbent capacity was determined. For that purpose the pipe that introduced it into the brass experimental tube was so constructed that the gas had first to pass over fragments of pumice-stone wet with strong sulphuric acid, which absorbed its moisture and dried it. Common atmospheric air, however, was not only dried in this manner, but it was deprived of its carbonic acid by passing over caustic potash, and many other precautions were taken to prevent the possibility of error.

Under the ordinary pressure of the atmosphere, when the experimental tube was exhausted, the needle of the goniometer stood at zero, but as soon as pure dry atmospheric air was introduced into the tube its absorption caused the needle to move from zero to 1°.

The tube was again exhausted; the needle stood at zero, but was deflected from zero to 1° as soon as the tube was filled with oxygen. A similar experiment was made with nitrogen and hydrogen with the same result. Thus, dry air and the elementary gases, oxygen, nitrogen, and hydrogen, have the same absorptive power, and consequently they all deflected the needle of the goniometer one degree. The whole amount of radiant heat that passed through the exhausted tube produced a deflection of 71° 5ʹ; hence taking as unit of heat the amount that would deflect the needle one degree, the number of units expressed by 71° 5ʹ is 308, consequently the absorption of each of these four gases amounts to ¹⁰⁰⁄₃₀₈ or 0·3 per cent. The most delicate tests could not show any difference between the three first, but Professor Tyndall had reason to believe that hydrogen has the lowest absorptive power of all gases and vapours, though he was unable to express the amount. The absorptive power of all four is very much less than that of every other gas or vapour, and invariably deflects the needle to 1°, which thus becomes the unit of comparison.

Olefiant gas, the most luminous of the constituents of coal gas, possesses the highest absorptive power of the permanent gases. When sent into the exhausted tube it deflected the needle of the goniometer from 0° to 70° 3ʹ, which is equivalent to 290 units. The whole heat that passed through the exhausted tube before the gas was admitted produced a deflection of 75° or 360 units, consequently more than ⁷⁄₁₀ths or 81 per cent. of the whole heat was cut off by the olefiant gas. Such opacity to heat in so transparent a gas is quite marvellous. A current of it was sent into the open air between the thermo-electric pile and one of the sources of heat, and although it was perfectly invisible, it instantly deflected the needle of the goniometer from 0° to 41°.

In order to ascertain the relation between the density of the gas and the quantity of heat extinguished or absorbed, an ordinary mercurial gauge was attached to the air-pump. The experimental tube was exhausted, and the needle of the goniometer stood at zero. Then, from a graduated glass vessel, measures of olefiant gas, each amounting to the ¹⁄₅₀th of a cubic inch, were successively sent through the drying pipe into the exhausted tube. The amount of the heat absorbed and the depression of the mercurial column corresponding to each measure of gas as it was introduced, was registered from one to fifteen measures. This experiment showed that for very small quantities of gas, the absorption is exactly proportional to the density or tension. One measure of the gas only produced a depression of the mercurial column amounting to the ¹⁄₃₆₇th part of an inch, or about the ¹⁄₁₅th of a millimetre.

In many of the vapours of volatile liquids, the preceding law only prevails to a certain amount of pressure differing in each case, beyond which increase of tension produces diminished effects. In sulphuric ether the change begins at the eleventh term.

In bisulphide of carbon the law changes after the sixth measure, &c.

In order to adapt the apparatus for experiments on coloured gases, a glass experimental tube 2 ft. 9 in. long, and 2 ft. 4 in. in diameter, was substituted for the brass tube, and, instead of boiling water, sources of radiant heat having a constant temperature of 270° Cent. were adopted.

The following table shows the absorption of a number of gases at a common pressure or tension of one atmosphere.

The absorptive power of ammonia is so great, that although as transparent in the glass tube as if it had been a vacuum, a length of three feet of it would be perfectly impervious or black to the heat here employed, yet even this does not express the energy which it exhibits under one inch of pressure.

When the relative absorptive actions of gases and vapours is compared, it must be under the same amount of pressure. Hence, for one inch of tension, the absorptive action of

Thus, for a tension of an inch of mercury, the absorption of ammonia exceeds that of air more than 7000 times; the action of olefiant gas is 7950 times, and that of sulphurous acid 8800 times, greater than the absorption of air.

The effect produced by ¹⁄₃₀th of an inch of tension of air and the elementary gases is equivalent to that produced by one inch in the others, so the unit representing the absorption of these four gases is only the ¹⁄₃₀th part of the unit in the preceding table.

It appears from the preceding tables of comparative absorption that chlorine, a highly-coloured gas with a specific gravity of 2·45, has an absorptive power expressed by 39° under the pressure of one atmosphere, while, at the same tension, hydrochloric acid, a chemical compound of chlorine and hydrogen which is perfectly transparent, with a specific gravity of only 1·26, has an absorptive action amounting to 62, whence it appears that the chemical change which renders chlorine more transparent to light, makes it more opaque to obscure heat. Again, bromine, which is far less permeable to light than chlorine, and has a specific gravity of 5·54, has an absorptive power of 160 under a tension of one inch; while hydrobromic acid, which is perfectly transparent to light, has an absorptive action for obscure heat amounting to 1005. This is a striking instance of transparency to light and opacity to heat being produced by the very same chemical art.

The enormous difference between the absorptive power of compound and simple gases and vapours is ascribed to their atomic structure; in fact the radiant and absorptive powers augment as the number of atoms in the compound molecule augments. The three elementary gases are formed of simple atoms, the compound gases and vapours consist of different kinds of atoms chemically united into groups. Both are free to receive the vibratory motions of the ether which constitute heat; but single atoms must produce a less effect than when a number of them are united into a molecule. The atoms are loaded by their chemical union, which offers a greater surface of resistance to the vibrations of heat, and renders the motion of the molecule more sluggish and more fit to accept the slowly recurrent waves of the obscure heat that strike upon it.

Thus when atoms of hydrogen and nitrogen are mixed in the proportion of three to one, the absorption of the mixture is represented by unity; but when they are chemically united in ammonia, the absorption is 1190 times as great. Atoms of hydrogen and oxygen mixed in the proportion of two to one absorb very feebly; when chemically united into a molecule of aqueous vapour the absorptive power is enormous. The absorptive power of nitrous oxide, a chemical compound of oxygen and nitrogen, exceeds that of dry air 250 times; a convincing proof that the atmosphere is a mixture and not a compound gas. Olefiant gas at five inches of tension absorbs 1000 times that of its constituent hydrogen. In fact all the compound gases and vapours far surpass the simple elementary gases and dry atmospheric air in their capacity for absorption.

Chlorine and bromine, which have so many singular properties in common, have this peculiarity also, that though simple substances respectively formed of homogeneous atoms, their absorptive powers are similar to those of compound substances, for the absorptive power of chlorine is 60 times that of the elementary gases, and that of bromine 160 times. This high absorptive power is ascribed by Professor Tyndall to their atoms being united into groups which act powerfully as oscillating systems, instead of the feeble action of single atoms.

Ozone is an analogous instance of the presumed union of homologous atoms into oscillating groups. By comparing the absorptive effect of ozonized oxygen obtained from the electric decomposition of water with that of the same oxygen deprived of its ozone by passing it over a very strong solution of iodide of potassium, Professor Tyndall found that ozonized oxygen possesses an absorption force 136 times greater than that of pure oxygen. The quantity of ozone producing this astonishing effect was too small even to admit of estimation, far less of measurement. This result induced Professor Tyndall to believe that ozone is produced by the packing of the atoms of elementary oxygen into oscillatory groups; and that heating dissolves the bond of union and allows the atoms to swing singly, thus disqualifying them from either intercepting or generating the motion which as systems they were competent to intercept and generate.

The indefinitely small and invisible constituents of perfumes of plants and flowers are proved to be compound bodies by their absorptive and radiating properties. The dried leaves of a flower or aromatic plant such as thyme were stuffed into a glass tube 18 inches long and a quarter of an inch in diameter. It was then inserted between the drying pipe of the machine and the experimental glass tube, which was exhausted, and the needle of the goniometer stood at zero. Then when the air admitted into the drying pipe passed over the thyme and carried its aroma into the experimental tube, the needle was deflected, and from thence the absorption of the thyme was computed to be 33 times greater than that of the air which carried it. By the same process it was found that the absorption of peppermint was 34 times, spearmint 38 times, lavender 32 times, and wormwood 41 times greater than that of the dry air, which was unity as usual. When small equal squares of bibulous paper rolled into cylinders and moistened with an aromatic oil, were substituted for the dried herbs, the absorption corresponding to the deflection of the needle was for dry air, equal to 1,—

The absorption of thyme and lavender shows how much aroma is lost when plants are dried. So great is the absorption of heat, that the perfume of a flower-bed may be more efficacious than the entire oxygen and nitrogen of the atmosphere above it.

The enormous absorption and consequently radiating power of the perfumes of plants and flowers is a proof that their constituent parts are molecules and not simple atoms, incredible as it may seem. The absolute weight of the substances producing these wonderful effects is unknown, but there must be great differences: some perfumes are carried to vast distances, others are less volatile, and that of mignonette was remarked by Dr. Wollaston to be absolutely so heavy that it was quite as powerful below a balcony containing a box of that plant, as in the balcony itself.

The perfumes during the experiments adhered to all parts of the apparatus so pertinaciously, that after a continued stream of dry air had been pumped through the tube till the exhaustion seemed to be complete and the needle stood at zero, after a few minutes’ repose, the residue of the perfume came out so powerfully from the crannies of the apparatus as almost to restore the original deflection. ‘The quantities of those residues must be left to the imagination to conceive. If they were multiplied by billions they probably would not obtain the density of the air.’

The absorptive power of the odour of musk was 72 or 74 times that of the air that conveyed it into the experimental tube; the quantity that produced it was quite inappreciable, yet the perfume was so persistent that the pieces of the apparatus through which it had passed had to be boiled in a solution of soda before they were fit for other experiments.

The absorption of many gases and vapours having been determined, their radiation was measured by a very simple arrangement. The thermo-electric pile was raised on a stand with a screen of polished tin in front of it. A heated copper ball in a perforated ring on a low stand was placed behind the screen; all direct radiation from the ball was thus cut off, but the heated air rising in a column above the screen radiated its heat on the pile and deflected the needle of the goniometer 60° when the ball was red-hot; but the radiation of the hot air was neutralized by another source of radiant heat on the opposite side of the pile which kept the needle steadily at zero. Then a purified gas or vapour conveyed by a pipe into the perforated ring which held the ball rose mixed with the heated air above the screen, but the radiation of the gas or vapour alone was shown by the deflection of the needle, because that of the air was compensated. With this apparatus Professor Tyndall proved that the amount of the absorption of each gas and vapour is exactly equal to the amount of its radiation. He has shown that this result is a necessary consequence of the dynamical nature of heat. For as no atom or molecule is capable of existing in vibrating ether without accepting a portion of the motion, the very same quality whatever it may be that enables it to do so, must enable it to impart its motion to still ether when plunged into it. ‘Hence from the existence of absorption we may on theoretic grounds infallibly infer a capacity for radiation; from the existence of radiation we may with equal certainty infer a capacity for absorption, and each of them must be regarded as the measure of the other.’ This reasoning, founded simply on the mechanical relations of the ether and the atoms immersed in it, is completely verified by experiment.

Hitherto the absorption and radiation of heat by the same thickness of different gases and vapours have been compared with each other, but in a recent series of experiments Mr. Tyndall has compared the action of different thicknesses of the same gas or vapour on radiant heat. The experiments extend from a thickness of 0·01 of an inch to that of 49·4 inches. The instrument employed for ascertaining the action of the smaller thickness was a horizontal hollow cylinder closed at one end by a plate of rock-salt. A second cylinder was fitted into this with its end also closed by a plate of rock-salt. This cylinder moved within the other like a piston, so that the two plates of rock-salt could be brought into flat contact with one another, or could be separated to any required distance, and the distance between the plates was measured by a vernier. At one end of the cylinder there was a source of constant heat, and the differential goniometer already described at the other. With this apparatus Mr. Tyndall found that olefiant gas maintains its great superiority over the other gases in absorptive power at all thicknesses. A layer of that gas not more than 0·01 of an inch thick intercepted about one per cent. of the total radiation. This great absorption corresponded to a deflection of 11° of the needle of the goniometer, and such was the delicacy of the apparatus that it would be possible to measure the action of a layer of this gas of less thickness than a sheet of writing paper. A layer of olefiant gas two inches thick intercepts nearly 30 per cent. of the entire radiation. A shell of olefiant gas two inches thick surrounding our globe would offer no appreciable hindrance to the solar rays in coming to the earth, but it would intercept, and in great part return, 30 per cent. of the terrestrial radiation; under such a canopy the surface of the earth would probably be raised to a stifling temperature.

The apparatus for measuring the action of the greater thicknesses of gas was a hollow brass cylinder 49·4 inches long, closed at both ends by plates of rock-salt, and divided internally into two compartments or chambers by a third plate of rock-salt movable in the interior; the source of heat being at one end and the differential goniometer at the other.

Carbonic oxide and carbonic acid are pervious to a vast majority of the rays of radiant heat. When the cylinder was filled with carbonic oxide gas and so divided, by moving the internal plate of rock-salt, that a stratum of the gas 8 inches long was next to the source of heat, and that 41·4 inches long farthest from it, the 8 inches of gas intercepted 6 per cent. of the whole radiation. But when the plate of rock-salt was moved till the column 41·4 inches long was next to the source of heat, and that of 8 inches farthest from that source, or behind the long one, the absorption of the 8 inches was sensibly zero. In like manner eight inches of carbonic acid gas when in front of a column of 41·4 inches of the same gas absorbed 6¹⁄₄ per cent. of the whole radiation, while placed behind that column the effect was nearly zero. The reason is that when the 8 inch stratum is in front, it stops the main portion of the rays which give it its thermal colours,[^[7]] while placed behind these same rays have been almost wholly withdrawn, and to the remaining 94 per cent. of the radiation the gases are sensibly permeable.

It is inferred from an extension of this reasoning that the sum of the absorptions of the two chambers taken separately must always be greater than the absorption effected by a single column of the gas of a length equal to the sum of the two chambers; this conclusion is illustrated in a striking manner by the experiments. It is also found that when the mean of the sums of the absorptions is divided by the absorption of the sum, the quotient is sensibly the same for all gases. It may farther be inferred that the sum of the absorptions must diminish and approximate to the absorption of the sum as the two chambers become more unequal in length, and that the sum of the absorptions of the two chambers is a maximum when the medial plate of rock-salt divides the long tube into two equal parts.

When air enters an exhausted tube it is heated dynamically by the collision of its particles on the sides of the tube as it rushes in to fill the vacuum; and when the tube is exhausted again by the air pump, chilling is produced by the application of a portion of the heat of the air to generate vis viva. This dynamic principle occurred in some of the experiments, and was dexterously adopted and applied to the solution of a striking and unprecedented problem: ‘To determine the radiation and absorption of gases and vapours without any source of heat external to the gaseous body itself.’

The two external sources of heat being therefore dispensed with in the absorptive apparatus, the thermo-electric pile was presented to the cold glass tube which was exhausted, and the needle of the goniometer stood at zero. Nitrous acid on entering the exhausted tube became heated and radiated its heat upon the adjacent face of the pile which deflected the needle of the goniometer through 28° in the direction that indicates absorption. As the heat of the gas became gradually exhausted, the needle returned slowly to zero. The pump was now worked, the rarefied gas in the tube was chilled, and the adjacent face of the pile gradually poured its heat on the chilled tube till the temperature of the pile was so much lowered, that the needle was deflected 20° on the negative side of zero, that is on the side denoting radiation.

When olefiant gas entered the exhausted tube, the needle showed an absorption of 67°, and when the gas was pumped out again, the needle showed a radiation amounting to 41°. When the gas was then pumped out, very dry atmospheric air was introduced into the tube,—the needle pointed to 59° indicating absorption; and when it was pumped out again the needle swung to nearly 40° on the other side of zero, indicating radiation. Remembering that the radiation and absorption of dry air only produce a deflection of 1°, it is evident that the preceding great deflection of the needle is entirely owing to the action of the small residue of olefiant gas that remained in the exhausted tube. In order to ascertain how much the quantity of a gas or vapour might be reduced before its action became insensible, the vapour of boracic ether, which has the greatest absorptive energy, was chosen.

The mercurial gauge for measuring the pressure or tension of the vapour already mentioned remained attached to the apparatus. When one-tenth of an inch of the vapour of boracic ether was admitted into the exhausted tube, the barometer stood at 30 inches: hence the tension of the vapour within the tube was the ¹⁄₃₀₀th part of an atmosphere. Dynamically heated by dry air the radiation of the vapour produced a deflection of 56°. Again the tube was exhausted to 0·2 of an inch and the quantity of vapour was thereby reduced to ¹⁄₁₅₀th of its first amount; the needle was allowed to come to zero, and the residue of the vapour produced a deflection of 42°. The pump was again worked till a vacuum of 0·2 of an inch was obtained, this residue containing of course the ¹⁄₁₅₀th of the quantity of ether present in the tube; and on dynamically heating the residue, its radiation produced a deflection of 20°.

Thus it is evident that the tension of the ether in these experiments was continually diminished by the 0·2 of an inch, consequently its quantity was continually diminished by its ¹⁄₁₅₀th part, accompanied by a corresponding decrease in the deflections of the needle. The final result of this process showed that the radiation of an amount of vapour in the tube possessing a tension of less than the thousand millionth of an atmosphere is perfectly measurable. The temperature imparted to this infinitesimal quantity of matter did not exceed 0·75 of a centesimal degree. The molecules which constituted this intensely attenuated vapour, though inconceivable, had as true an existence as the suns which constitute the star-dust of the nebulæ. ‘A platinum wire raised to whiteness in a vacuum by an electric current, becomes comparatively cold in a second after the current has been interrupted; yet that wire, while ignited, was the repository of an immense amount of mechanical force. What has become of this? It has been conveyed away by a substance so attenuated that its very existence must for ever remain an hypothesis. But here is matter that we can weigh, measure, taste, and smell; that we can reduce to a tenuity which, though expressible by numbers, defeats the imagination to conceive of it. Still we see it competent to arrest and originate quantities of force which on comparison with its own mass are almost infinite, a small fraction of this force causing the double needle of the galvanometer to swing through considerable arcs. When we find ponderable matter producing these effects, we have less difficulty in investing the luminiferous ether with those mechanical properties which have long excited the interest and wonder of all who have reflected upon the circumstances involved in the undulatory theory of light.’

The dynamical principle was next applied to determine the radiation of a gas through itself; or through any other gas having the same period of vibration. For that purpose Mr. Tyndall made use of the hollow cylinder 49·4 inches long already mentioned, closed at both ends by plates of rock-salt, and divided internally into two chambers by a movable plate of the same substance. All sources of heat being dispensed with, the chamber next the voltaic pile contained the gas which was to act as an absorber, and the more remote as a radiator.

Heat is evolved in air when its motion is arrested; on entering an exhausted tube, the more rapid the motion the greater the heat. Both chambers of the cylinder were at first filled with the vapour to be examined, the usual pressure being the ¹⁄₆₀ part of an atmosphere. But the vapour entered so slowly, and the quantity was so small that the radiation due to the warming of the vapour by its own collision was insensible. The needle of the goniometer being at zero, dry air was allowed to enter the chamber most distant from the pile; this air became heated dynamically by the collision of its particles against the sides of the tube, communicated its heat to the vapour, and the vapour immediately discharged the heat thus communicated to it against the pile. This case not only resembles, but is actually of the same mechanical character as, that in which a vibrating tuning fork is brought into contact with a surface of some extent. The fork, which before was inaudible, becomes at once a copious source of sound. What the sounding board is to the fork, the compound molecule is to the elementary atom. The tuning fork vibrating alone is in the condition of the atom radiating alone; the sound of the one and the heat of the other being insensible. But in association with sulphuric or acetic ether vapour the elementary atom is in the condition of the tuning fork applied to its sounding board, communicating motion to the luminiferous ether through the molecules, as the fork through the board communicates its motion to the air.

Mr. Tyndall’s experiments show the great opacity of a gas to radiations from the same gas, and may likewise show the remarkable influence of attenuation in the case of vapour. The individual molecules of a vapour may be powerful absorbers and radiators, but in their strata they constitute an open sieve through which a great quantity of radiant heat may pass. In such thin strata, therefore, the vapours as used in the experiments were generally found far less energetic than the gases, while in thick strata the same vapours showed an energy greatly superior to the same gases, but the gases were always employed at a pressure of one atmosphere.

Lastly Mr. Tyndall examined the diathermancy of the liquids from which his vapours were derived, and the result leaves not a doubt that both absorption and radiation are phenomena irrespective of aggregation. If any vapour is a strong absorber and radiator, the liquid from whence it comes is also a strong absorber and radiator.

Perfectly dry pure air is as pervious to light and heat as a vacuum itself; consequently, if the atmosphere was quite pure and dry, the rays of the sun would fall on the earth with unmitigated force during the day, and would be radiated back again and dissipated in space during the night to the destruction of vegetation. But the earth is protected from these extremes by the absorptive power of aqueous vapour, which is always present more or less in the atmosphere; even when the air is so transparent that distant objects seem to be near, it is loaded with vapour in an elastic invisible state, which a change of temperature may condense into cloud or precipitate in rain.

The absorptive power of aqueous vapour was determined by placing tubes containing fragments of glass moistened with water between the drying apparatus and the experimental glass tube of the instrument, so that perfectly pure dry air in passing over the wet fragments of glass carried a portion of aqueous vapour with it into the exhausted experimental tube, and the deflection of the needle of the goniometer showed that the absorptive power of the aqueous vapour exceeded that of the dry air 80 times. Now since in the atmosphere there is one molecule of aqueous vapour with an absorptive power of 80 for every 200 atoms of oxygen and nitrogen whose absorptive power is 1 like that of one of its constituent atoms, it follows by comparison that the absorptive power of the molecule is 16,000 times greater than that of an atom of either oxygen or nitrogen. From this enormous opacity to obscure heat ‘it is certain that more than ten per cent. of the terrestrial radiation from the soil of England is stopped within ten feet of the surface of the soil; remove for a single summer night the aqueous vapour from the air which overspreads the country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature.’

The quantity of vapour in each place varies with the latitude, the season, and other circumstances; but whenever the amount of heat radiated from the earth surpasses the absorption, the remainder passes through the vapour into space, and for the same reason the residue of that coming from the sun passes through the vapour and comes to the earth, so that whatever may be the local differences it has been decidedly proved with regard to the whole globe, that the quantity of heat annually received from the sun is annually radiated into space; the latter is a force lost to the earth, nevertheless it does not interfere with the law of the conservation of force which extends to the universe.

By observations made during ten scientific ascents in a balloon to very great altitudes, Mr. Glaisher has proved, that the theory of the uniform decrease of temperature with increase of elevation is no longer tenable. Since the absorptive force of aqueous vapour is 16,000 times that of dry air, the whole of the heat radiated by the full moon is intercepted by our atmosphere. It raises the temperature of the higher regions, dissolves the vapour, dissipates the clouds, prevents the formation of more, and allows the heat radiated from the earth to pass freely into space: thus confirming the common, and almost universal, belief that the full moon dispels the clouds. The absorptive power of aqueous vapour is so enormous that even the planet Mercury may be habitable should his atmosphere contain a sufficient quantity of it to mitigate the heat of the sun.

No doubt all the heat from the stars must be absorbed by the atmosphere, but their photographs show that it is pervious to the chemical rays. Those from Sirius, the nearest and brightest of the stars, travelling through 180 millions of millions of miles and decreasing in quantity inversely as the square of the distance, still have sufficient energy to give a perfect photographic impression of its spectrum; but Sirius is sixty times larger than the sun, and is many times more luminous. A photograph of the spectrum of Capella has been taken, though three times more distant than Sirius. Photographs of double stars of the sixth and seventh magnitude show that actinic rays from immeasurable distances in space have power sufficient to decompose matter in unstable equilibrium on the surface of the earth.

The chemical power of the moon’s light only surpasses that of Jupiter in the ratio of 6 to 4 or 5, and Jupiter’s light has twelve times more actinic energy than that of Saturn. For such comparisons a standard of photographic intensity is requisite.

A paper coated with chloride of silver can be prepared which has a constant degree of sensitiveness, and Dr. Roscoe has proved that a constant dark tint is produced on this standard paper by a constant quantity of light, the tint being the same, whether light of the intensity represented by 1 acts for the time represented by 50, or light represented by 50 acts for the time represented by 1; or in other words the amount of the chemical action of light is directly proportional to the intensity of the light, and when the light is constant, the amount of action is exactly proportional to the time of exposure.

The ratio of the chemical action of the rays of light falling directly from the sun to the chemical action of the light diffused over the whole sky can be determined by means of an instrument, in which the shadow of a little ball is made to fall on a sensitive paper so as to intercept the direct rays of the sun, and allow it to be impressed by an action of the light diffused over sky alone; this compared with a similar paper, on which both the direct and indirect light has fallen, gives the ratio required. From this it appears, that the relative amount of chemically active light which comes directly from the sun, is very much less than the amount of his direct visible light. For while Professor Roscoe was making experiments at Manchester on the maximum effect of the chemical action of light, he found when the sun had an altitude of 20°, that of 100 chemical rays which fell on a piece of standard paper, only about 8 came from the direct light of the sun; while on the contrary, of 100 rays of visible light, 66 came directly from the sun, and only 40 from the light diffused over the whole sky, so that the diffused light is richer in chemical rays than the direct solar beam, ‘a startling result,’ but borne out by observations not only made at Manchester and in its vicinity, but at Kew, Heidelberg, and at Pará on the Amazon nearly under the equator.

On account of the increasing rarity of the atmosphere, the greater the height above the level of the sea, the less the amount of diffused light and consequently of actinic power. Hence photographers have to expose their plates for a much longer time to the light on the snowy peaks of the Alps and other great heights than in England or at the level of the sea. During Mr. Glaisher’s tenth balloon ascent simultaneous observations were made at Greenwich Observatory and in the balloon, when at more than three miles above the surface of the earth, the standard paper exposed to the full rays of the sun was not as much coloured in half an hour as the corresponding paper at Greenwich in one minute.

By a series of observations at Heidelberg, Kew, and Manchester, it has been proved that the very small relative chemical action of the sun’s direct light decreases rapidly with his altitude, and at these three places of observation, it has frequently happened when the sun’s altitude was very low, as at 12°, that his direct light made no impression on a sensitive paper. ‘The sun’s light had been robbed of its chemical power in passing through the air.’ This singular result is ascribed by Professor Roscoe to what he calls the opalescence of the atmosphere.

Opalescent glass, slightly milky liquids, pure water with particles of sulphur floating in it, are impervious to the chemical rays, whence Professor Roscoe infers that the atmosphere, more especially its lower regions, possesses that property in consequence of multitudes of solid particles floating in it. What they are is unknown, but infinitesimal particles of soda seem to be everywhere, and no doubt particles of other substances mixed with them may be often seen as motes dancing in the sunbeams. Besides, it is clearly proved that myriads of the eggs and germs of organized beings, though invisible to the naked eye, are continually floating in the air, and that they are more abundant in the lower than in the higher strata of the atmosphere. Since opalescent matter reflects the blue rays of light and transmits the red, Professor Roscoe ascribes the blue colour of the sky and the bright tints at sunrise and sunset to the opalescent property of the air.

The atmosphere is permeable to every kind of chemical rays, which is far from being the case with bodies on earth, some of which though transparent to all the visible rays, vary greatly in their transparency to the chemical rays.

The atoms and molecules of matter not only have the power of turning the rays of the solar beam out of their rectilinear path, but of changing their refrangibility.

The myriads of ethereal waves or rays of light that constitute the seven colours of the solar spectrum, decrease in refrangibility and increase in rapidity of vibration and length of wave from the extreme violet to the end of the red; each ray having its own rate of vibration, its own length of wave, and its own colour. From the middle of the yellow, which is the luminous part of the spectrum, the chemical spectrum extends invisibly, but with increasing refrangibility and increasing velocity of vibration, to a point far beyond the violet. On the contrary, the heat spectrum, which may also be said to begin in the yellow light, extends invisibly but with decreasing refrangibility, and decreasing velocity of vibration to some distance beyond the visible red.

The rays of heat are absorbed by the humours of the eye, but were they to reach the retina we should see that they differ from one another as much as those of the luminous spectrum; the chemical spectrum from its greater length is still more diversified.

The whole of the solar spectrum, visible and invisible, is crossed at right angles to its length by innumerable dark rayless lines, differing in breadth and intensity. Sir John Herschel discovered vacant spaces in the extra-luminous part of the heat spectrum, and more recently M. Edouard Becquerel, by throwing the solar spectrum upon a daguerreotype plate, discovered that the chemical spectrum given by a glass prism, from its beginning in the yellow to its extreme point beyond the violet, is crossed by rayless lines, and that the lines in the part passing through the visible spectrum coincide exactly with the rayless lines in the luminous part. This coincidence was confirmed by the independent researches of Dr. Draper at New York. By means of the rayless spaces or black lines in the visible spectrum, M. Kirchhoff has proved that thirteen terrestrial substances are constituents of the sun’s atmosphere.

The length of the undulations of the ether which produce the impression of the extreme violet rays of the solar spectrum on our eyes, is the seventeen millionth part of an inch; the length of the ethereal undulation that produces the sensation of the extreme red is the twenty-six millionth part of an inch; the ethereal undulations beyond these limits are invisible to human eyes. Nevertheless certain substances have the power of increasing the length of the vibrations, and reducing the rays of the spectrum to a lower grade in the scale of refrangibility, so that the invisible rays of the chemical spectrum have thus been brought within the limit of human vision.

For example, the chemical rays shine as visible light when they fall on glass tinged with the oxide of uranium. When these dark rays fall upon the glass, they put the whole of its molecules into vibrations, the same with their own, while at the same time they give a more rapid vibration to a certain number of the same molecules. The whole of the molecules restore their vibrations to the surrounding ether. Those having the same velocity with the chemical rays make no sensible impression on our eyes; but the more rapid vibrations come within the limits of the visible spectrum; they have consequently a lower refrangibility, and shine as visible light. It is called degraded light on account of its lower position in the prismatic scale, but more frequently fluorescent light, because fluor spar was the first solid known to possess the property. A number of substances are fluorescent, both solid and liquid, organic and inorganic.

If in a dark room a non-fluorescent body be illuminated by a sunbeam passing through glass stained deep blue by cobalt, it will reflect blue light; but it will appear to be perfectly black if it be viewed through glass tinged yellow by silver; while a piece of canary glass, which is highly fluorescent, will shine with a vivid light under the same circumstances. All the molecules of the canary glass give back to the ether the undulations that have been impressed on them by the blue light; while a certain number of them possess the power of receiving and giving back more rapid vibrations to the ether. The yellow glass held before the eye is impervious to the undulations of the blue rays, but transmits those of the fluorescent light, which emanate from the smaller number of molecules, and which thus become in reality new centres of light, different from the sun’s light, though dependent upon it: the one terrestrial, the other celestial. Since the vibrations of the fluorescent light are more rapid than those of the blue light their colour is lower in the prismatic scale. The vibrations of the molecules in a fluorescent substance are analogous to those of a musical cord, which give the fundamental note or pitch and its harmonics, for the whole of the musical cord while vibrating the fundamental note divides itself spontaneously into parts having more rapid vibrations, which give the harmonics. Professor Stokes of Cambridge, who made this beautiful experiment, computed that the vibrations which produced the fluorescent light were a major or minor third below the pitch or vibrations of the blue light.

One of the first discoveries of fluorescence was made by Sir John Herschel—certainly the first who observed the property in a liquid. He found that the blue light which emanates from all parts of a solution of the sulphate of quinine, especially from its surface, is fluorescent, and that the light transmitted through the liquid, though sensibly like the incident white light, is no longer capable of producing fluorescence; it has been deprived of its chemical rays by absorption.

The chemical rays having been rendered visible by an increase in the length of the periods of vibration, unsuccessful attempts have been made to change the periods of the rays of heat beyond the red end of the spectrum so as to bring them within the limits of vision. The idea of effecting such a change by employing a substance opaque to light, but pervious to heat, is due to Dr. Akin; but it has since been accomplished by Dr. Tyndall, who, in the course of his experiments on radiant heat, found that a solution of iodine in the bisulphide of carbon excludes the most dazzling light, but transmits the rays of heat freely. He employed a mirror, lined in front with silver, to concentrate the rays emitted from the charcoal points of the electric lamp, and interposed a vessel containing the solution in question, so that the rays of heat alone were brought to a focus almost undiminished. When the solar spectrum was examined, the point of maximum heat was found to be as far beyond the extreme red on one side as the green rays on the other. In the spectrum of the electrical light the point of maximum heat was also found to lie beyond the extreme red, but the augmentation of intensity was so sudden and enormous as far to exceed the maximum heat of the sun previously determined by Professor Müller. Aqueous vapour powerfully absorbs radiant heat; so a solar spectrum beyond the earth’s atmosphere might probably exhibit as great intensity as the electrical light. With the apparatus described oxidizable substances burst into the flame of common combustion when put into the focus; but when the chemical action of the oxygen of the atmosphere was excluded by igniting substances in vacuo by the invisible rays of heat, their periods of vibration were so changed as to bring them within the limits of vision. When the electric light is very powerful, a plate of platinized platinum in vacuo is raised to white heat at the focus of invisible rays; and when the incandescent platinum is looked at through a prism, its light yields a complete and brilliant spectrum. ‘In all these cases we have a perfectly invisible image of the charcoal points formed by the mirror; and no experiment illustrates the change of heat into light’ more strongly than the following:—When the plate of platinum or one of charcoal is placed in the focus, the invisible image raises it to incandescence, and thus prints itself visibly on the plate. On drawing the coal points of the lamp apart, or causing them to approach each other, the thermograph follows their motion. By cutting the plate of carbon along the boundary of the thermograph, a second pair of coal points may be formed of the same shape as the original ones, but turned upside down; and thus by the rays of the one pair of coal points which are incompetent to excite vision, we may cause a second pair to emit all the rays of the spectrum. Fluorescence and calorescence act in contrary directions. Fluorescence causes the molecules of a fluorescent substance to oscillate in slower periods than the incident light, while calorescence causes the molecules of a substance to oscillate in longer periods than the incident light. The refrangibility of the rays is lowered in the first case, and raised in the second.

Substances differ as much in their transmission of the chemical rays as those of light and heat. Glass is impervious to the most highly refrangible chemical rays, while rock crystal transmits them with the greatest facility; and on that account the absolute length of the spectrum was not known till the light was refracted by prisms of rock crystal. Besides, the number, position, and intensity of the chemical rays vary with the source of light. Some flames have scarcely any chemical rays; that of the oxy-hydrogen blowpipe, though intensely hot, has very few, and even the solar light is inferior in that respect to electricity. The electric spark from the prime conductor of a common electrifying machine, or the discharge of a Leyden jar, emits rays of very high refrangibility, far surpassing those which emanate from the sun. For, when the electric light from a highly charged Leyden jar was refracted by two quartz prisms and thrown by Professor Stokes on a plate of uranium glass, the chemical spectrum was highly luminous, and six or eight times as long as the visible spectrum. An equally extensive spectrum was obtained from the voltaic arc taken between copper points; it consisted entirely of bright lines. The long spectrum also appeared on the uranium glass when the spark refracted by quartz prisms was obtained from the secondary terminals of an induction coil in connection with the coatings of a Leyden jar. It consisted of bright lines, but was not so luminous as that from a powerful voltaic battery. On changing the metals of the points between which the sparks passed, the bright lines were changed, which showed that they were due to the particular metals.

The heat of the electric spark volatilizes the metals which form the points of the conducting wires; and all volatilized metals give characteristic spectra, both visible and chemical. The visible part differs from that of the solar spectrum in being crossed by bright lines instead of dark ones; but the number, intensity, and position of both the visible and invisible lines change with each metal. The changes in the invisible part under consideration may be readily observed by throwing the spectra either on a fluorescent or collodion plate. For example: in the spectrum from the spark between thallium points thrown on the latter, Dr. Miller found that there were two strong groups of lines in the least refrangible part of the spectrum; at a little distance from these there were three groups, the two first less intense than the third; several rows of feeble dots followed, and the chemical spectrum terminated rather abruptly with four nearly equidistant groups. This spectrum bears a resemblance to those of zinc and cadmium, less strongly to that of lead. Dr. Miller found that the photographic spectra of iron, cobalt, and nickel, also have a strong analogy, but that the metals arsenic, antimony, and tin showed as great a difference in the invisible as in the visible part of their spectrum.

The fluorescent spectra of seventeen metals were examined by Professor Stokes of Cambridge; several of them showed luminous lines of extraordinary strength, especially zinc, cadmium, magnesium, aluminium, and lead, which in a spectrum not generally remarkable contains one line surpassing perhaps all other metals in brilliancy. Some other metals exhibit in certain parts of their spectra lines that are both bright and numerous; on the whole some parts of the spectra are strong and tolerably continuous, while in others they are weak. This grouping of the lines is most remarkable in copper, nickel, cobalt, iron, and tin. Of all the metals examined, magnesium gave the shortest spectrum, ending in a very bright line, beyond which however excessively faint light extended to a distance equal to that of the long spectra. Aluminium, on the other hand exceeded all the other metals in richness of the rays of the very highest refrangibility. All the strong lines mentioned lie in that part of the spectra.

In the course of these experiments Professor Stokes observed that even quartz of a certain thickness is not transparent to invisible lines of the highest refrangibility, for the highest aluminium line, which is double, could only be seen by rays passing through the edge of the prism. This leads to another branch of the subject, namely, the absorption of the invisible rays by solids, liquids and gases. Mr. Wm. Allen Miller has shown from his own experiments that bodies pervious to the chemical rays in the solid form, are so also in the liquid and gaseous form; that colourless transparent solids which absorb the photographic rays, absorb them more or less also in their liquid and gaseous states. He has moreover found that the following substances have the same maximum transparency:—rock crystal, ice, and fluor spar among solids, water among liquids, the three elementary gases and carbonic acid among gaseous substances. The most opaque to the invisible rays are, nitrate of potash, bisulphide of carbon, and sulphuretted hydrogen. It appears that a thin plate of mica is intensely opaque to all the invisible rays except a small portion of them of the lowest refrangibility.

The absorptive property however is partial: an absorptive substance either cuts off a portion of the light of a fluorescent spectrum or stripes it with dark lines: each substance absorbs rays peculiar to itself. Those employed by Professor Stokes were the alkaloids and glucosides, and he assumed the spectrum of tin for their examination because it has a long interval of continuity.

The fluorescent property of yellow uranite was discovered by Professor Stokes some years ago, and now he has added another fluorescent mineral in adularia or moonstone; from its natural faces and planes of cleavage alike a beautiful blue fluorescence emanated under the induction spark. As the same was observed in colourless felspars generally, Professor Stokes concluded that fluorescence is an inherent property in the silicate of alumina and potash constituting the crystal of moonstone. The blue fluorescence extended to a very sensible though small depth within the substance.

A particular variety of fluor spar found at Alston Moor in Cumberland, which is very pale by transmitted light, shows a strong blue fluorescence, and is eminently phosphorescent on exposure to the electric spark. It is the same kind of crystal in which Sir David Brewster originally discovered the property of fluorescence. On presenting such a crystal to the spark passing between aluminium terminals, besides the usual blue fluorescence, there was another of a reddish colour extending not near so far into the crystal, produced by the rays belonging to the strong lines of aluminium of extreme refrangibility.

The cube of fluor spar which showed these effects was externally colourless to the depth of ¹⁄₂₀ of an inch; then came one or two strata parallel to the faces of the cube showing the ruddy fluorescence, while the blue fluorescence extended to a much greater depth and had a stratified appearance. This crystal was eminently phosphorescent, its blue phosphorescence being arranged in strata parallel to the face of the cube like the blue fluorescence, but it was not perceptible beyond a very moderate distance below the surface at which the exciting cause entered, that cause being the photographic rays of extremely high refrangibility of the electric spark—taken, as in all these experiments, between the secondary terminals of an induction coil in connection with the coatings of a Leyden jar, and refracted by quartz prisms.

Mr. Stokes has employed fluorescence as a means of tracing substances in impure chemical solutions. When a pure fluorescent substance is examined in a pure spectrum it is found that on passing from the extreme red to the violet and beyond, the fluorescence commences at a certain point of the spectrum, varying from one substance to another, and continues from thence onwards more or less strongly in one part or another according to the particular substance. The colour of the fluorescent light is found to be nearly constant throughout the spectrum. ‘Hence when in a solution examined in a pure spectrum we notice the fluorescence taking as it were a fresh start with a different colour, we may be pretty sure that we have to deal with a mixture of two fluorescent substances.’

Experience as well as theory shows that rapid absorption is accompanied by copious fluorescence. But experience has hitherto also shown what could not have been predicted, and may not be universally true, that conversely absorption is accompanied in the case of a fluorescent substance by fluorescence.

The phosphorescent light of insects, fish, and plants is owing to chemical action, which produces many luminous phenomena; but a great number of inorganic and organic substances shine in the dark with a phosphorescence which is nearly allied to fluorescence. It is produced by exposure to the sun, by heat, electricity, insulation, cleavage, friction, and motion. For if a bottle containing nitrate of uranium be shaken, it shines spontaneously with a vivid light; even the hand shows phosphorescence in the dark after being exposed to the sun.

The essential difference between fluorescence and phosphorescence consists in the time during which the light lasts. Fluorescence ceases almost immediately after the exciting cause is withdrawn, while a phosphorescent body whether excited by heat, solar light, or electricity, lasts a much longer time; besides, the fluorescent rays are generally of lower refrangibility. Light and heat are temporarily absorbed and given out again by every body on the surface of the earth, more or less, that are exposed to the sun’s light. The nights would be much darker even when illuminated by the stars were it not for earth light, for the molecules restore to the ether, in the form of phosphorescence, the undulations they have received from the sun’s light during the day. The snow and ice blink of the sailors is a striking instance; generally, however, it is of much shorter duration. The phosphorescent property is nearly allied to electricity, for bodies that are bad conductors are apt to become phosphorescent, while good conductors of electricity rarely if ever show it. Ozone must be phosphorescent, for oxygen exhibits persistent light when electric discharges are sent through it, and Mr. Faraday saw a flash of lightning leave a luminous trace on a cloud which lasted for a short time.

In the solar spectrum the chemical or actinic rays produce phosphorescence, which the red rays have the power to extinguish. M. Nièpce de St-Victor found that solar light impresses its vibrations so strongly on substances exposed for a short time to its influence that they not only shine in the dark, but that the phosphorescent light they radiate has chemical energy enough to decompose substances in unstable equilibrium, and leave daguerreotype impressions of great delicacy and beauty.

The polarization of light and heat affords a remarkable instance of the elective power of matter. Light and heat are said to be polarized, which, having been once reflected, are rendered incapable of being again reflected at certain angles. For example, a ray incident on a plate of flint glass at an angle of 57° is rendered totally incapable of being reflected at that same angle from another plate of flint glass in a plane at right angles to the first. At the incidence of 57° the whole of the ray is polarized: it is the maximum of polarization for flint glass, but there is a partial polarization for every other angle; the portion of the ray polarized increases gradually up to the maximum, as the incidence approaches to 57°. All reflecting surfaces are capable of polarizing light and heat, but the angle of incidence at which the ray is totally polarized is different in each substance. Thus, the angle of incidence for the maximum polarization of crown glass is 56° 55ʹ, and no ray can be totally polarized by reflection from the surface of water unless the angle of incidence is 53° 11ʹ. As each substance has its own maximum polarizing angle, the effect is evidently owing to the action of the molecules of matter, and not to any peculiarity in the light or heat.[^[8]]

Light and heat are also polarized by refraction, for certain substances, especially irregularly crystallised minerals like Iceland spar, possess the property of dividing a ray of light or heat passing through them in certain directions into two pencils, namely, the ordinary and extraordinary rays. The first of these is refracted according to the same law as in glass or water, never quitting the plane perpendicular to the refracting surface, while the second does quit that plane, being refracted according to a different and more complicated law. Hence, if a crystal of Iceland spar be held to the eye, two images of the same object will generally be seen of equal brightness. But when they are viewed through a plate of tourmaline it will be found that while the spar remains in the same position the images vary in relative brightness as the tourmaline is made to revolve in the same plane; one increases in intensity till it arrives at a maximum, at the same time that the other diminishes till it vanishes, and so on alternately at each quarter revolution of the tourmaline, proving both rays to be polarized. For in one position the tourmaline transmits the ordinary ray and reflects the extraordinary, and after revolving 90°, the extraordinary ray is transmitted and the ordinary ray is reflected.

The undulations of the ethereal medium which produce the sensation of common light, are performed in every plane at right angles to the direction in which the ray is moving, but the case is very different after the ray has been polarized by passing through a substance like Iceland spar, for the light then proceeds in two parallel pencils whose undulations are still indeed transverse to the direction of the rays, but they are accomplished in planes at right angles to one another. The ray of common light is like a round rod, whereas the parallel polarized rays resemble two long flat rulers, one of which lies on its broad surface, and the other on its edge. By a simple mechanical law, each vibratory motion of the common light is resolved into vibratory motions at right angles to one another.

The polarization of light and heat by refraction is not owing to the chemical composition, but to a want of homogeneity in the molecular structure of the substances through which they pass; for regular crystals and substances which are throughout of the same temperature, density, and structure, are incapable of double refraction. The effect of molecular structure is strikingly exhibited by the circular polarisation in the dimorphic crystals of quartz. In one form the plane of polarization revolves from right to left, and in the other that plane revolves from left to right, although the crystals themselves differ apparently by a very slight and often almost imperceptible variety of forms.

Thus polarization forms the most admirable connection between light, heat, and crystalline structure; showing peculiar arrangements of the molecules in regions otherwise unapproachable, and too refined for our perceptions. Besides, the gorgeously coloured images displayed by depolarization are splendid examples of the power of matter in decomposing light.

The perfect correspondence of the properties of the symmetrical, elastic, and optical axes of crystals with light and heat is another instance of the connection between the latter and crystalline form.

The axis of symmetry is that direction or imaginary line within a crystal, round which all the parts or particles are symmetrically arranged. A medium is said to be elastic which returns to its original form with a resilient force after being relieved from compression, and the axis of elasticity of a crystal is that direction in which it is most elastic. The optic axis is that line or direction through which light passes in one beam according to the law of ordinary refraction. Crystals may have one, two or more optical axes according to their form. Doubly refracting crystals such as Iceland spar have only one principal optic axis in which the whole beam passes according to the ordinary law; in every other direction the beam of light is divided into two polarized rays, one of which called the ordinary ray passes according to the ordinary law, while the other, known as the extraordinary ray, traverses the crystal in a different direction, with more rapidity and according to a different and more complicated law. The velocity of this extraordinary ray is a maximum when at right angles to the principal optical axis, and a minimum when parallel to it.

In perfectly regular crystals like the cube or die, the octohedron, &c., there are three axes of symmetry and of equal elasticity at right angles to one another. In these regular crystals all the axes are optical, so that they have no double refraction.

Right square prisms have two equal rectangular axes of symmetry, two axes of equal elasticity, and one optical axis.

All crystals of the pyramidal and rhomboidal systems have one axis of symmetry, two axes of elasticity, one optical axis; and form coloured circular rings traversed by a black cross when viewed by depolarized light.

Lastly oblique prismatic crystals which have three unequal axes of symmetry have three axes of unequal elasticity, two optical axes; and by depolarization give coloured lamnescata, that is coloured figures having the form of the figure 8 which are traversed by a black cross in two opposite quadrants, and when the crystal is made to revolve, the same figure, but in the complementary colours and traversed by a white cross, appears in the other two quadrants.

The right and left-handed circular polarization of quartz, according as certain facettes of the crystal are turned to the right or left, and the property of double refraction being exclusively possessed by crystals of the rhomboidal form, are striking instances of the connection between the geometrical arrangement of the molecules of matter and the optical and thermal forces, for the polarization of heat and all its consequences are in every respect analogous to those of light, and similar phenomena would be seen were heat visible.

Heat changes the position of the optical axes of crystals. When applied to a crystal of sulphate of lime, the two optical axes gradually approach to each other and at last coincide; if the heat be continued and increased, the axes open again, but in a direction at right angles to their former position. Thus the force of heat throws every molecule in the body into correlative motion. The angles of all crystals that are not of the octohedral group are changed by heat and vary with the intensity; the difference between the length of the greatest and least optic axes in such crystals diminishes as the temperature is raised, increases when it is lowered, and is constant at a given heat. In Iceland spar heat indirectly affects the doubly refracting power, for the expansion of the crystal in the direction of its axis is accompanied by contraction at right angles to it, which brings the crystal nearer to the cubical form, and consequently diminishes its doubly refracting power.

According to the researches of M. Angström, in crystals with different axes of elasticity the velocity of the molecular vibrations is different in different directions when they are heated. In rock crystal and tourmaline the heat radiates from a surface cut parallel to the axis of the crystal; in felspar the radiating surface is at right angles to the symmetrical axis.

The optical axes of crystals are also affected by pressure. Doubly refracting crystals with one principal axis acquire two when the pressure is perpendicular to it. The new principal axis coincides with the line of pressure or is at right angles to it according as the crystal is positive or negative, that is, according as the extraordinary ray is refracted to or from the optic axis of the crystal. The colours produced by polarization are affected by compression and dilatation according as the crystal is positive or negative.

Sir David Brewster is of opinion that all the properties of double refraction and the gorgeous phenomena of polarization, whether by crystals or produced in various substances permanently or transiently by heat, cold, rapid cooling, compression, dilatation, and induration, are wholly the result of the forces by which the atoms are held together; but these phenomena may rather be said to depend upon a reciprocal action between an irregular molecular structure and the agency of light and heat: which indeed seems to be confirmed by the transit of these two forces through right and left-handed quartz, for there is no reason to believe that there is any difference in the form of the particles in these two crystalline substances.

The experiments of M. Becquerel show that electricity is a power which makes the atoms of matter aggregate in crystalline forms; for he has succeeded in forming crystals of gold, silver, cobalt, nickel, platinum, and a variety of the gems undistinguishable from those in nature, by exposing saturated solutions of these substances for a very long time to feeble voltaic electricity; and crystals of earthy matter have been obtained in the same manner. The electric and magnetic state of mineral veins in mines which contain a vast variety of crystals, metallic and non-metallic, strongly favours this view of the origin of crystalline form.

M. Regnault has proved that the ratio between the specific heat and the weight of the atoms of matter is intimately connected with the mode of their aggregation; and indeed if it be considered that the atoms have not only specific heat and weight, but specific affinity, electricity, magnetism, consequently polarity, and probably specific forms, these peculiar forces must necessarily influence the structure of crystals according as they combine with or oppose the natural or artificial forces acting upon them, or upon their dissimilar faces, and this may be the cause of the great variety of forms that matter appears under. Carbonate of lime alone assumes more than 1,200 different modifications of its primitive type, but whatever be the variety of forms which any one substance may take, they are found to be all compatible with and derivative from a common type. The circumstances which have caused dimorphous crystals to deviate from the general law have not yet been explained.

It is very singular that when chlorate of soda is dissolved in water the solution does not possess the property of circular polarization, but when evaporated and allowed to crystallise, some of the crystals turn light to the right, and others to the left. Now if all the crystals that have the same property be picked out and dissolved in water a second time, the liquid will still have no circular polarization, but when allowed to crystallise, some of the crystals make light revolve through them to the right and others to the left as before. From this it is supposed that the atoms of liquids, which are free to move in every direction, already possess part of the characters which the change to solidity renders evident and permanent.

Although the relations between the force of magnetism and the atoms of matter do not exhibit such brilliant phenomena as light does, they are nevertheless most interesting and wonderful. Mr. Faraday discovered that all substances, whether solid, liquid, or aëriform, are either magnetic like iron, or diamagnetic like bismuth, the latter being by far the most numerous. Thus if a bar of iron be freely suspended between the poles of an extremely powerful magnet or electro-magnet, it will be attracted by both poles and will rest or sit axially, that is, with its length between the poles or in the line of magnetic force; whereas an equal and similar bar of bismuth so suspended will be repelled by both poles and will rest or sit equatorially, that is with its length perpendicular to the line of magnetic force. Magnetism and diamagnetism are both dual forces, but they are in complete antithesis to one another, which is strikingly illustrated by their action on crystalline matter.

A sphere of amorphous substance freely suspended under magnetic influence is indifferent, that is to say it has no tendency to set one way more than another; but a sphere cut out of a crystal whether magnetic or diamagnetic, is more powerfully attracted or repelled in one direction than in any other, which shows a connection between the magnetic forces and crystalline structure.

Crystals of carbonate of iron and carbonate of lime are isomorphous, that is, they have exactly the same crystalline form, but the carbonate of iron being highly magnetic is most powerfully attracted in the direction of its greatest optical axis which therefore sets axially, that is, in the line of magnetic force; while the principal optic axis of the carbonate of lime, which is diamagnetic, is most powerfully repelled and therefore sets equatorially. In both cases the antithetic forces follow the same law of decrease in intensity from the greatest optical axis to the least.

A bar of soft iron sets with its longest dimensions axially, but a bar of highly compressed iron-dust, whose shortest dimensions coincide with the line of pressure, sets equatorially, because it is most powerfully attracted in the line of greatest density. A bar of bismuth sets equatorially, but a bar of highly compressed bismuth dust, whose shortest dimensions coincide with the line of pressure, sets with its length axially, because it is most strongly repelled in the direction of its greatest density. Hence the action of magnets upon matter is most powerful in the line of maximum density, the force being attractive or repulsive according to the kind of magnetism possessed by the atoms. It follows therefore that the density is greatest in the line of the principal optical axis, and gradually decreases to the least optical axis, where it is a minimum.

The position which crystals take with regard to the magnetic force depends also upon their natural joints of cleavages, and upon their power of transmitting electricity. The diamagnetic force is inversely as the conducting power of bodies, and the conducting power of crystals is a maximum in the planes of their principal natural joints. Hence the action of the diamagnetic power is least in the natural joints, and conversely the magnetic force is greatest. In fact, the magnetic phenomena of crystals depends upon unequal conductibility in different directions, and their set is determined by the difference between the forces of attraction and repulsion of the poles, for one pole of the magne-crystallic axis is attracted and the other repelled. It is unnecessary to give more examples to show the action of the magnetic forces upon the atomic structure of crystals.[^[9]]

Magnetism changes the relations and distances between the ultimate atoms of matter, a circumstance which probably depends upon their polarity. It changes steel permanently, iron temporarily, and it elongates a bar of iron, which loses in breadth what it gains in length; and as heat is developed in one direction and absorbed in the other, the temperature of the bar remains the same. Heat being an expansive force, diminishes the magnetism of iron and nickel in proportion as it increases the distance between their atoms, till at length they lose their cohesive force altogether. But there seems to be a temperature at which the magnetic force is a maximum, above and below which temperature it diminishes. Thus the magnetism of cobalt increases with the temperature up to a certain point; it then decreases as the temperature increases, and it loses its magnetism altogether when the heat amounts to 1996°.

Sir Humphry Davy and M. Arago noticed that the voltaic arc takes a rotatory motion on the approach of a magnet; and the effect of magnetism on the stratified appearance of the electric light in highly rarefied air shows how powerful its action is. In the year 1858, Mr. Gassiot published a series of observations on stratified light; subsequently various publications appeared on the subject both by Mr. Gassiot and by Professor Plücker, who made a series of very interesting observations on the nature of the stratifications, but more especially on the effects produced when they are under the influences of magnetism. Since that time, Mr. Gassiot has published several papers on the subject, and still continues his experiments on the stratifications of electric light, which give a visible proof of the connection between electricity and magnetism. He first showed that the stratified character of the electric discharge through highly attenuated media is remarkably developed in the Torricellian vacuum; latterly he has made his experiments by passing electricity through closed glass tubes of various lengths and internal diameters, filled with highly attenuated gases and vapours.[^[10]] Two among the many brilliant experiments of this gentleman may be selected as illustrations of the property of electric light.

One of these closed glass tubes containing a highly attenuated gas was 38 inches long with an internal diameter of about an inch, and had the extremities of two platinum wires fused into the same side 32 inches apart. When these wires were put in connection with the wires of an induction battery and brought into contact, and the electricity passed through the tube, the luminous appearances at the extremities or poles of the platinum wires were very different, but simultaneous. A glow surrounded the negative pole, and in close approximation to the glow, a well defined black space appeared, while from the positive pole there issued in rapid succession a series of alternate dark and brilliantly luminous curved strata, which formed a column of stratified light, the concavities of the strata being turned to the positive pole. The stratifications do not extend to the black band round the negative wire or ball, which is quite different to the dark intervening space between the stratified discharge and the luminous negative glow. On making and breaking the electric circuit, the stratified discharge emanates from each pole alternately, the concavities of the strata turning alternately in different directions; in fact the whole phenomena are reversed, but not changed. ‘The stratified discharge arises from the impulses of a force acting on highly attenuated but resisting media,’ a new proof of the wonderful power inherent in highly attenuated gases; the number of stratifications given out at each discharge, depending upon the intensity of the electricity and rarity of the gas.

Fig. 1.

[Fig. 1] represents the form which the stratified discharge assumes in a vacuum tube one inch diameter and 38 inches in length, + and - representing platinum wires attached to the terminals of a Ruhmkorff’s induction coil.

When the tube, with its stratifications just described, was laid horizontally on the pole of a magnet, the stratified column showed a tendency to rotate as a whole round it. According to the theory of Ampère, the polarity of a magnet is owing to a superficial current of electricity perpetually circulating in a direction perpendicular to its axis; and he also showed that currents of electricity flowing in the same direction attract one another, while currents flowing in opposite directions repel each other. Hence, since the currents of electricity in the magnet and tube were flowing in the same direction on one side of the magnet, and in opposite directions on the other side, the stratified column was attracted at one end and repelled at the other, so as to take the form

, in consequence of its tendency to rotate as a whole round the pole of the magnet.

When narrow bands of tin foil wrapped round the glass tube near the platinum wires were put in communication with the poles of the induction battery, brilliant stratifications filled the whole tube between the tin coatings every time the electric circuit was broken or renewed; and when the tube was placed horizontally on the pole of a magnet, the stratifications no longer showed a tendency to rotate as a whole, they were divided into two parts tending to rotate in opposite directions; when the tube was placed between the poles of a powerful electro-magnet, one half of the stratifications were repelled and the other half attracted. When the tube was placed on the north pole, the divided stratifications arranged themselves on each side of the tube, changing their respective positions when placed on the south pole, but in every case each half was concave in opposite directions.

[Fig. 2] (p. 81) represents the form which the induced stratified discharges assume when the vacuum tube is placed on or between the poles of a powerful electro-magnet—the tin foil coatings C+ C- being attached by wires to the terminals of an induction coil.

Fig. 2.

If a vacuum tube with or without wires or tin coatings be laid upon the induction coil of a battery, or upon the prime conductor of an electrifying machine, stratifications are produced by induction which are divided by a magnet. Thus there are two distinct forms of the stratified discharge, one direct, the other induced.

When Professor Plücker of Bonn sent an induced current of electricity from Ruhmkorff’s coil through a vacuum tube having a platinum wire fused into each extremity, and extending a little way into the interior of the tube, electric light radiated from every point of the negative wire, and when exposed to the action of an electro-magnet the whole tube was filled with a luminous atmosphere. But when all the negative platinum wire except its extreme point was insulated by a coating of glass, the rays of electric light which radiated from the point were united into one single and perfectly regular magnetic curve, upon the approach of an electro-magnet; when the negative platinum wire was partially insulated by glass coating, electric light emanated from every exposed part, and assumed the form of magnetic curves under electro-magnetic action. Whence Professor Plücker concluded that the luminous atmosphere in the first experiment was the locus of an infinite number of magnetic curves, and consequently that magnetic light emanates from the negative or warmth pole, and electric light from the positive or light pole. These magnetic curves of light are precisely similar to those assumed by iron filings from magnetic action.

The most remarkable of these experiments is the absolute extinction of a powerful electric discharge by magnetic action. Mr. Gassiot sent a discharge from a voltaic water battery, containing 3,520 insulated cells, into a tube filled with attenuated carbonic acid gas. The discharge was so strong that it was capable of passing through more than six inches of the gas, yet, on the approach of a very powerful electro-magnet, the stratifications were arrested as soon as they appeared, as if blown out, and finally extinguished. A stratified discharge, in vacuo, from 400 insulated cells of a nitric acid battery, was extinguished by the large electro-magnet of the Royal Institution; the luminous strata rushed from the positive pole of the battery, but under the magnetic force they retreated; cloud followed after cloud with deliberate motion, appearing as if swallowed up by the positive terminal. The amount of electricity that passed through the tube appeared to be materially increased by exciting the electro-magnet; the discharge was so intense on one occasion as to fuse half an inch of the positive terminal. A very powerful magnet is also capable of extinguishing a stratified discharge. In fact, according to the law of the reciprocal action of magnetism, the forces are equal in intensity and opposite in direction.

The electric discharge from an induction coil is discontinuous, or eruptive sparks of high tension are given out producing stratified discharges.

The discharge of the voltaic battery had hitherto been considered absolutely continuous; and so it is for chemical action, whether of analysis or combination; nevertheless certain phenomena gave reason to doubt its continuity. Mr. Gassiot has proved that the tension of a single cell of a galvanic battery increases in force according to the chemical energy of the exciting liquid, and in all his experiments he found that ‘the higher the chemical affinities of the elements used, the greater was the development of evidence of tension.’ These observations induced him to institute a series of experiments with galvanic batteries of different chemical affinities, and to compare the resulting phenomena with those produced by the induction coil, whose sparks are of high tension. The same carbonic acid vacuum tubes were made use of in all the experiments; a copper wire formed the positive terminal, and a copper plate was fixed at the extremity of the negative terminal. In other tubes platinum terminals extended into the interior, coated with glass, except the points, to which charcoal balls were fixed. One end of the tubes was of small diameter and contained caustic potash.

When a discharge from an induction coil was sent through these tubes, there were either minute luminous spots, narrow stratifications, or a well defined cloud-like discharge at the positive pole, according to the size and structure of the terminal, but the characteristic phenomenon in all the tubes was a large cloud-like luminosity or circular glow on the brass plate or charcoal ball at the negative terminal.

With 512 insulated cells of copper and zinc of Daniell’s constant battery, the exciting liquid being dilute sulphuric acid, a brilliant glow appeared round the charcoal ball of the negative terminal on the passage of the electric discharge through the tube, with very trifling luminosity of the positive pole.

Two copper plates that could be separated or closed by a screw, were placed between the poles of a nitric acid battery, so that the circuit could be made or broken gradually, and spark discharges were obtained between them. The vacuum tubes were placed between one of these plates and a pole of the battery; one of these tubes was 24 inches long, 18 in circumference, and had a circular copper disc 4 inches in diameter on its negative terminal. On completing the circuit, the discharge of the battery passed with a display of magnificent strata of dazzling brightness; on separating the plates by the screw, the luminous discharges presented the same appearance as when taken from an induction coil, but brighter. On the copper disc within the vacuum tube, there was a white layer, then a dark space about an inch broad, and then a bluish atmosphere curved like the disc, evidently three negative envelopes on a great scale. When the disc was made the positive pole, the effect was feeble.

In vacuum tubes 6 inches long and 1 inch diameter, with carbon balls on the terminals, the discharge of the nitric acid battery elicits extreme heat. In one of these the discharge presented a stream of light of intolerable brightness, but when viewed through a plate of green glass strata could be seen. This soon changed to a sphere of light on the positive ball, which became red hot, the negative being surrounded by magnificent envelopes; with a horse-shoe magnet the positive light was drawn out into strata. The needle of a galvanometer in circuit was violently deflected and the polarity reversed. When the caustic potash was heated, the discharge burst into a sunlike flame, subsequently subsiding into three or four large strata of a cloud-like shape, but intensely bright. This is called the arc discharge: it occurs in vacuum tubes with charcoal balls; when the potash is heated intensely, dazzling stratifications suddenly emanate from the positive ball, and powerful chemical action takes place in the battery, after which the discharge ceases.

This process facilitates the discharge and assists the disintegration of the carbon particles, and these in a minute state of division are subsequently found attached to the sides of the glass. It is these particles which produce the arc discharge with its intense vivid light so suddenly observed with far more brilliant effects than the usual stratified discharge. During its passage the conducting power of the vacuum tube is greatly enhanced.

It was already mentioned that a stratified discharge was obtained from 3,520 insulated cells of a water battery, which differs but little in intensity from 400 cells of the nitric acid battery. On one occasion the electricity seemed to pass through the vacuum tubes in a continuous stream, but when examined with Mr. Wheatstone’s revolving mirror it was decidedly stratified. Mr. Gassiot never could obtain a continuous discharge in air, whether between the points or metallic plates of the water battery. The discharge was invariably in the form of minute clearly defined and separate sparks.

Thus it was proved by the preceding experiments that a spark discharge could be obtained in air from both the nitric acid and water battery; and that when these discharges were passed through the highly attenuated matter contained in carbonic acid vacua, the same luminous and stratified appearance was produced as by an induction coil; a proof that whatever may be the cause of the phenomena it could not arise from any peculiar action of that apparatus.

Mr. Gassiot finally concludes that the cause of the stratified discharge arising from the impulses of a force acting upon highly attenuated but resisting media is also applicable to the discharge of the voltaic battery in vacuo; while the fact of this discharge even in its full intensity having been now ascertained to be also stratified leads to the conclusion that the ordinary discharge of the voltaic battery, under every condition, is not continuous but intermittent, that it consists of a series of pulsations or vibrations of greater or less velocity, according to the resistance in the chemical or metallic elements of the battery or the conducting media through which the discharge passes.

Caustic potash absorbs the carbonic acid gas by degrees, and at last so completely exhausts a vacuum tube that electricity cannot be conducted. Air is a non-conductor, and an electric discharge that will not pass through an inch of air, will pass through more than 30 or 40 inches of attenuated gas.

It has already been mentioned that the stratified discharge can be obtained by a single discharge of the primary current of an inductive coil, however long may be the vacuum tube through which the discharge is passed. If no addition be made to the battery and no alteration be made in the arrangement of the coil so as to increase or diminish the intensity of the discharge, the stratifications will always present the same appearance and form, occupying the same spaces and positions in the vacuum tube; but if any change be made so as to alter the intensity, then a corresponding alteration will appear in the discharge, the striæ assuming a different shape, and the bright and dark divisions occupying different positions.

In order to try what effect a change of intensity would produce, three separate insulated voltaic batteries, in which the exciting liquid was brine, formed an electric circuit which was completed by two long wires. It was so arranged that the discharge of one, two, or all the three batteries could be separately employed. In order to vary the resistance at pleasure, two tubes 18 inches long containing distilled water and connected at their base were introduced into the circuit. By varying the depth to which the terminal wires of the circuit were plunged into the water, the resistance could be regulated at pleasure, and it was immaterial in what part of the circuit the vacuum tube was introduced provided the circuit was completed.

The first experiments were made with a carbonic acid vacuum tube 20 inches long and 4 inches in diameter. The negative terminal at one extremity of the tube was of aluminium, cup-shaped, about 3 inches in diameter; the positive terminal was a wire of the same metal fused into the other extremity of the tube; the point of the wire and cup were about four inches and a half apart. With this tube and 2,240 cells of the battery the discharge when the resistance was introduced had the appearance of a positive and negative discharge, impinging on and intermingling with each other, without any dark space intervening. Around the negative terminal the luminosity extends to the sides of the tube and tapers to the point of the positive wire. The light round the negative terminal becomes brighter, a dark space appears next to it when the resistance is diminished, and increases as the resistance decreases, by the rolling back of the light in bright clouds to the point of the positive terminal. These changes can be perfectly regulated by the resistance, and various luminous phenomena occur at each stage.

With 2,240 cells distinct sounds were heard in the tube; with the whole battery of 3,360 series the sounds were not heard till a magnet was applied to the striæ, when they again became audible and the striæ were spread over the surface of the tube.

A carbonic acid vacuum tube with platinum terminals fused into the same side far apart was now put into the circuit, the part of the wires that penetrated within the tube being coated with glass up to the carbon balls in which they terminated. When a discharge from all the three batteries passed through the tube, changes occurred in the form and number of the striæ corresponding to the greater or less amount of the resistance offered in the circuit.

At the commencement of the experiments there were 18 inches of water in each of the tubes, which formed the maximum resistance. The wires attached to the terminal wires of the battery were placed inside of these tubes, and as soon as they touched the surface of the water a faint luminous discharge was seen at each ball in the vacuum tube. As the wire attached to the negative end of the battery was slowly depressed, the two luminous discharges appeared to travel towards or attract each other, and at times a portion of the positive luminosity passed over and mingled with the negative; in this state the discharge was extinguished by a magnet.

When the wire was pressed farther into the water a dark space about an inch in length divided the light into two parts, the positive glow being sharply defined, the negative glow having an irregular edge. When the wire had been about three inches deep in the water, the positive and negative glows became more brilliant, and a single clearly defined luminous disc burst from the positive side and occupied the middle of the dark space. When the wire was pressed down till 13 inches of it were in the water, a second luminous disc travelled from the positive side, and then the two luminous discs or striæ occupied the dark space at a little distance from one another. As the wire was pressed more into the water, three parallel luminous striæ appeared, then four, then five, and so on till as many as thirteen or fourteen striped the dark central space. With the full power of the battery, the adjacent disc impinged on the glow that surrounded the negative ball. This disc was of a pale green, those adjacent were reddish, while the negative glow was of a bluish white; minute bright scintillations emanated from the negative ball, while distinct luminous flash discharges took place through the striæ. Thus by the amount of resistance introduced into the circuit, the number of striæ can be regulated, their position fixed, separating or closing up the dark space between the luminous glows round the balls.

In these experiments there is indication of a force emanating from the negative wire. The actual disruption of the particles from the negative terminal also indicates a force, and the disruption is as freely obtained by the continuous discharge of the battery as it is by the intermittent discharge of the induction coil. Besides, when Mr. Gassiot sent discharges from the induction coil through Torricellian vacua, he several times observed that while a cloud-like discharge issued from the positive terminal, a long tongue of the most brilliant blue phosphorescent light emanated from the upper part of the negative terminal, and a brilliant white tongue of light was also seen close to the negative wire: so there is reason to believe that force emanates from both terminals.

Some of the preceding striated discharges ‘present an appearance somewhat analogous with the stationary undulations (or nodes) which exist in a column of air when isochronous progressive undulations meet one another from opposite directions, and on the surface of water by mechanical impulses similarly interfering with each other.’[^[11]]

‘May not the dark bands be the nodes of undulations arising from similar impulses proceeding from positive and negative discharges? or can the luminous stratifications which we obtain in a close circuit of the secondary coil of an induction apparatus, and in the circuit of a voltaic battery, be the representations of pulsations which pass along the wire of the former, and through the battery of the latter, impulses probably generated by the action of the discharge along the wires?’

The action of magnetism and electricity on light is similarly illustrated by the rotation of the plane of polarization. Sir John Herschel was the first who tried to rotate the plane of polarization of a ray of light by surrounding it with a spiral wire electrized by the great battery of two enormous plates of copper and zinc at the London Institution, but he obtained no evidence of any such action. Long afterward Professor Faraday succeeded by sending a ray of light through a piece of silico-borate of lead, which formed the core of a magnetic helix. The silico-borate took on a quasi-crystallised state during the passage of the electric current round it, giving it for the moment the property of circular polarization, analogous to that of glass in a state of tension or compression.

Substances vary exceedingly in the facility with which they transmit electricity; even the same substance under another form differs remarkably in that property: charcoal, which next to the metals is the best conductor known, when under the form of diamond is quite impervious to electricity. In general, substances that are the best conductors of heat are also the best conductors of electricity, as for example the metals, which however, possess the transmissive property in very different degrees. Silver and copper are the best conductors, lead one of the worst; its resistance to the passage of electricity is twelve times greater than that of silver and copper, consequently it becomes twelve times as hot, for when a current of electricity is impeded it is changed into heat. So great is the resistance offered by a fine platinum wire, that the heat amounts to 3280° and the wire is melted, a striking instance of the correlation of electricity and heat, and of the power of the cohesive force.

When electricity is passing through conducting substances or when it is static, it induces an electric state in bodies at a distance by transmission through non-conducting substances or air, for it gives polarity and tension to the adjacent atoms, and these to the next, and the next in succession, throughout the whole intervening mass,—a strong proof of the individuality and polarity of the atoms of matter.

Motion, which is the result of all the physical powers, has itself a strong action upon the ultimate elements of matter; in cases of unstable equilibrium it accelerates and even determines their chemical union. Some substances will remain merely mixed as long as they are at rest, but no sooner is their inertia disturbed by a slight motion than they rush into permanent combination. In newly sublimed iodide of mercury the vibration impressed by the scratch of a pin is so rapidly transmitted through the mass that its colour is immediately changed from yellow to bright red. By a new arrangement of the molecules their action on light is altered.

Catalysis or the chemical decomposition and composition of substances by the contact of a foreign body, is well illustrated by the chloride of nitrogen, that explodes when touched by substances which at ordinary temperatures would neither combine with the chlorine nor with the nitrogen. The iodide of nitrogen explodes if touched by a feather, and M. Becquerel decomposed the iodide of nitrogen by the vibrations of sound. When substances only exist in consequence of the inertia of their atoms, the instability of their chemical attractions and repulsions is only increased by an external agent, so that a great effect is produced by a slight cause, as in an avalanche, the snowy mass is on the point of falling, and the smallest motion, a breath of wind, hurls it down. In such cases the potential energy of the unstable mass is in a moment changed into vis viva or impetus. Daguerreotype impression shows the power of the chemical rays on substances in unsteady equilibrium, and the length of time required to make the impression under the same circumstances is a measure of the instability.

Most of the fulminates are compounds of nitrogen; of that the fulminate of aniline is a recent instance, since it is formed by the slow action of nitrous acid on aniline. Explosion takes place on the sudden evolution of gas, or the sudden change of a solid into vapour. In these cases fire or percussion are the foreign causes of change. They are all particular instances of the general principle of catalysis, which is the chemical combination of heterogeneous atoms by the action of a substance that does not participate in the change. Thus it has long been known that when platinum is plunged into a mixture of oxygen and hydrogen it combines these gases into water. Acids in some cases seem to have the same effect; for when rags or starch are dissolved in an acid the starch is changed to dextrine and the liquid has acquired the power of turning the plane of polarised light to the right. The acid has undergone no alteration, but it has changed the properties of the starch though not its chemical composition. After a time, a second transformation takes place, the liquid ceases by degrees to turn the plane of polarisation to the right, and ends by turning it to the left. The acid is still unchanged, but the dextrine has now disappeared: it has combined with the water and is transformed into glucose or sugar of grapes.

The quantity of the physical powers, active and latent, is inappreciably great. The quantity of heat or potential energy generated by chemical combination alone is enormous.

SECTION III.
ATOMIC THEORY, ANALYSIS AND SYNTHESIS OF MATTER, UTILITY OF WASTE SUBSTANCES—COAL-TAR COLOURS, ETC.

The chemical combination which forms the infinite variety of substances in the organic and inorganic creation consists in an intimate union of their ultimate atoms which produces substances differing from their constituent parts in every respect except gravitation, the sum of the weights of their constituent parts being invariably equal to the weight of the resulting substance. Thus the chemical union of oxygen and hydrogen forms water, and the weight of the water so formed is exactly equal to the sum of the weights of the two gases.

All chemical changes whether of analysis or composition are subject to definite unalterable laws of weight, measure and number; nothing is by chance or casual, the relative weights of the invisible atoms of matter, and their combination in definite proportions reveal the laws which prevailed in the primeval structure of created things. By the wonderful discovery of these laws Dr. Dalton has placed chemistry on a strictly numerical basis.

The chemical union of different kinds of atoms and volumes of matter in the definite proportions of whole numbers entirely changes their character and properties, as for example the chemical combination of one atom of hydrogen and one atom of oxygen into water. The condensation is often unexpected and wonderful; two different liquids are often condensed into a solid, and the result of the chemical combination of two different gases or vapours in quantitative proportions may be solid, liquid or aëriform, a fact which could only have been discovered by experiment. The powers of the atoms are changed and often highly exalted by chemical union as in ammonia, a chemical compound of three atoms of hydrogen and one of nitrogen, which absorbs 1,195 times more radiant heat than its constituents whether simple or mixed. During chemical combination light and electricity are often evolved, heat always. The quantity given out is exactly proportional to the energy of the chemical action, and is often so great and so rapidly evolved as to produce an explosion by the sudden expansion of the air around. Whatever the temperature may be, which is given out during the union of the atoms, the very same quantity of heat is requisite to dissolve their union, and the atoms are separated in the same definite proportions in which they were combined.

Voltaic electricity both combines and resolves substances into their component parts, strictly according to the law of definite proportions. It combines eight parts by weight of oxygen and one part by weight of hydrogen into water; and again when it decomposes water, one part by weight of hydrogen is given out at the negative pole of the battery, and eight parts by weight at the positive or zinc pole. For an electric current weakens or neutralizes the force of affinity in one direction and strengthens it in the other, so that the heterogeneous atoms of the substance under its influence have a tendency to go in different directions and appear at opposite poles. Mr. Faraday has established as a general law, that the quantity of electricity requisite to unite the atoms of matter, is precisely equal to the quantity requisite to separate the same atoms again. Electro-chemical action, or the power of electricity to combine and separate the heterogeneous atoms of matter, is in direct proportion to the absolute quantity of electricity that passes in the current. Hence the superior analytical power of voltaic over static electricity, which has enormous intensity, but is very small in quantity. The electric current separates molecular combinations which yield to no other means: it is the most powerful instrument of analysis; light is the most delicate.

Two simple substances are only capable of a certain number of chemical combinations, which form a regular series of new substances; as for example oxygen and nitrogen. Two measures of nitrogen gas will unite with one measure of oxygen to form the protoxide of nitrogen; with two measures of oxygen it unites to form the binoxide of nitrogen; with three measures of oxygen it forms the hyponitrous acid; with four it forms nitrous oxide; and with five measures of oxygen it forms nitric acid. Thus there are five compounds of nitrogen and oxygen, no more. Affinity of kind is merely the attraction of one element or atom of matter for another; affinity of degree consists in the grades and limits of combination; the preceding series is of the fifth degree; the limit is the last term, for no further combination of these two gases can take place, and these are accomplished by art. All the five substances are deleterious, most of them deadly poisons, for the protoxide of nitrogen, which is the laughing gas, could not be long inhaled with impunity. For a long time the middle term of the preceding series was wanting, but Gay-Lussac formed it by attending to the laws of definite proportion and sequence.

The atoms of different kinds of matter possess an affinity, or attractive force, which binds them together chemically in different and very unequal degrees. Two substances may unite and form a third differing from both, as water does from oxygen and hydrogen; but if a new substance be added which has a greater attraction for one of the substances than for the other, it will dissolve their union, combine with that for which it has the strongest attraction, and set the other free. Thus the metal potassium, which has a greater attraction for oxygen than it has for hydrogen, decomposes water, combines with the oxygen, and sets the hydrogen free. Both chlorine and ozone have the property of liberating the iodine in a weak solution of the iodide of potassium; the liquid stains starch blue, a proof of the free iodine. The facility with which acids and alkalies combine affords the means of eliminating either the one or the other from a compound so as to liberate what remains.

The constituents of compound substances may be separated from one another by a variety of means depending upon their greater or less fusibility, volatility, and other properties. Water, acids, alcohols and other liquids hot or cold, different degrees of temperature, sublimation, solution, distillation, evaporation, together with static and voltaic electricity, are the most powerful means of analysis.

But the animal and vegetable creation rear their fabrics by a synthetic process. A plant after having absorbed carbonic acid and water, decomposes the carbonic acid, returns the oxygen to the atmosphere, and combines the carbon and water into wood, leaves, and a variety of organic substances. Now MM. Berthelot, Wöhler, and other distinguished chemists, by following this example of nature, have established a system of synthetic chemistry, by which they have produced from the chemical combination of the three elementary gases and carbon alone more than 1,000 complete organic substances, precisely the same with those formed within the living plants and animals. Yet we are as far as ever from any explanation of the mystery of life, whether animal or vegetable.

Carbon and hydrogen will not combine at any artificial heat however great; but when the electric arc between highly purified charcoal terminals passes through hydrogen gas, acetylene, a new carburet of hydrogen, is formed, consisting of four equivalents of carbon and two of hydrogen. This substance, which no organized being is capable to form, was discovered by M. Berthelot, and being assumed as a base, yielded an extensive series of organic substances. Thus when two atoms of carbon are added to acetylene it becomes olefiant gas; when two equivalents of oxygen are added to olefiant gas, the result is alcohol, which is transformed into acetic acid by the addition of two atoms of oxygen, and from this by a similar process have been obtained the malic, tartaric, succinic, and the other acids; glycerine also, which is the sweet principle of the oils, wax, essential oils, the perfumes of fruit and flowers, the principle of the balms, the essential oil of mustard, and numerous other organic substances, simply from carbon, oxygen and hydrogen; but nitrogen was introduced by combining alcohol with ammonia, an inorganic substance consisting of three equivalents of hydrogen and one of nitrogen, from whence a vast number of nitrogenized substances were derived, both animal and vegetable.

Chemical combination, which has from the beginning of created things, and still is, building up organic and inorganic matter in the earth, in the air, and the ocean, exerts forces of transcendent power, though silent, unperceived, and for the most part unknown. Professor Tyndall has given a striking instance of this in water, the most simple compound of oxygen and hydrogen, a constituent alike of organic and inorganic nature. ‘In the combustion of the two gases to form a gallon of water weighing ten pounds, an energy is expended, the atoms clash together with a force, equal to that of a ton weight let fall from a height of 23,757 feet; and in the change from the state of vapour to water, an energy is exerted equal to that of a ton weight falling from a height of 3,700 feet, or of a hundredweight falling from a height of 74,000 feet. The moving force of the stone avalanches of the Alps is but as that of snowflakes compared with the energy involved in the formation of a cloud. In passing finally from the liquid to the solid state,’ that is from water to ice, ‘the atoms of ten pounds exercise an energy equal to that of a ton weight falling down a precipice of 550 feet of perpendicular height.’

From Mr. Joule’s investigation of the relation existing between chemical affinity and mechanical force, it appears that when affinity is feeble it can be overcome mechanically. He formed amalgams of different metals, that is he combined them with mercury, by electricity. The affinity of iron for mercury is so feeble that the amalgam is speedily decomposed when left undisturbed by the pressure of the atmosphere, and if a greater pressure be added, almost all the mercury is driven out. The efficacy of mechanical force to overcome feeble chemical affinities is strikingly illustrated by the amalgam of tin, out of which nearly the whole of the mercury is driven by long continued pressure. In these cases the force of affinity did not amount to chemical equivalency, otherwise the mercury could not have been driven out by so small a force. Instances from the weakest to the strongest affinity show that it is only when the power reaches a definite point that the law of chemical equivalents comes in. The intense energy which then begins to be exerted has just been shown.

It is vain to hope for a knowledge of the absolute weight of the ultimate atoms of matter, and nothing seems to be more beyond the power of man than to determine even their relative weights; yet the definite proportions in which they combine have enabled him to do so. Thus, an atom of oxygen unites with an atom of hydrogen to form water; but as every drop of water, however small, contains eight parts by weight of oxygen, and one part by weight of hydrogen, it follows that an atom of oxygen is eight times heavier than an atom of hydrogen. Now, since hydrogen gas is the lightest body known, its atom has been assumed as the unit of comparison. Hence, if the unit of hydrogen be represented by 1, that of oxygen may be represented by 8. Again, carbonic acid gas contains six parts by weight of carbon, and eight parts by weight of oxygen, and as an atom of oxygen is eight times heavier than an atom of hydrogen, therefore an atom of carbon is six times heavier than an atom of hydrogen, and consequently may be represented by 6. In this manner the relative weights of many substances have been determined. But the property of isomorphism also affords the means of ascertaining the atomic weights of certain substances with unerring certainty. It is exactly the contrary of dimorphism, for in the latter substances are chemically the same under different forms; whereas isomorphic bodies are chemically different under the same form. Now the peroxide of manganese contains one atom of oxygen for one atom of metal; but in 100 parts of the protoxide there are 21·94 parts of oxygen and 78·06 of manganese. Comparing these numbers with 8 the atomic weight of oxygen, the result is 28 the weight of an atom of manganese. The same number is obtained from two other isomorphic compounds of oxygen and manganese, which proves the accuracy of this result. The atomic weights of many bodies have been determined, of which the following are the most important.

Atomic Weights, an Atom of Hydrogen being the Unit.

In the determination of atomic weights a few cases have occurred of fractional numbers, and although it cannot yet be affirmed that no such cases exist, yet it seems to be established by the new and more perfect analyses of MM. Dumas, Isidore, Williamson, and others, that the atomic weights of substances compared with an atom of hydrogen are in whole numbers.

This law leads to very important results. For example, the equivalent weights of the chemical elements of bodies derived from their specific gravities are either identical with, or simple multiples or sub-multiples of, their relative weights. Thus the specific gravity of hydrogen is 0·0693, and that of oxygen is 1·111; hence taking hydrogen as the unit of comparison, it is easy to see that 0·0693 : 1·111 :: 1 : 16, the simple multiple of 8, the relative atomic weight of oxygen. In fact since each substance has its own specific gravity or weight, that weight must depend upon the weight of its atoms, so that the weights of equal bulks of different substances are proportional to the weights of their atoms, and thus a relation is established between the atomic weights and specific gravities of bodies, so that one being given the other may be found.

Atoms like their substances have many different capacities for heat and electricity. It was proved by MM. Petit and Dulong, that specific heat, or the quantity of heat required to raise a simple substance to a given temperature, is inversely as the weight of its atoms, so that the specific heat or repulsive force of simple substances multiplied by their atomic weights is a constant quantity. Such is the condition requisite for the equilibrium or equality of force; or the law may be thus expressed: A given quantity of heat will raise to the same number of degrees a portion of every simple substance represented by its atomic weight. For instance, the atomic weight of sulphur is 16, that of zinc 32·5; hence it requires twice as much heat to raise a pound of sulphur ten degrees as it does a pound of zinc. It has also been proved that the atoms of compound bodies of analogous composition are endowed with the same capacity for heat, so that there is a perfect correspondence between the weight of atoms and their specific heat. The numbers representing the atomic weights derived from the specific heat of bodies are connected with their equivalent atomic weights by the simple ratios of equality, multiples or sub-multiples.

Mr. J. Croll has made experiments showing that the specific heat of compound gases and liquids is generally less, and those of solids more, than that of their component elements, which is contrary to the hitherto received opinion. Moreover it appears that the changes in the specific heat of bodies which occur during combination are not only due to chemical action, but also to molecular changes; the real specific heat of a simple atom probably remaining the same under all conditions.

Mr. Faraday has proved that the specific electricity of different substances is also in proportion to their atomic weights, that is to say, a given quantity of electricity will separate combined substances into parts represented by their atomic weights. For example, 32·5 parts of zinc will generate voltaic electricity enough to separate nine parts of water into eight parts of oxygen and one part of hydrogen gas. The weights thus derived from decomposition are exactly the same with those determined by composition, and thus the atomic weights derived from electro-decomposition accord exactly with those obtained from chemical composition. Moreover, Mr. Faraday, as already mentioned, proved that the very same quantity of electricity necessary to decompose a body into its elementary atoms, is requisite to unite them again. The analysis and synthesis of compound matter, solid or fluid, show a constant and definite proportion of the component elements expressed by number, and by an equivalent or multiple ratio of parts in every chemical change.

The atomic theory unites, by a common bond, specific gravity, chemical affinity, heat, and electricity. Taking atmospheric air at the temperature of 60° Fahr. and a barometric pressure at 30 inches as the standard unit of specific gravity; the quantity of heat required to raise a volume of water 1° Fahr. as the unit of specific heat; hydrogen gas as the unit of atomic weight; and atomic electro-chemical electricity as the unit of specific electricity, the following numbers have been established:

The distances between the atoms of the gases are equal, hence the atomic weights of simple gases are proportional to their densities; and for the same reason, equal volumes of the same fluid contain an equal number of atoms, and the number of atoms in the same volume of different fluids is in the simple ratio of one to one, one to two, one to three, &c.