The _Oxides_.--By ignition of salts of volatile acids, Auer von Welsbach[255] obtained an oxide to which he assigned the formula Pr₄O₇. More recent work[256] has shown that the composition of the oxide obtained depends upon the conditions under which the various salts are decomposed. By fusing the nitrate in presence of potassium nitrate at 400-450°C., Meyer obtained the dioxide, PrO₂; at higher temperatures this decomposes, giving the intermediate oxides. The formation of the dioxide is greatly influenced by the presence of other oxides,[257]--ceric oxide, acting as an oxygen carrier, favouring whilst the other oxides hinder. The pure dioxide is a brownish-black powder, which resembles manganese dioxide, but is less stable. It liberates halogens from the halogen acids, and oxidises manganese salts to permanganates, but does not completely oxidise ferrous or stannous salts, losing instead a part of its oxygen in the gaseous form. The dioxide cannot be obtained in the wet way. [255] _Monats._ 1885, ~6~, 477. [256] See, _e.g._ Meyer, _Zeitsch. anorg. Chem._ 1904, ~41~, 94. [257] Brauner, _Monats._ 1882, ~3~, 1; Marc, _Ber._ 1902, ~35~, 2370; Meyer and Koss, _ibid._ 3470. When heated in a stream of hydrogen, the dioxide yields the _sesquioxide_, Pr₂O₃, as a greenish-yellow powder,which readily absorbs oxygen from the air, becoming brown, with formation of the intermediate oxide. The _chloride_, PrCl₃,7H₂O, forms large green prisms, very readily soluble in water; 100 parts of the solvent at 13° take up 334·2 parts of the hydrated salt, the solution having the specific gravity 1·687. The anhydrous chloride is a pale green deliquescent powder, which melts at a red heat to a clear green liquid; ebullioscopic measurements show that in alcoholic solution it has the simple molecular formula PrCl₃. The _Bromate_, Pr(BrO₃)₃,9H₂O, has been obtained by James and Langelier[258] by dissolving the oxide in aqueous bromic acid, and also by double decomposition. It forms greenish hexagonal prisms, melting at 56·5°, and is readily soluble; 100 parts of water dissolve 190 parts of this salt at 25°. At 100° it loses five molecules of water, forming the tetrahydrate Pr(BrO₃)₃,4H₂O, which loses all its water at 130°. The anhydrous salt begins to decompose at 150°. [258] _J. Amer. Chem. Soc._ 1909, ~31~, 913. The _sulphate_ crystallises with 8 molecules of water of crystallisation at ordinary temperatures, but hydrates with 15¹⁄₂, 12, and 5 molecules of water respectively have been described. The octohydrate is considerably more soluble than lanthanum sulphate enneahydrate. The anhydrous salt is a bright green powder. _Praseodymium acetylacetone_ melts at 146°.