These salts are obtained when the element is dissolved in hydrochloric and sulphuric acids, and by reduction of the compounds of tetravalent titanium in solution by means of zinc and hydrochloric acid, or by electrolysis. According to Diethelm and Forster[448] the reduction may also be effected by hydrogen in presence of finely divided platinum. The salts have strong reducing properties, transforming nitro-bodies to amines and decolourising azo-derivatives very rapidly; they reduce unsaturated bodies, and reduce dyes to the leuco-bases; they reduce sulphurous acid to sulphur, precipitate gold, silver and mercury from their salts, and reduce cupric and ferric salts to cuprous and ferrous compounds respectively. The salts are green or violet in solution, showing the phenomenon of hydrate-isomerism which is exhibited by the chromic salts; they are to some extent hydrolysed in aqueous solution, as shown by the acid reaction of the chloride. They resemble the salts of ferric iron and aluminium in giving precipitates of basic salts when boiled with sodium acetate or sodium formate, and in giving no precipitate with alkalies in the presence of organic hydroxy-acids. Ferrocyanide and ferricyanide give brown precipitates. [448] _Zeitsch. physikal. Chem._ 1908, ~62~, 129. The _hydroxide_, Ti(OH)₃,_x_H₂O, is thrown down as a dark precipitate with strong reducing properties; it attacks water with evolution of hydrogen, forming the dioxide; when an aqueous suspension is shaken with air, autoxidation occurs, hydrogen peroxide being formed. The _sesquioxide_, Ti₂O₃, has been prepared by Friedel and Guérin[449] by heating the dioxide to a white heat in a current of hydrogen and titanium tetrachloride; it forms black lustrous crystals, isomorphous with hæmatite. The _sulphide_, Ti₂S₃, is best obtained by reduction of the disulphide, at a moderate temperature, in a stream of hydrogen or nitrogen, but is also prepared by the action of a mixture of carbon disulphide and sulphuretted hydrogen on the dioxide at a high temperature. It is a dark grey metallic powder, stable towards air, water, alkalies and dilute acids. [449] _Loc. cit._ _Titanium Nitride_, TiN, is obtained in all reduction processes in which titanium compounds are used, if air or nitrogen is admitted; it is formed when the element is heated in nitrogen, and by the action of ammonia on the chloride. It forms lustrous, bronze-coloured leaflets, which appear blue or violet when powdered. It is extremely hard, and very stable, but is attacked by alkalies with evolution of ammonia. It reduces the oxides of copper and lead in the fused state. Ruff and Eisner have shown that it is a true nitride of the trivalent element, and that only one nitride exists.[450] [450] _Ber._ 1905, ~38~, 742; 1908, ~41~, 2250.