HYDROCHLORIC ACID

hydrochloric acid, also known in commerce as "spirits of salts" and "muriatic acid," a compound of hydrogen and chlorine. its chemistry is discussed under chlorine, and its manufacture under alkali manufacture. hydrodynamics (gr. [greek: hydor], water, [greek: dynamis], strength), the branch of hydromechanics which discusses the motion of fluids (see hydromechanics). hydrogen [symbol h, atomic weight 1.008 (o = 16)], one of the chemical elements. its name is derived from gr. [greek: hydor], water, and [greek: gennaein], 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. lemery 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-beaupre, _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 elektrizitats-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°