Hydrogen (Gr.Hydrogen 090029 water, andHydrogen 090030 to produce), an elementary gaseous body, named from its property of forming water by combining with oxygen. Its symbol is H; chemical equivalent 1; weight compared with air 0.06926; 100 cubic inches weigh under ordinary pressure and temperature 2.14 grains, being 16 times less than an equal volume of oxygen, and 14.4 times less than air. One litre of hydrogen gas at 0° C. and 760 mm. pressure weighs 0.08936 gramme. It was known near the close of the 17th century, and was termed inflammable air from its burning with a flame; it was also called phlogiston, from the supposition of its being the matter of heat. Its real nature was first described by Cavendish in 1766. The gas is not found uncombined, but is readily obtained by decomposing water, of which it constitutes about one ninth by weight, the remainder being oxygen. This process is effected very much as metallic oxides are decomposed, some substance being presented to the compound which has a strong affinity for the oxygen, and combining with it liberates the hydrogen or other element. The vapor of water passed through an iron tube filled with iron shavings and kept at a red heat is thus decomposed, the oxygen uniting with the iron, and the hydrogen escaping.

The common method of preparing the gas is to place some bits of zinc in oil of vitriol or sulphuric acid diluted with five or six times its bulk of water. Chemical action immediately takes place, and the zinc is dissolved with effervescence, owing to the bubbles of hydrogen separating from the liquid. The reaction is represented by the formula Zn + H2SO4 = ZnSO4 + H2. With an ounce of zinc there may be obtained 615 cubic inches of hydrogen. A common flask answers very well for the apparatus, by inserting a bent tube through the cork for the exit of the gas, and a straight tube, terminating above in a small funnel, and reaching below the cork nearly to the bottom of the flask, at least so as to be covered by the liquid. Through this tube the acid is poured in as required, the zinc and water being first introduced. The sulphur and carbon which are present in almost all zinc appear in the hydrogen as traces of sulphuretted hydrogen and carbonic acid. They may be separated by agitating the gas with lime water. When pure, hydrogen has neither taste, smell, nor color. It is destructive to animal life when inhaled for a short time, and extinguishes a burning taper plunged into it.

Yet it is itself highly combustible, burning with a faint bluish yellow flame at its contact with atmospheric air or oxygen; and when mixed with proper proportions of ether and ignited by flame, an electric spark, or a glass rod heated hardly to redness, its combustion is instantaneous and explosive. A piece of spongy platinum introduced into the mixture also causes combustion to take place. The most violent effects are produced by a mixture of two volumes of hydrogen and one of oxygen. The only product of the combustion of hydrogen is water. The gas is made to enter into combination with the oxygen of the air, producing heat sufficient to cause its ignition, by directing a jet of it upon a piece of spongy platinum, or even upon a perfectly clean surface of sheet platinum. The metal becomes red hot, the gas ignites, and thus a light may be instantaneously obtained. A little apparatus was devised for this purpose by Prof. Dobe-reiner, which would be an excellent means of obtaining a flame in the absence of the cheap matches in common use. Though the flame of hydrogen is very slightly luminous, a bright light is emitted from the heated platinum; and an apparatus based on this principle has been applied to purposes of illumination in the place of ordinary gas lights.

Such lights were at one time in practical use in France and England. The hydrogen was produced by the decomposition of water, effected by passing its vapor over incandescent charcoal contained in a tube; some carbonic oxide and carburetted hydrogen were generated, which burned with the hydrogen, the jet of mixed gases being directed against a basket constructed of fine gauze of platinum, which became intensely hot and highly luminous. Hydrogen produces intense heat by its combustion, taking up more oxygen than is required by the same weight of any other combustible. It is this property that has led to its application in the oxyhydrogen blowpipe for melting the most refractory substances. (See Blowpipe.) The levity of hydrogen early suggested its use for filling balloons. The quantity required to fill one of the capacity of 2,000 cubic feet would weigh only 10.57 lbs., while the same volume of air would weigh 153.26 lbs., giving an ascensional power of 142.69 lbs. Illuminating gas is heavier, but is commonly used instead of hydrogen only on account of its greater cheapness. Hydrogen is so subtle and penetrating a gas that it passes with facility through paper and also through gold and silver leaves. A stream of the gas directed against one side of the leaf may be ignited on the other.

Hydrogen combines with one equivalent of oxygen to form hydrogen monoxide or water; with two equivalents to form the dioxide or oxygenated water, a liquid discovered by Thenard in 1818, and now prepared by chemists for medicinal purposes; also with one equivalent of nitrogen to form ammonia; and with one of chlorine to form hydrochloric acid. From his researches on the occlusion of hydrogen by palladium, Prof. Graham was led to infer the existence of an alloy of palladium and hydrogen gas condensed to a solid form, to which he gave the name of hydrogenium. Assuming that the hydrogen enters into the combination with the density which it would exhibit if solidified in the free state, he calculates, from the observed density of this so-called alloy of palladium and hydrogenium, and of similar alloys containing in addition gold, silver, or nickel, that the density of this hypothetically solidified hydrogen varies between the limits 0.711 and 0.7545; mean, 0.733. The presence of hydrogen in the atmosphere of the sun and in the planets has been shown by spectrum analysis.

On the sun four lines are attributed to hydrogen.