_Thorium dioxide_, ThO₂, is obtained by the ignition of the hydroxide or of suitable salts as a white powder, of which the properties and appearance depend largely on the method and temperature used in its formation. Whilst the residue obtained by ignition of the nitrate is an extraordinarily voluminous and light flaky mass, the sulphate yields a dense thick powder; the nitrate was therefore always preferred in the manufacture of incandescent mantles (_q.v._), as it was thought that the oxide obtained from it was the most suitable for illumination. In the crystalline form the oxide has been obtained in the laboratory by fusion with borax and with potassium phosphate. The first method gives tetragonal crystals, probably isomorphous with those of rutile and cassiterite; the phosphate fusion is said to give cubic crystals (see p. 74). The oxide is insoluble in acids, but can be transformed into the sulphate by evaporation with concentrated sulphuric acid, or fusion with alkali bisulphate. It does not liberate carbon dioxide when fused with alkali carbonates. By repeated evaporation with small quantities of acids, thoria can be transformed into a gel soluble in water (thorium meta-oxide). The sol is an opalescent fluid, orange-red by transmitted light, and contains small quantities of the acid employed. The hydroxide may also be obtained in this form by carefully washing it, and boiling with small quantities of acids, or with thorium or other salts, or even by long continued washing with pure water; similarly, continued dialysis of thorium salts eventually yields such gels. The colloid is positively charged, and resembles the zirconium oxide gel in its relation to negatively charged colloids. The gel is easily precipitated by electrolytes. Ignited thorium oxide has found considerable application in recent years a catalyst in the preparation of ketones by the contact method of Sabatier and Senderens.[478] By passing mixtures of the vapours of appropriate acids over the catalyst heated to the necessary temperature, good yields of the required ketones are obtained.[479] [478] Cf. Senderens, _Ann. Chim. Phys._ 1913 [viii.], ~28~, 143. [479] Cf. Pickard and Kenyon, _Trans. Chem. Soc._ 1913, ~103~, 1923. The _sulphide_, ThS₂, is obtained, together with the oxysulphide, ThOS, according to Duboin,[480] by passing a current of sulphuretted hydrogen over a mixture of thorium chloride with excess of sodium chloride, at a red heat. The former forms large brown crystals, from which the small orange-yellow crystals of the oxysulphide may be separated by means of a sieve; the latter is purified by treatment with warm nitric acid, which dissolves the sulphide very readily. The oxysulphide is also obtained when the anhydrous sulphate is heated in sulphuretted hydrogen.[481] [480] _Compt. rend._ 1908, ~146~, 815. [481] Hauser, _Zeitsch. anorg. Chem._ 1907, ~53~, 74.