An interesting series of addition compounds of the rare earth nitrates with antipyrine (dimethylphenylpyrazolone, C₁₁H₁₂ON₂) has been described recently by Kolbe.[164] Those of the cerium metals have the general formula R(NO₃)₃,3C₁₁H₁₂ON₂; the yttrium nitrates appear to combine with four molecules of the base. [164] _Zeitsch. anorg. Chem._ 1913, ~83~, 143 The tendency to form double nitrates with nitrates of the metals of Group IA and Group IIA also varies with the basic strength of the hydroxides. In the most positive elements of the cerium group, the tendency is very pronounced, and there are a large number of stable, crystalline double salts; but the stability decreases rapidly as the atomic weight of the element rises, and in the terbium and yttrium groups crystallised double nitrates cannot be obtained. The solubility of these double salts increases rapidly in the same direction, the lanthanum double nitrates being the least soluble. For this reason, these compounds are of great importance for the purpose of separation, especially in the cerium group. Bismuth nitrate and the various bismuth double nitrates are isomorphous with the corresponding compounds of the cerium group, and the double bismuth ammonium and bismuth magnesium salts have been largely used by Urbain in the separation of samarium and the elements of the terbium group. ~Phosphates.~--Addition of phosphoric acid, or an alkali phosphate to solutions of rare earth salts throws down the phosphates as gelatinous precipitates, which slowly become crystalline on standing. The precipitate is soluble in excess of phosphoric acid, and in other mineral acids, a fact of great importance in the commercial treatment of monazite. The composition of the precipitate is not known with certainty; both neutral and acid phosphates can probably be obtained according to the conditions. Double salts with the alkali phosphates can be prepared by fusion methods. The naturally occurring phosphates, monazite and xenotime, are mixtures of the orthophosphates of the cerium and yttrium elements respectively. _Phosphites_ are known in a few cases only; _arsenates_ and _arsenites_ of lanthanum have been prepared. _Vanadates_ of some of the rare earth elements have been described.