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Rh HYDROCHLORIC ACID, also known in commerce as “spirits of salts” and “muriatic acid,” a compound of hydrogen and chlorine. Its chemistry is discussed under, and its manufacture under.

HYDRODYNAMICS (Gr. , water,  , strength), the branch of hydromechanics which discusses the motion of fluids (see ).

HYDROGEN [symbol H, atomic weight 1.008 (O = 16)], one of the chemical elements. Its name is derived from Gr.  , water, and , to produce, in allusion to the fact that water is produced when the gas burns in air. Hydrogen appears to have been recognized by Paracelsus in the 16th century; the combustibility of the gas was noticed by Turquet de Mayenne in the 17th century, whilst in 1700 N. Lémery showed that a mixture of hydrogen and air detonated on the application of a light. The first definite experiments concerning the nature of hydrogen were made in 1766 by H. Cavendish, who showed that it was formed when various metals were acted upon by dilute sulphuric or hydrochloric acids. Cavendish called it “inflammable air,” and for some time it was confused with other inflammable gases, all of which were supposed to contain the same inflammable principle, “phlogiston,” in combination with varying amounts of other substances. In 1781 Cavendish showed that water was the only substance produced when hydrogen was burned in air or oxygen, it having been thought previously to this date that other substances were formed during the reaction, A. L. Lavoisier making many experiments with the object of finding an acid among the products of combustion.

Hydrogen is found in the free state in some volcanic gases, in fumaroles, in the carnallite of the Stassfurt potash mines (H. Precht, Ber., 1886, 19, p. 2326), in some meteorites, in certain stars and nebulae, and also in the envelopes of the sun. In combination it is found as a constituent of water, of the gases from certain mineral springs, in many minerals, and in most animal and vegetable tissues. It may be prepared by the electrolysis of acidulated water, by the decomposition of water by various metals or metallic hydrides, and by the action of many metals on acids or on bases. The alkali metals and alkaline earth metals decompose water at ordinary temperatures; magnesium begins to react above 70° C., and zinc at a dull red heat. The decomposition of steam by red hot iron has been studied by H. Sainte-Claire Deville (Comptes rendus, 1870, 70, p. 1105) and by H. Debray (ibid., 1879, 88, p. 1341), who found that at about 1500° C. a condition of equilibrium is reached. H. Moissan (Bull. soc. chim., 1902, 27, p. 1141) has shown that potassium hydride decomposes cold water, with evolution of hydrogen, KH + H2O = KOH + H2. Calcium hydride or hydrolite, prepared by passing hydrogen over heated calcium, decomposes water similarly, 1 gram giving 1 litre of gas; it has been proposed as a commercial source (Prats Aymerich, Abst. J.C.S., 1907, ii. p. 543), as has also aluminium turnings moistened with potassium cyanide and mercuric chloride, which decomposes water regularly at 70°, 1 gram giving 1.3 litres of gas (Mauricheau-Beaupré, Comptes rendus, 1908, 147, p. 310). Strontium hydride behaves similarly. In preparing the gas by the action of metals on acids, dilute sulphuric or hydrochloric acid is taken, and the metals commonly used are zinc or iron. So obtained, it contains many impurities, such as carbon dioxide, nitrogen, oxides of nitrogen, phosphoretted hydrogen, arseniuretted hydrogen, &c., the removal of which is a matter of great difficulty (see E. W. Morley, Amer. Chem. Journ., 1890, 12, p. 460). When prepared by the action of metals on bases, zinc or aluminium and caustic soda or caustic potash are used. Hydrogen may also be obtained by the action of zinc on ammonium salts (the nitrate excepted) (Lorin, Comptes rendus, 1865, 60, p. 745) and by heating the alkali formates or oxalates with caustic potash or soda, Na2C2O4 + 2NaOH = H2 + 2Na2CO3. Technically it is prepared by the action of superheated steam on incandescent coke (see F. Hembert and Henry, Comptes rendus, 1885, 101, p. 797; A. Naumann and C. Pistor, Ber., 1885, 18, p. 1647), or by the electrolysis of a dilute solution of caustic soda (C. Winssinger, Chem. Zeit., 1898, 22, p. 609; “Die Elektrizitäts-Aktiengesellschaft,” Zeit. f. Elektrochem., 1901, 7, p. 857). In the latter method a 15% solution of caustic soda is used, and the electrodes are made of iron; the cell is packed in a wooden box, surrounded with sand, so that the temperature is kept at about 70° C.; the solution is replenished, when necessary, with distilled water. The purity of the gas obtained is about 97%.

Pure hydrogen is a tasteless, colourless and odourless gas of specific gravity 0.06947 (air = 1) (Lord Rayleigh, Proc. Roy. Soc., 1893, p. 319). It may be liquefied, the liquid boiling at −252.68° C. to −252.84° C., and it has also been solidified, the solid melting at −264° C. (J. Dewar, Comptes rendus, 1899, 129, p. 451; Chem. News, 1901, 84, p. 49; see also ). The specific heat of gaseous hydrogen (at constant pressure) is 3.4041 (water = 1), and the ratio of the specific heat at constant pressure to the specific heat at constant volume is 1.3852 (W. C. Röntgen, Pogg. Ann., 1873, 148, p. 580). On the spectrum see . Hydrogen is only very slightly soluble in water. It diffuses very rapidly through a porous membrane, and through some metals at a red heat (T. Graham, Proc. Roy. Soc., 1867, 15, p. 223; H. Sainte-Claire Deville and L. Troost, Comptes rendus, 1863, 56, p. 977). Palladium and some other metals are capable of absorbing large volumes of hydrogen (especially when the metal is used as a cathode in a water electrolysis apparatus). L. Troost and P. Hautefeuille (Ann. chim. phys., 1874, (5) 2, p. 279) considered that a palladium hydride of composition Pd2H was formed, but the investigations of C. Hoitsema (Zeit. phys. Chem., 1895, 17, p. 1), from the standpoint of the phase rule, do not favour this view, Hoitsema being of the opinion that the occlusion of hydrogen by palladium is a process of continuous absorption. Hydrogen burns with a pale blue non-luminous flame, but will not support the combustion of ordinary combustibles. It forms a highly explosive mixture with air or oxygen, especially when in the proportion of two volumes of hydrogen to one volume of oxygen. H. B. Baker (Proc. Chem. Soc., 1902, 18, p. 40) has shown that perfectly dry hydrogen will not unite with perfectly dry oxygen. Hydrogen combines with fluorine, even at very low temperatures, with great violence; it also combines with carbon, at the temperature of the electric arc. The alkali metals when warmed in a current of hydrogen, at about 360° C., form hydrides of composition RH (R = Na, K, Rb, Cs), (H. Moissan, Bull. soc. chim., 1902, 27, p. 1141); calcium and strontium similarly form hydrides CaH2, SrH2 at a dull red heat (A. Guntz, Comptes rendus, 1901, 133, p. 1209). Hydrogen is a very powerful reducing agent; the gas occluded by palladium being very active in this respect, readily reducing ferric salts to ferrous salts, nitrates to nitrites and ammonia, chlorates to chlorides, &c.

For determinations of the volume ratio with which hydrogen and oxygen combine, see J. B. Dumas, ''Ann. chim. phys.'', 1843 (3), 8, p. 189; O. Erdmann and R. F. Marchand, ibid., p. 212; E. H. Keiser, Ber., 1887, 20, p. 2323; J. P. Cooke and T. W. Richards, ''Amer. Chem. Journ., 1888, 10, p. 191; Lord Rayleigh, Chem. News'', 1889, 59, p. 147; E. W. Morley, ''Zeit. phys. Chem.'', 1890, 20, p. 417; and S. A. Leduc, Comptes rendus, 1899, 128, p. 1158.

Hydrogen combines with oxygen to form two definite compounds, namely, (q.v.), H2O, and hydrogen peroxide, H2O2, whilst the existence of a third oxide, ozonic acid, has been indicated.

Hydrogen peroxide, H2O2, was discovered by L. J. Thénard in 1818 (Ann. chim. phys., 8, p. 306). It occurs in small quantities in the atmosphere. It may be prepared by passing a current of carbon dioxide through ice-cold water, to which small quantities of barium peroxide are added from time to time (F. Duprey, Comptes rendus, 1862, 55, p. 736; A. J. Balard, ibid., p. 758), BaO2 + CO2 + H2O = H2O2 + BaCO3. E. Merck (Abst. J.C.S., 1907, ii., p. 859) showed that barium percarbonate, BaCO4, is formed when the gas is in excess; this substance readily yields the peroxide with an acid. Or barium peroxide may be decomposed by hydrochloric, hydrofluoric, sulphuric or silicofluoric acids (L. Crismer, Bull. soc. chim., 1891 (3), 6, p. 24; Hanriot, Comptes rendus, 1885, 100, pp. 56, 172), the peroxide being added