[162] Moissan stated that 24-30 per cent. of methane was formed in this action; compare _Compt. rend._ 1900, ~131~, 595. ~Halogen Salts.~--The halides of the rare earth elements show a close analogy with the corresponding compounds of the alkaline earth elements. The _fluorides_ are insoluble in water and dilute mineral acids, and are obtained as gelatinous precipitates by the addition of hydrofluoric acid, or a soluble fluoride, to solutions of the salts. They may be prepared in the crystalline condition by heating the carbides in a stream of fluorine, or by the action of hydrofluoric acid upon the hydroxides in aqueous suspension. The rare earth elements, as well as thorium, may be separated from zirconium by taking advantage of the insolubility of their fluorides in excess of hydrofluoric acid or alkali fluorides, since zirconium fluoride is readily soluble in excess of the precipitant. The solubility of the fluorides in a large excess of concentrated acid increases with the electropositive character of the metal, the fluorides of the more negative elements being the least soluble. Thorium and scandium may, therefore, be concentrated to a large extent by repeated precipitation with hydrofluoric acid in acid solution. The _silicofluorides_ of the rare earth elements have been used by R. J. Meyer in the extraction of scandium from wolframite (see Chapter I and under Scandium, p. 215). They are thrown down as gelatinous precipitates on addition of potassium or sodium silicofluoride to boiling, neutral solutions of rare earth salts. In presence of mineral acids, however, they are not thrown down in the cold; on boiling, the cerium metals are precipitated as fluorides, by hydrolysis of the silicofluorides--the yttrium elements, with the exception of scandium, being held in solution by the mineral acid. With the exception of the fluorides, the halogen salts of the rare earth metals are readily soluble in water, and crystallise from the concentrated solutions in the hydrated form. The bromides and iodides have not been so fully studied as the chlorides; they are hygroscopic salts, and decompose rather easily. The iodides have been obtained by Moissan in the anhydrous state, by the action of iodine vapour on the carbides at high temperature.