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 of the matte see Christofle and Bouilhet, French Patent 111591 (1876). L. Mond (Jour. Soc. Chem. Ind. 1895, p. 945) has obtained metallic nickel from the Canadian mattes by first roasting them and then eliminating copper by the action of sulphuric acid, the product so obtained being then exposed to the reducing action of producer gas at about 350° C. The reduced metal is then passed into a “volatilizer” and exposed to the action of carbon monoxide at about 80° C., the nickel carbonyl so formed being received in a chamber heated to 180–200° C., where it decomposes, the nickel being deposited and the carbon monoxide returned to the volatilizer. For an electrolytic method of treating mattes, see T. Ulke, Moniteur scient., 1897, 49, p. 450. The metal as obtained by industrial methods rarely contains more than about 99–99·5% of nickel, the chief impurities being copper, iron, cobalt, silicon and carbon.

The following tables show the output of nickel from Canada and the shipments of nickel ore from New Caledonia in recent years:—

The metal may also be obtained on the small scale by the reduction of the oxide by hydrogen or by carbon, by ignition of the oxalate or of nickel ammonium oxalate (J. J. Berzelius), by reduction of the chloride in a current of hydrogen (E. Péligot), by electrolysis of nickel ammonium sulphate (Winkler, Zeit. anorg. Chem. 1894, 8, p. 1), and by reduction of the chloride with calcium carbide.

It is a greyish white metal, and is very malleable and ductile. Its specific gravity varies according to the method employed for its preparation, the extreme values being 8·279 and 9·25. It melts between 1400–1600° C. Its specific heat increases with rise of temperature, the mean value from 15° to 100° C. being 0·1084 (A. Naccari, Gazz., 1888, 18, p. 13). It is magnetic, but loses its magnetism when heated, the loss being complete at about 340–350° C. On the physical constants see H. Copaux, Comptes rendus, 1905, 140, p. 651. Nickel occludes hydrogen readily, is attacked by the halogen elements, and oxidizes easily when heated in air. In the massive state it is unacted upon by dry air, but if moistened with acidified water, oxidation takes place slowly. When obtained by reduction processes at as low a temperature as possible the finely divided metal so formed is pyrophoric, and according to P. Schutzenberger (Comptes rendus, 1891, 113, p. 177) dry hydrochloric acid gas converts this form into nickel chloride and a volatile compound of composition NiHCl. It decomposes water at a red heat. According to E. St Edme (Comptes rendus, 1886, 106, p. 1079) sheet nickel is passive to nitric acid, and the metal remains passive even when heated to redness in a current of hydrogen. On the reduction of organic compounds by hydrogen in the presence of metallic nickel see P. Sabatier and J. B. Senderens, ''Ann. Chim. Phys.'', 1905 [8], 4, pp. 319, 433.

It rapidly oxidizes when fused with caustic soda, but is scarcely acted upon by caustic potash (W. Dittmar, Jour. Soc. Chem. Ind., 1884, 3, p. 103). Hydrochloric and sulphuric acids are almost without action on the metal, but it dissolves readily in dilute nitric acid. Nickel salts are antiseptic; they arrest fermentation and stop the growth of plants. Nickel carbonyl, however, is extremely poisonous. On the toxic properties of

nickel salts see A. Riche and Laborde, ''Jour. Pharm. Chem.'', 1888, [5], 17, pp. 1, 59, 97.

Nickel is used for the manufacture of domestic utensils, for crucibles, coinage, plating, and for the preparation of various alloys, such as German silver, nickel steels such as invar (nickel, 35·7%; steel, 64·3%), which has a negligible coefficient of thermal expansion, and constantan (nickel, 45%; copper, 55%), which has a negligible thermal coefficient of its electrical resistance.

Compounds.

Nickel Oxides.—Several oxides of nickel are known. A suboxide, Ni2O (?), described by W. Muller (Pogg. Ann., 1869, 212, p. 59), is not certainly known. The monoxide, NiO, occurs naturally as bunsenite, and is obtained artificially when nickel hydroxide, carbonate, nitrate or sulphate is heated. It may also be prepared by the action of nickel on water. by the reduction of the oxide Ni2O3, with hydrogen at about 200° C. (H. Moissan, Ann. Chim. Phys., [5], 21, p. 199), or by heating nickel chloride with sodium carbonate and extracting the fused mass with water. It is a green powder which becomes yellow when heated. It dissociates at a red heat, and is readily reduced to the metal when heated with carbon or in a current of hydrogen. It is readily soluble in acids, forming salts, the rate of solution being rapid if the oxide is in the amorphous condition, but slow if the oxide is crystalline. The hydroxide, Ni(OH)2, is obtained in the form of a greenish amorphous powder when nickel salts are precipitated by the caustic alkalis. It is readily soluble in acids and in an aqueous solution of ammonia. Nickel sesquioxide, Ni2O3, is formed when the nitrate is decomposed by heat at the lowest possible temperature, by a similar decomposition of the chlorate, or by fusing the chloride with potassium chlorate. It is a black powder, the composition of which is never quite definite, but approximates to the formula given above. When heated with oxy-acids it dissolves, with evolution of oxygen, and with hydrochloric acid it evolves chlorine. Numerous hydrated forms of the oxide have been described (see W. Wernicke, Pogg. Ann., 1870, 217, p. 122). A peroxide, NiO2, has been obtained in the form of dinickelite of barium, BaO·2NiO2, by heating the monoxide with anhydrous baryta in the electric furnace (E. Dufau, Comptes rendus, 1896, 123, p. 495). G. Pellini and D. Meneghini (Zeit. anorg. Chem., 1908, 60, p. 178) obtained a greyish green powder of composition NiO2·xH2O, by adding an alcoholic solution of potassium hydrate to nickel-chloride and hydrogen peroxide at −50°. It has all the reactions of hydrogen peroxide, and S. Tanatar (Ber., 1909, 42, p. 1516) regards it as NiO·H2O2. An oxide, Ni3O4, has been obtained by heating nickel chloride in a current of moist oxygen at about 400° C. (H. Baubigny, Comptes rendus, 1878, 87, p. 1082), or by heating the sesquioxide in hydrogen at 190° C. (H. Moissan, Ann. Chim. Phys., 1890 [5], 21, p. 199). The former method yields greyish, metallic-looking, microscopic crystals, the latter a grey amorphous powder. A hydrated form, Ni3O4·2H2O, is obtained when the monoxide is fused with sodium peroxide at a red heat and the fused mass extracted with water.

Nickel Salts.—Only one series of salts is known, namely those corresponding to the monoxide. In the anhydrous state they are usually of a yellow colour, whilst in the hydrated condition they are green. They may be recognized by the brownish violet colour they impart to a borax bead when heated in an oxidizing flame. The caustic alkalis added to solutions of nickel salts give a pale green precipitate of the hydroxide, insoluble in excess of the precipitant. This latter reaction is hindered by the presence of many organic acids (tartaric acid, citric acid, &c.). Potassium cyanide gives a greenish yellow precipitate of nickel cyanide, Ni(CN)2, soluble in excess of potassium cyanide, forming a double salt, Ni(CN)2·2KCN, which remains unaltered when boiled with excess of potassium cyanide in presence of air (cf. ). Ammonium sulphide precipitates black nickel sulphide, which is somewhat soluble in excess of the precipitate (especially if yellow ammonium sulphide be used), forming a dark-coloured solution. Ammonium hydroxide gives a green precipitate of the hydroxide, soluble in excess of ammonia, forming a blue solution. Numerous methods have been devised for the separation of nickel and cobalt, the more important of which are:—the cobaltinitrite method by which the cobalt is precipitated in the presence of acetic acid by means of potassium nitrite (the alkaline earth metals must not be present); the cyanide method (J. v. Liebig, Ann., 1848, ,65, p. 244; 1853, 87, p. 128), in which the two metals are precipitated by excess of potassium cyanide in alkaline solution, bromine being afterwards added and the solution warmed, when the nickel is precipitated. The latter method has been modified by adding potassium cyanide in slight excess to the solution of the mixed salts, heating for some time and then adding mercuric oxide and water, the whole being then warmed on the water bath, when a precipitate of mercuric oxide and nickel hydroxide is obtained