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.