HYDROGEN, a chemical element, appears to have been re cognized by Paracelsus in the 16th century, but the first definite experiments concerning its nature were made in 1766 by H. Cavendish, who showed that it was produced by the action of hydrochloric or sulphuric acid on certain metals. (Symbol H, atomic number I, atomic weight 1.008.) Cavendish called it "inflammable air," and it was confused with other inflammable gases, all of which were supposed to contain "phlogiston," the inflammable principle, until he showed that water was the only substance produced when hydrogen was burned in air or oxygen, and that, contrary to A. L. Lavoisier's views, no acid was formed. W. Prout's hypothesis, that all other elements were built up from atoms of hydrogen (1815), remained unsubstantiated till the work of F. W. Aston, over loo years later, showed that the proton, i.e., the hydrogen atom deprived of its electron, does in fact fulfil this function.
Hydrogen is found in the free state in some volcanic gases, in certain stars and nebulae, in some meteorites and in the atmos phere of the sun. Although it forms only about o•oo1 % of our air at ordinary altitudes, it may constitute a much larger propor tion at very great heights. In combination, it is found as a con stituent of water, in the gases from certain mineral springs, in many minerals, and in most animal and vegetable tissue. It may be prepared by the electrolysis of acidified water, by the decom position of water by various metals such as sodium, and by the action of acids or caustic alkalis on many metals. The decom position of steam by red-hot iron was first studied by H. St. C. Deville; the reaction takes place in a series of stages but may be expressed as The lower the tempera ture the larger the ratio of hydrogen to water, but actually the equilibrium is set up very slowly below about 800° C. Calcium hydride, or "hydrolith," prepared by passing hydrogen over heated calcium, is used as a portable source of the gas for filling balloons, etc., since water decomposes it with the production of a cu. metre of gas per kg. of solid. In the "silical" process, a mixture of silicon (usually as ferro-silicon) and caustic soda (sometimes mixed with slaked lime and then known as "hydrogenite"), is used for the same purpose : Aluminium amalgam and water, or aluminium, caustic soda and a little mercuric oxide, are also used in the production of hydrogen.
The gas obtained from hydrochloric or sulphuric acid and metals (usually zinc or iron) is often very impure, and if pure acids and pure metals are used, the reaction is very slow but may be accel erated by the addition of a copper or platinum salt, the accelera tion in either case being due to the formation of, e.g., a zinc copper couple.
Hydrogen is often prepared technically by the action of steam on red-hot coke and ± see CARBON), but unless the resulting "water-gas" is to be utilized as such, the oxides of carbon may be harmful. F. Bergius claims that if the water is kept as a liquid at 34o° C by high pressure, and 1 % of thallium chloride is mixed with the coke, only the second reaction takes place, i.e., no carbon monoxide is formed. The hydrogen for synthetic ammonia (q.v.) in Claude's process is obtained by the action of steam on calcium carbide at a red heat: and the gas is freed from carbon dioxide by cooling and expansion (see LIQUEFACTION OF GASES). Hydrogen of 97% purity is prepared on a large scale by the electrolysis of a 15% solution of caustic soda at 7o° C.
Pure hydrogen is a colourless, tasteless, odourless gas of density 0.06947 (air=1), i.e., I litre at o° C and 76o mm. weighs 0.089873 gram (Lord Rayleigh; E. W. Morley). The liquid, which has a specific gravity of only 0.07, boils at —252.6° C, and the solid melts at —259° C. The gas obeys Boyle's law at low pres sures (up to 15omm.) but at higher pressures it is not sufficiently compressible, the deviation being o• 1 % at o° C and 76omm. ; its specific heat at constant volume is 2.39 at o° C and that at con stant pressure is 1.41 times as great. Hydrogen is only slightly soluble in water (about 2% by volume under ordinary conditions), but is fairly soluble in liquid air. It diffuses rapidly through por ous materials and also through some metals at a red heat. Palla dium and some other metals absorb large volumes of the gas, especially if they are finely divided as in palladium black; it was once thought that a hydride, was formed, but C. Hoit sema showed that the process was purely one of adsorption (q.v.) ; moreover, E. B. Maxted showed that the extent of adsorp tion was influenced by impurities in the metal, and J. B. Firth found that it depended on the proportion of crystalline and amorphous palladium.
Hydrogen burns with a pale blue, non-luminous flame, but it does not support combustion. Its mixture with air or oxygen is highly explosive, especially if the volume of oxygen is half that of hydrogen (compare but H. B. Baker has shown that the perfectly dry, pure gases will not combine (see DRYNESS [CHEMICAL] ). Hydrogen combines violently with fluo rine even at —25o° C, and its reaction with chlorine is greatly accelerated by sunlight or other actinic light; it combines with carbon at 1,20o° C to give methane under certain conditions (W. A. Bone and H. F. Coward), but at higher temperatures increas ing amounts of acetylene are formed, e.g., at 3,000° C in the elec tric arc (M. Berthelot) . The alkali and alkali-earth metals give hydrides when heated in a current of hydrogen, and these have the formulae NaH, etc. ; that of barium (q.v.) is the least stable. Hydrogen is a powerful reducing agent, especially when occluded in palladium (see above) or when in the "nascent" state, i.e., when generated in the presence of the substance to be reduced, as in the reduction of ferric salts to ferrous by the addi tion of pure zinc to the acid solution, or as in the use of sodium amalgam in organic reductions. Gaseous hydrogen is not so effective, although its efficiency is sometimes increased by pres sure, as in the reduction of mercuric chloride solution under 100 atmos. pressure; in conjunction with colloidal palladium, it has been applied by C. Paal to many reductions in organic chemistry. P. Sabatier and J. B. Senderens effected a number of reductions by the use of hydrogen in the presence of finely divided reduced nickel at 150-200° C, and a similar method is used commercially in reducing liquid (unsaturated) fats to solid (saturated) fats. (See HYDROGENATION.) Hydrogen is used in the reduction of metallic oxides to the metal, in the oxy-hydrogen flame for weld ing, etc., and in the fixation of atmospheric nitrogen. (See NITRO