HYDROGEN. It had been long known to the chemists, that a vapour or air is disengaged in some processes, which kindled on the approach of an ignited body. Van Helmont gave this the name of gas igneum, and it seems to have at tracted the attention of Boyle, Mayow, and Hales. The chemists knew, that such a vapour or air was commonly disengaged during the solution of certain metals in muriatic or dilute sulphuric acid, that it burnt at the mouth of the phial, and if mixed with atmospheric air, exploded when kindled by a match.
Mr.Cavendish, however, first examined its properties fully, show. d that it is per manently elastic, not absorbed by water, and that it is much lighter than atmos pheric air. (Philos. Trans. vol. lvi p. 141). This substance forming water when com bined with oxygen, and being therefore the radical of that compound, the name hydrogen was given to it at the formation of the new nomenclature.
It is always obtained from the decom position of water, as it cannot, from other substances, in which it exists, be easily disengaged in perfect purity. Some sub stance is made to act on water, which exerts an attraction to the oxygen, with out combining with the hydrogen, when, of course, the hydrogen is disengaged, and passes into the elastic form.
At the common temperature of the globe, this decomposition cannot be ef fected with rapidity.by any single affinity. Iron, moistened with water, decomposes it very slowly, and evolves hydrogen ; but at the temperature of ignition, the de composition is more rapid. If a coil of iron wire, or a quantity of iron filings, be put into an iron or coated glass, or earthen tube, which is placed across a small furnace, and surrounded with burn. ing fuel, so as to be brought to a red heat, on distilling water from a retort connect ed with it, the vapour, in passing over the surface of the ignited iron, is decomposed, the iron attracts its and hydro gen gas issues from the extremity of the tube.
This process is a troublesome one, and by the agency of an acid, water is decom posed as rapidly by iron or zinc, at a na tural temperature. Zinc affords the hy
drogen in the greatest purity. One part of it, in small pieces, is put into a retort, or a bottle with a bent tube adapted to it ; two parts of sulphuric acid, previously diluted with five times its weight of water, are poured upon it, an effervescence is immediately excited, hydrogen gas es capes, and is to be collected in jars filled with water, and placed on the shelf of the pneumatic trough. Its disengagement continues until the zinc is dissolved, Iron may be employed in place of zinc, but containing generally a little carbon, which is dissolved by the hydrogen, it affords a gas less pure. Muriatic acid serves the same puppose as sulphuric acid, but must be diluted with only twice or three times its weight of water.
In the experiment, the hydrogen gas is derived entirely from the decomposition of the water, the oxygen of which is at tracted by the metal. That the acid suf fers no decomposition is proved, by the liquor, at the end of the experiment, be ing capable of saturating as much of an alkali as the quantity of acid employed would have done in a pure state. The agency of the acid was formerly explain ed, on the absurd doctrine of disposing affinity,—that it had no attraction to the pure metal, but to the oxide of the metal: that, to satisfy this affinity, it caused the oxidation of the metal at the expense of the water, and then combined with the oxide thus formed. In conformity to Berthollet's speculations, it may be re ferred to the affinities of the acid to iron, and to oxygen, conspiring with the affi nity of iron to oxygen : these co-operat ing produce a ternary combination, while the hydrogen gas is disengaged.
Hydrogen gas is permanently elastic. When collected over water, it is obsefted to have a peculiar smell, slightly fetid, which is not so perceptible when it is col lected over quicksilver, and which is lost when the gas is exposed to substances which powerfully attract humidity. It is not the only substance in which water appears requisite to develope odour.