The _hydroxide_, or _hydrated oxide_, is capable of existing in two modifications, according to the conditions under which it is thrown down, though the two can hardly be said to be very definitely differentiated. The α or ortho form is obtained as a voluminous white precipitate by the addition of ammonia or alkali hydroxide in the cold to a freshly prepared solution of a titanium salt. It is insoluble in water and alcohol, but dissolves readily in dilute mineral acids, and to some extent also in dilute alkalies. The water content is very variable, and no definite hydrate or hydroxide can be prepared; when the substance is heated, it loses water continuously, and at a definite temperature glows, doubtless by reason of some polymeric change. If it be maintained for some time at a temperature somewhat below the normal temperature of glowing, this phenomenon no longer occurs when the temperature is further raised. The β modification, or metatitanic acid, as it is called, is obtained by hydrolysis of the salts by boiling, or by addition of alkali at 100°, as a fine white precipitate. It is almost insoluble in dilute acids and alkalies, but dissolves in water to a colloidal solution; when heated it does not glow. The β form is also obtained when the metatitanates are treated with water; these compounds hydrolyse very readily, but the precipitated dioxide carries down alkali by adsorption. The _dioxide_, TiO₂, occurs crystalline in nature in the three forms Rutile, Brookite, and Anatase, all of which can be prepared by laboratory methods;[454] the amorphous form is obtained by ignition of the hydrated oxide, and of suitable salts. The oxide melts at 1560°, forming a mobile (?) liquid of density 4·1; for the physical properties, see the accounts of the naturally occurring forms in Chapter V. When heated in a current of hydrogen or carbon monoxide, it gives rise to intermediate oxides, Ti₃O₄, Ti₇O₁₂, etc., which are not very well known, and are of doubtful individuality. It reacts when heated in chlorine, and with many non-metallic chlorides, forming the tetrachloride; with carbon disulphide at high temperatures it gives the disulphide, ammonia at a red heat forms the nitride. It is exceedingly resistant to acids, but is attacked slowly by boiling sulphuric acid, more quickly by fused bisulphate. [454] See p. 79; also Hautefeuille, _Ann. chim. phys._ 1863, [iv.], ~4~, 129. _Titanium disulphide_, TiS₂, is obtained in the pure state when a mixture of the vapour of the tetrachloride and sulphuretted hydrogen is led through a strongly heated porcelain tube. It is a fairly stable substance, forming metallic crystals which yield the dioxide when heated in air. When heated in a stream of hydrogen or nitrogen it yields one or other of the lower sulphides according to the temperature employed. It is not attacked by water, but dissolves in acids, and is decomposed by boiling potash, forming a titanate; it is insoluble in alkali sulphides.