Ignited lanthana resembles quicklime in that it readily absorbs carbon dioxide from the air, and hisses when slaked with water; as the basicity becomes weaker, the affinity for water and carbon dioxide becomes less marked. All the oxides are soluble in dilute acids, even after prolonged ignition; but the ease with which solution occurs is naturally much influenced by the treatment to which the oxide has been subjected, as well as by its strength as a base. The rare earth oxides are capable of existing in more than one modification, the compounds obtained by ignition of the hydroxides differing in appearance and reactivity from those prepared by ignition of the oxalates or nitrates, and so on; they are probably highly polymerised. Cerium dioxide, CeO₂, is remarkable for its power of combining with the other oxides, R₂O₃, of the rare earth metals. The pure dioxide is insoluble in nitric acid, but mixtures of earths containing up to 50 per cent. of the dioxide dissolve readily. The various colours of mixtures of the ceria earths may sometimes be attributed to a similar combination,[160] and there can be little doubt that the dioxide sometimes functions as an acid in the rare earth minerals. [160] The brown colour of a mixture of ceria oxides containing praseodymium is generally attributed to the presence of the strongly coloured peroxide of that element. ~Sulphides.~--These compounds cannot be prepared in the wet way, that is, by the action of hydrogen sulphide or ammonium sulphide on the salts in solution; the former reagent gives no precipitate, the latter throws down the hydroxides. In this behaviour, the rare earth elements resemble aluminium and chromium. The normal sulphides, R₂S₃, are obtained by reduction of the anhydrous sulphates, or from the oxides at high temperatures, by treatment with hydrogen sulphide. They are strongly coloured compounds, fairly stable towards cold water, but readily hydrolysed on boiling. Disulphides, RS₂, are known in the cases of cerium, lanthanum, and praseodymium; these are to be regarded as polysulphides, since on treatment with dilute acids they yield hydrogen persulphide, H₂S₂. ~Carbides.~--By reduction of the oxides with carbon in the electric furnace, Moissan obtained the carbides in the form of microscopic yellow crystals. They have the general formula RC₂, and are attacked by water and dilute acids, with evolution of very complex mixtures of gases.[161] The principal product is acetylene, with various higher homologues, and in smaller quantities ethylene and ethane and their homologues. No methane is formed,[162] but hydrogen is always present, the olefines and paraffins probably arising from its action on the acetylenic hydrocarbons. The relation of the rare earth elements to the calcium group is here very close; calcium carbide when attacked by water yields pure acetylene, whereas aluminium carbide gives pure methane. [161] Damiens, _Compt. rend._ 1913, ~157~, 214.