PART I.ATOMS AND MOLECULES OF MATTER.
SECTION I.ELEMENTARY CONSTITUTION OF MATTER.
The investigationswhich 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 constituentsof 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 tobothlight and heat, however thin.
By far the greater number of terrestrial substances consist of heterogeneous atoms chemically combinedinto 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 ductilemetal 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 uraniumform 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 insulatedby 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-hottube, 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 whichorganic 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 ofthe 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 321⁄2pounds 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, eachhaving 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 orpoisonous, 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 acandle 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, wereit 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 irreconcilablewith 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; itstendencies 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. Siliconexists 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 ofpotassium, 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 formsmuriatic 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 notsimple 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 continuouslythrough 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.