The cerium compounds obtained by these methods are purified by transformation into the anhydrous sulphate, which is dissolved in ice-water; when this solution is allowed to come slowly to room temperature, the pure octohydrate separates. Pure cerium salts should show no trace of absorption when concentrated solutions are examined spectroscopically; on ignition, the oxide obtained should be almost colourless, having at most a faint yellow tinge. A reddish or brownish-red shade indicates the presence of praseodymium. An arc spectrum examination will generally show the presence of lanthanum, which occurs in traces even in the most carefully purified cerium preparations. The preparation and properties of metallic cerium have already been described (see p. 114); for an account of the pyrophoric alloys, see p. 314. THE CEROUS COMPOUNDS The salts of trivalent cerium are very similar to those of the other rare earth elements, and a detailed description of them is therefore unnecessary. The _sesquioxide_, Ce₂O₃, cannot be obtained by ignition of the oxalate, nitrate, or other similar salt, since these decompose at high temperatures with formation of the dioxide, CeO₂. It has been prepared by the reduction of the dioxide with calcium;[200] it has a great affinity for oxygen, and readily absorbs the gas when exposed to moist air. _Cerous hydroxide_, Ce(OH)₃, obtained by addition of alkali to solutions of cerous salts, has also strong reducing properties,[201] and can only be prepared and preserved when oxygen is carefully excluded. It has been obtained as a perfectly white solid by the action of water on the carbide;[202] when dried in an inert atmosphere, it yields a perfectly white oxide. In presence of air, it darkens, assuming a reddish-violet colour, which passes into yellow as the oxidation becomes complete. The oxidation proceeds more quickly in presence of potash or soda, ceric hydroxide, Ce(OH)₄, being formed; in presence of potassium carbonate, however, a dark-coloured peroxyhydrate is formed by autoxidation. The colour so produced disappears on shaking if an ‘acceptor’ is present, ceric hydroxide being left; if the acceptor cannot reduce this, the solution after shaking loses the power of re-forming the dark peroxide, but if the acceptor can reduce the ceric compound to cerous hydroxide, the solution after shaking regains the power of forming the peroxide which is a property of the lower hydroxide. [200] Burger, _Ber._ 1907, ~40~, 1652. [201] Dennis and Magee, _J. Amer. Chem. Soc._ 1894, ~16~, 649; also Biltz and Zimmerman, _Ber._ 1907, ~40~, 4979. [202] Damiens, _Compt. rend._ 1913, ~157~, 214.