The ignited oxide is soluble in nitric or hydrochloric acid only in presence of a reducing agent. Concentrated sulphuric acid converts it into ceric sulphate; fused bisulphate attacks it more readily. In the crystalline form, obtained by fusing the amorphous form with borax, or a suitable salt,[213] it is extremely resistant to acids and to alkalies. [213] See, _e.g._ Sterba, _Ann. Chim. Phys._ 1904, [viii.], ~2~, 193. By heating the dioxide in a stream of hydrogen, care being taken to exclude air, a dark blue oxide, of which the composition corresponds approximately to that required by the formula Ce₄O₇, is obtained.[214] This substance has strong reducing properties; when warmed in air, it glows, forming the dioxide, and reduces carbon dioxide when heated in a current of that gas. This _intermediate oxide_ is said to correspond in composition to the violet hydroxide which is obtained as an intermediate product in the oxidation of cerous to ceric hydroxide, and which is said to yield the blue oxide, Ce₄O₇, when dried _in vacuo_. [214] Sterba, _Compt. rend._ 1901, ~133~, 221; Meyer, _Zeitsch. anorg. Chem._ 1903, ~37~, 378. The _disulphide_, CeS₂, has been obtained by Biltz[215] by prolonged heating of anhydrous cerous sulphate in a current of sulphuretted hydrogen at a dull red heat; it is a dark, yellowish-brown, crystalline solid, which on treatment with hydrochloric acid yields hydrogen persulphide. [215] _Ber._ 1908, ~41~, 3341. _Halogen salts._--No halogen compounds are known in the free state, except the _fluoride_, CeF₄,H₂O, which was obtained by Brauner as a yellowish-brown mass, by the action of hydrofluoric acid on the hydroxide. A _double fluoride_, 2CeF₄,3KF,2H₂O, was prepared by the same author by dissolving the hydroxide in potassium hydrogen fluoride; it is insoluble in water. By dissolving a ceric salt in concentrated hydrochloric acid, a dark red solution is obtained, which is believed to contain the unstable complex acid, H₂CeCl₆; this decomposes slowly in the cold, more quickly on warming, with evolution of chlorine, and formation of cerous chloride. Several double compounds of ceric chloride with hydrochlorides of organic bases have, however, been obtained.