HYDROGEN (symbol H, equiv. 1), so called from the Greek words hydtir, water, and genndo, to generate, is an elementary substance, which exists as a colorless and inodorous gas (regarded as permanent till 1878). One of its most striking peculiarities is its specific gravity, it being the lightest of all known bodies. Assuming the weight of a given volume of atmospheric air to he 1, the weight of the same volume of hydro gen under similar conditions is 0.0692; hendelydrogen is 14i tittles lighter than atmo spheric air; while, on the other hand, it is 241.573 times lighter than platinum, the heaviest body known. Its refractive power is greater then that of any other gas, and is more than 6 times as great as that of atmospheric air. It is combustible; that is to say, it is capable of combining with oxygen, and developing light and heat. When a lighted taper is passed up into an inverted jar of hydrogen, the gas burns quietly with a pale-blue, scarcely visible flame, and the taper is extinguished. The flame only occurs at the line of junction of the hydrogen and the external air. If the hydrogen be mixed with air or oxygen prior to the application of the taper, the whole mixture is simultaneously inflamed, and there is a loud explosion, which is most violent when 2 volumes of hydro gen are mixed with 1 volume of oxygen, or with 5 volumes of atmospheric air. The hydrogen and oxygen in these cases combine to form watery vapor or steam, which suddenly expands from the high temperature attendant on the combustion, but imme diately afterwards becomes condensed; this condensation causes a partial vacuum, into which the surrounding air rushes, and by the collision of its particles produces the report. At ordinary temperatures, water dissolves rather less than 2 per cent of its volume of hydrogen. Hydrogen was liquefied for the first• time in 1878, and even solidified (see GASES). Pure hydrogen, though it cannot support life, is not poisonous, and when mixed with a sufficient quantity of atmospheric air or oxygen, may be breathed for some time without inconvenience.
Hydrogen does not possess very marked chemical properties. The only substances with which it combines directly at ordinary temperatures are chlorine and .oxygen. Hydrogen and chlorine, mixed together, and exposed to direct sunlight, combine with explosion; in diffused daylight, they gradually unite; but in the dark do not act on one another. Hydrogen and oxygen do not combine spontaneously even in direct sunlight, but require the presence of a red-hot solid, of flame, or of spongy platinum.
It is generally stated that hydrogen does not exist naturally in a pure or uncombined state, but Bunsen recognized its presence in variable proportions in the gases evolved from the solfataras of Iceland, and it will probably be detected in other localities where similar geological relations hold good. In combination with oxygen, as water, it not only forms a very considerable part of the earth, and of the atmosphere, but enters largely into the structure of every animal and vegetable organism. It is an essential ingredient of many inflammable minerals, such as coal, amber, and petroleum; and of certain gases, such as marsh gas, ammonia, and hydrosulphuric acid (or sulphureted hydrogen). It likewise enters into the composition of a large number of manufactured substances and products used in the arts, medicine, etc., as for instance, sal-ammoniac, starch, sugar, vinegar, alcohol, olefiant gas, aniline, indigo, morphia, strychnia, hydrocyanic acid, etc.
There are numerous ways in which hydrogen may be prepared, but the usual and most convenient process is by the action of diluted sulphuric acid on zinc. About half an ounce of granulated zinc is placed in a retort, and a dilute acid, prepared by gradu ally mixing an ounce of oil of vitriol with six ounces of cold water, is poured on the zinc. Hydrogen gas is rapidly evolved in great abundance, but the first portions should not be collected, since they are mixed with the atmospheric air which was contained in the retort. The rest of the gas may be collected in the ordinary way over water. In this process the zinc takes oxygen from the water, and forms oxide of zinc, which combines with the sulphuric acid, forming sulphate of zinc, which remains in sohiTion, while the hydrogen of the decomposed water escapes. The reaction is shown in the formula, Zn +HO,SO. = ZnO,S03 + H. A precisely similar reaction ensues if we use iron in place of zinc, but in this case the gas is generally less pure.
Hydrogen gas, under the name of combustible air, was obtained in the 16th c. by Paracelsus by treating certain metals with dilute acids, and was more or less known to Boyle and others; but Cavendish, in his paper on "Factitious Airs," published in the Transactions of the Royal Society for 1766, was the first to describe accurately the proper ties of this gas, and the methods of obtaining it; hence he is usually mentioned as its discoverer.