Hydrogen (sym. H; atom. wt. 1; Gr. hydōr, 'water,' and gennāō, 'to produce') is an elementary gas and the lightest substance known. It is colourless, odourless, and non-poisonous, although as ordinarily prepared it frequently contains traces of disagreeably smelling or of poisonous impurities. The gas when subjected to enormous pressure at an extremely low temperature can be reduced to the liquid state. In combination with oxygen it forms one-ninth part by weight of water, and it is a most important constituent of the tissues of animals and plants. It enters into the composition of a large number of manufactured substances and products used in the arts, medicine, &c., as, for instance, starch, sugar, vinegar, gutta-percha, alcohol, ether, benzene, aniline, indigo, morphia, &c. It is not found largely in nature in the free or uncombined state, but it does occur in some gaseous emanations from the earth, as in the solfataras of Iceland and in the petroleum regions of Pennsylvania. Being the lightest gas known, its density is often adopted as the standard of comparison for the densities of other gases. The density of atmospheric air compared with that of hydrogen as unity is nearly 14.5. As hydrogen possesses the lowest atomic weight of all the elements, the atomic weight of hydrogen is almost universally adopted by chemists as unity, and those of the other elements referred to it; but at present there is a decided movement in favour of the adoption as standard of an atomic weight which can be determined with more rigid accuracy than that of hydrogen can.
Although hydrogen is usually classed amongst the non-metallic elements, it is in its chemical behaviour more closely related to the metals. It combines with oxygen, at a red heat forming water, this combination being accompanied by the giving out of a great deal of heat. A jet of hydrogen burns in air or oxygen with a non-luminous flame, which is, however, sufficiently hot to heat to whiteness a fine platinum wire held in it. The behaviour of hydrogen towards chlorine is extremely interesting. The two gases can be mixed in equal volumes and preserved without combination taking place for an indefinite period if kept in the dark, but on exposure to diffused daylight combination begins, and its progress depends upon the brightness of the light and the duration of the exposure. Momentary exposure to direct sunlight causes combination to take place with explosive violence, and a similar effect is produced by raising any portion of the mixture to a red heat. Hydrogen as a rule combines with those things with which the metals in general combine, forming compounds which are analogous to those of the metals. Compounds containing hydrogen and one other element are common decomposition products of decaying vegetable and animal matters; as, for instance, marsh-gas, ammonia, and sulphuretted hydrogen, which contain hydrogen combined with carbon, nitrogen, and sulphur respectively.
Hydrogen gas, under the name of combustible air, was obtained in the 16th century by Paracelsus by treating certain metals with dilute acids, and was more or less known to Boyle and others; but Cavendish (q.v.) in his paper on 'Factitious Airs,' published in the Transactions of the Royal Society for 1766, was the first to describe accurately the properties of this gas, and the methods of obtaining it, hence he is usually mentioned as its discoverer.
The ordinary methods for preparing and purifying hydrogen will be found in any elementary treatise on chemistry. See GASES.