The _hydroxide_ is thrown down from solutions by addition of alkali, alkali carbonate, alkali cyanide, or ammonium sulphide, as a black precipitate. It cannot be transformed to the corresponding oxide by drying, since it attacks the water with evolution of hydrogen, forming the dioxide. The _monoxide_, TiO, has probably never been obtained in the pure state; it is formed by reduction of the dioxide with zinc or magnesium. Moissan[442] obtained it in the form of black prismatic crystals by treating the dioxide with the calculated amount of charcoal in the electric furnace. The _sulphide_, TiS, is an extremely stable compound; it can be prepared by heating the higher sulphides in a stream of hydrogen to a very high temperature, and then forms pseudomorphs after these.[443] It is a dark red metallic mass, which reacts in the air only when heated, forming the dioxide; dilute acids and alkalies have no action on it, concentrated nitric acid oxidises it slowly. [442] _Loc. cit._ [443] See v. d. Pfordten (_loc. cit._); Thorpe, _Chem. News_, 1885, ~51~, 260. The _dichloride_, TiCl₂, is obtained in the impure state as a black powder by decomposition of the trichloride at a red heat: the tetrachloride is formed at the same time, and volatilises.[444] According to v. d. Pfordten,[445] it is obtained by reduction of the tetrachloride by sulphuretted hydrogen or sodium amalgam in the cold. The latter author states that it dissolves in alcohol or water in absence of air to a dark brown solution; Friedel and Guérin, however, state that it acts energetically on these solvents with evolution of hydrogen, forming a yellow solution. When heated in the air it burns, evolving fumes of the tetrachloride and leaving a residue of the dioxide. The _iodide_, TiI₂, has been obtained by Defacq and Copaux[446] by reduction of the tetraiodide with silver or mercury, as a black, lustrous, infusible sublimate. It is insoluble in organic solvents, but reacts with water and aqueous alkalies, and is readily attacked by acids. Hydrogen at a bright red heat reduces it to amorphous titanium. [444] Friedel and Guérin, _Compt. rend._ 1875, _81_, 889; 1876, ~82~, 509, 872. [445] _Loc. cit._ [446] _Compt. rend._ 1908, ~147~, 65. COMPOUNDS OF TRIVALENT TITANIUM.[447] [447] Compounds of trivalent titanium are frequently referred to in English chemical and technical literature as ‘Titanous Compounds,’ the salts of the tetravalent element being tacitly recognised as ‘Titanic Compounds.’ In view of the existence of compounds of divalent titanium, to which the name ‘Titanous Compounds’ might be more logically applied, the former nomenclature cannot be regarded as altogether satisfactory, and it is therefore not adopted here.