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Rh  .—Although much has been written of late years about the sources of the Oxus within the region of the Pamirs, there is very little to be found in the writings of geographers of modern date descriptive of that part of its course which separates Darwaz and Afghan Turkestan from Bokhara, and that little is chiefly in the pages of reports and gazettes, &c., which are not available to the public. The following authorities may be consulted: The Report of the Pamir Boundary Commission of 1895, published at Calcutta (1897); Dr A. Regel, “Journey in Karateghin and Darwaz,” Investia, Russian Geog. Soc., vol. xiii. (1882); translation, vol. iv. ''Proc. R.G.S.''; Michell, “Regions of the Upper Oxus,” vol. vi. ''Proc. R.G.S.'' (1884); Griesbach, “Geological Field Notes,” No. 3, Afghan Boundary Commission (1885); C. Yate, Northern Afghanistan (London, 1888); Curzon, “The Pamirs,” vol. viii. ''Jour. R.G.S. (1896); Kropotkin, “Old Beds of the Oxus,” Jour.'' R.G.S. (September 1898); Cobbold, Innermost Asia (London, 1900). To the above may be added the Reports of the Russo-Afghan Boundary Commission of 1884-1885, and that of Lockhart's Mission in 1885, and the Indian Survey Reports.

 OXYGEN (symbol O, atomic weight 16), a non-metallic chemical element. It was apparently first obtained in 1727 by Stephen Hales by strongly heating minium, but he does not seem to have recognized that he had obtained a new element, and the first published description of its properties was due to J. Priestley in 1774, who obtained the gas by igniting mercuric oxide, and gave it the name “dephlogistigated air.” K. W. Scheele, working independently, also announced in 1775 the discovery of this element which he called “empyreal air” (Crells' Annalen, 1785, 2, pp. 229, 291). A. L. Lavoisier repeated Priestley's experiments and named the gas “oxygen” (from Gr., sour, , I produce) to denote that in a large number of cases, the products formed by the combustion of substances in the gas were of an acid character. Oxygen occurs naturally as one of the chief constituents of the atmosphere, and in combination with other elements it is found in very large quantities; it constitutes approximately eight-ninths by weight of water and nearly one-half by weight of the rocks composing the earth's crust. It is also disengaged by growing vegetation, plants possessing the power of absorbing carbon dioxide, assimilating the carbon and rejecting the oxygen. Oxygen may be prepared by heating mercuric oxide; by strongly heating manganese dioxide and many other peroxides; by heating the oxides of precious metals; and by heating many oxy-acids and oxy-salts to high temperatures, for example, nitric acid, sulphuric acid, nitre, lead nitrate, zinc sulphate, potassium chlorate, &c. Potassium chlorate is generally used and the reaction is accelerated and carried out at a lower temperature by previously mixing the salt with about one-third of its weight of manganese dioxide, which acts as a catalytic agent. The actual decomposition of the chlorate is not settled definitely; the following equations give the results obtained by P. F. Frankland and Dingwall (Chem. News, 1887, 55, p. 67):—at a moderate heat: 8KClO3 = 5KClO4+3KCl+2O2, succeeded by the following reactions as the temperature increases: 2KClO3 = KClO4+KCl+O2 and 2KClO3 = 2KCl+3O2 (see also F. Teed, ibid., 1887, 55, p. 91; H. N. Warren, ibid., 1888, 58, p. 247; W. H. Sodeau, Proc. Chem Soc., 1901, 17, p. 149). It may also be obtained by heating manganese dioxide or potassium bichromate or potassium permanganate with sulphuric acid; by the action of cobalt salts or manganese dioxide on a solution of bleaching powder (Th. Fleitmann, Ann., 1865, 134, p. 64); by the action of a ferrous or manganous salt with a salt of cobalt, nickel or copper on bleaching powder (G. F. Jaubert, Ger. pat. 157171); by passing chlorine into milk of lime (C. Winkler, Jour, prakt. Chem., 1866, 98, p. 340); by the action of chlorine on steam at a bright red heat; by the decomposition of hydrogen peroxide by bleaching powder, manganese dioxide, potassium ferricyanide in alkaline solution, or potassium permanganate in acid solution; by heating barium peroxide with an aqueous solution of potassium ferricyanide (G. Kassner, Zeit. angew. Chem., 1890, p. 44S) BaO2+2K3Fe(CN)6 = Ba[FeK3(CN)6]2+O2; by the decomposition of sodium and potassium peroxides with a solution of potassium permanganate in the presence of a trace of nickel salts (G. F. Jaubert, Comptes rendus, 1902, 134, p. 778).

Numerous methods have been devised for the manufacture of oxygen. The more important are as follows: by decomposing strongly heated sulphuric acid in the presence of a contact substance; by heating an intimate mixture of one part of sodium nitrate with two parts of zinc oxide (T. H. Pepper, Dingler's Jour., 1863, 167, p. 39): 2ZnO+4NaNO3 = 2Zn(ONa)2+2N2+5O2; by the use of cuprous chloride which when mixed with clay and sand, moistened with water and heated in a current of air at 100-200° C. yields an oxychloride, which latter yields oxygen when heated to 400° C (A. Mallet, Comptes rendus, 1867, 64, p. 226; 1868, 66, p. 349); by the electrolysis of solutions of sodium hydroxide, using nickel electrodes; by heating calcium plumbate (obtained from litharge and calcium carbonate) in a current of carbon dioxide (G. Kassner, Monit. Scient., 1890, pp. 503, 614); and from air by the process of Tessié du Motay (Ding. Jour., 1870, 196, p. 230), in which air is drawn over a heated mixture of manganese dioxide and sodium hydroxide, the sodium manganate so formed being then heated to about 450° C. in a current of steam, the following reversible reaction taking place:

$$

Oxygen is largely prepared by Brin's process (Mém. soc. des Ingén. civ., 1881, p. 450) in which barium monoxide is heated in a current of air, forming the dioxide, which when the retorts are exhausted yields up oxygen and leaves a residue of monoxide; but this method is now being superseded, its place being taken by the fractional distillation of liquid air (The Times, Engin. Suppl., April 14, 1909, p. 13) as carried out by the Linde method (Eng. Pat. 14111; 1902).

Oxygen is a colourless, odourless and tasteless gas. It is somewhat heavier than air, its specific gravity being 1.10523 (A. Leduc, Comptes rendus, 1896, 123, p. 805). It is slightly soluble in water and more so in alcohol. It also dissolves quite readily in some molten metals, especially silver. Oxygen does not burn, but is the greatest supporter of combustion known, nearly all the other elements combining with it under suitable conditions (cf. ). These reactions, however, do not take place if the substances are absolutely dry. Thus H. B. Baker (Proc. Chem. Soc., 1902, 18, p. 40) has shown that perfectly dry oxygen and hydrogen will not combine even at a temperature of 1000° C. It is the only gas capable of supporting respiration. For the properties of liquid oxygen see.

It is found, more especially in the case of organic compounds, that if a substance which oxidizes readily at ordinary temperature be mixed with another which is not capable of such oxidation, then both are oxidized simultaneously, the amount of oxygen used being shared equally between them; or in some cases when the substance is spontaneously oxidized an equivalent amount of oxygen is converted into ozone or hydrogen peroxide. This phenomenon was first noticed by C. F. Schonbein (Jour. prakt. Chem., 1858-1868), who found that on oxidizing lead in the presence of sulphuric acid, the same quantity of oxygen is used to form lead o.xide as is converted into hydrogen peroxide. In a similar manner M. Traube (Ber., 1882-1893) found that when zinc is oxidized in presence of water equivalent quantities of zinc hydroxide and hydrogen peroxide are formed at first, thus:, followed by . The oxygen uniting with the substance undergoing oxidation is generally known as “bound oxygen,” whilst that which is transformed into ozone or hydrogen peroxide is usually called “active oxygen.” C. Engler (Ber., 1897, 30, p. 1669) calls the substance which undergoes oxidation the “autoxidizer” and the substance which unites with the active oxygen the “acceptor”; in the oxidation of metals he expresses results as:, followed by MO2 → M·O + O, and if water be present,. Various theories have been developed in order to account for these phenomena. Schonbein (loc. cit.) assumed that the ordinary oxygen molecule is decomposed into two parts which carry electrical charges of opposite kinds, the one with the positive charge being called “antozone” and the other carrying the negative charge being called “ozone,” one variety being preferentially used up by the oxidizing compound or element and the other for the secondary reaction. J. H. Van't Hoff (Zeit. phys. Chem., 1895, 16, p. 411) is of the opinion that the oxygen molecule is to a certain extent ionized and that the ions of one kind are preferably used by the oxidizing compound. Traube (loc. cit.), on the other hand, concludes that the oxygen molecule enters into action as a whole and that on the oxidation of metals, hydrogen peroxide and the oxide of the metal are the primary products of the reaction. A. Bach (Comptes rendus, 1897, 124, p. 2) considers that the first stage in the reaction consists in the production of a peroxide which 