HY'DROGEN (from Gk. iAdp, hyd4r, water -gcnis, producing, from yiyuccreat, yigneslhai, to become). .A gaseous element, dis covered in 1706 by Cavendish, who called it 'inflammable air.' Paraeelsus had already ob tained hydrogen by treating iron with dilute mineral acids. But the existence of gases essen tially different from air was not generally recog nized until about the middle of the eighteenth century, and the properties of hydrogen gas were not definitely known before the researches of Cavendish. Lavoisier recognized its elementary nature, and gave it its present name. In the free state, hydrogen is found in the exhalations from volcanoes, in the gases issuing from the salt beds at Stassfurt and Wieliczka, in gases given off by oil-wells, in the intestinal gases of animals. etc. The spectroscope reveals its exist ence in the atmosphere of the sun and of many stars, and it has been found 'occluded' in meteor ites. In combination it occurs as water, of which it forms very near one-ninth by weight, and as a constituent of almost all organic mat ter. It may be obtained by the electrolysis of acidulated watcr; by the action of sodium, potas sium. and certain other metals on water; by passing steam over red-hot iron wire, etc. But the most convenient method, and the one gen erally employed. consists in allowing dilute hydrochloric acid, or preferably dilute sulphuric acid, to act on metallic zinc. Owing, however, to the impurities ordinarily present in zinc, as well as in mineral acids, the hydrogen thus ob tained is impure and has a disagreeable odor. A large pereentage of hydrogen is contained in ordinary eoal-gas, in which it is formed as a product of the destructive distillation of coal (organic matter). Mneh hydrogen is also con tained in ordinary water-gas, in which it is formed by the action of red-hot coal on water vapor.
Bydrogen (symbol If; atomic- weight-'-either 1, the standard of atomic weights, or 1.01, when the figure 16 for oxygen is adopted as the stand ard; see Anomie WmonTs) is a colorless, taste less, and odorless gas, whose critical temperature is —234.5°C., the critical pressure being 20 at mospheres (300 pounds to the square inch). It was first solidified in 1899, by causing it rapidly to evaporate when in the liquid state. In the gaseous state, it is the lightest substance known, being fourteen and one-half tittles lighter than ?tmospherie air, and about 256,000 times lighter than the mineral platiniridium. One liter, at 0° C. and under normal atmospheric pressure (760 millimeters of mercury), weighs .08952289 gram. In accordance with the atomic and molecular theory, the molecule of ordinary gaseous hydrogen is assumed to consist of two atoms, and is therefore usually represented by the formula H,. The gas burns in the air with
a non-luminous flame of very high temperature, the oxygen to form water. pure, it cannot support animal life, hydrogen is not poisonous, and when mixed with a sufficient quantity of oxygen or atmospheric air, may be inhaled for some time without incon venience; but it weakens the voice, and renders it high-pitched. Hydrogen gas is capable of be ing absorbed by certain metals, such as iron, platinum, and especially palladium. Graham, who studied this phenomenon, maintained that when thus 'occluded' hydrogen assumed a true metallic form, and named that form of the de Went hydrogenium. Troost and Hautefeuille, too, believed that hydrogen is capable of forming alloys with metals, and some of the alloys ob tained by them were found to have the composi tion of true chemical compounds: for example, the hydride of sodium, NaJI, and the hydride of potassium, If this is true. then hydro gen cannot be assumed to be invariably univ alent in its compounds. (See VALENCY.) At ordinary temperatures, hydrogen is chemically inert toward most of the other elements; with chlorine, however, it combines gradually in dif fused, and with explosive violence in direct, sun light. Its mixture with double its volume of oxygen explodes violently if ignited; the explo sion is somewhat less violent if instead of two volumes of oxygen, about two and a half volumes of air are mixed with one volume of hydrogen. At elevated temperatures, or when in the nascent state (i.e. while being formed, say by the action of dilute acid on zinc), hydrogen is a powerful reducing agent, i.e. it readily abstracts oxygen from compounds or adds itself to them. Nascent hydrogen is thus often employed in the prepara tion of organic compounds from given materials. With oxygen, hydrogen forms two different com pounds: water, H.0, and hydrogen peroxide, With sulphur it combines directly, at the boiling temperature of the latter, forming sul phureted hydrogen, H,S. The ordinary com pound of hydrogen with nitrogen is ammonia, NH,; another compound of hydrogen and nitro gen, termed hydrazine, has the formula N,H, Hydrochloric acid, HC1, is the compound of hydrogen with chlorine, the two elements com bining, as stated above, under the influence of sunlight at ordinary temperatures. At some what elevated temperatures, hydrogen similarly combines with the vapors of bromine and iodine, forming, respectively, hydrobromic acid, HBr, and hydriodic acid, HI. For the compounds of hydrogen with phosphorus. see PHOSPHORUS. For its compounds with carbon, see HYDRoomt