_Cyanides_ of the rare earth elements are not known; addition of potassium cyanide to solutions of the salts throws down the hydroxides. The _platinocyanides_ may be obtained by double decomposition of the sulphates with barium platinocyanide. They are very stable and characteristic bodies, of the general formula R₂[Pt(CN)₄]₃, with 18 or 21 molecules of water. The compounds of the cerium elements are yellow, with a strong blue fluorescence; they crystallise in the monoclinic system. The platinocyanides of the yttrium metals are red or crimson, with a splendid green fluorescence, and crystallise in the rhombic system. Scandium platinocyanide is of great interest from the fact that it exists in two modifications, which show the characteristic appearance of the two groups of compounds respectively. Potassium ferrocyanide precipitates _potassium earth ferrocyanides_ of the general formula KR(FeC₆N₆),3H₂O, from neutral solutions;[163] the precipitate is somewhat soluble in excess. The ferrocyanides have been proposed for the purification of yttrium; the method is useful where rapid concentration of the element is required, yttrium ferrocyanide being far more soluble than the analogous compounds of the erbium and ytterbium metals, but the precipitates are gelatinous, and very difficult to handle. [163] Compare Astrid Cleve, _Zeitsch. anorg. Chem._ 1902, ~32~, 129. ~Halogen Oxy-salts.~--_Perchlorates_ and _periodates_ of the rare earth elements, of the general formula R(XO₄)₃,_x_H₂O, have been obtained. The existence of _chlorates_ has been observed only in the yttrium group; yttrium chlorate, Yt(ClO₃)₃,8H₂O, has been prepared by double decomposition of the sulphate with barium chlorate. The _bromates_ are also prepared in this way. They are readily soluble compounds, of which several hydrated forms are known. They are of considerable importance for purposes of separation in the yttrium group. The _iodates_ are sparingly soluble bodies, precipitated by addition of the alkali compound to solutions of the rare earth salts. The rare earth iodates are soluble in nitric acid, the solubility increasing as the electropositive character of the element becomes stronger. A method for the purification of yttrium has recently been based upon this property of the iodates, whilst the fact that thorium iodate is completely insoluble in nitric acid allows of the easy separation and estimation of thorium in minerals or mixtures containing rare earth elements.