The _acetates_ are readily soluble in water, the yttrium salts being rather less easily soluble than those of the cerium group. They are therefore obtained by dissolving the oxides in acetic acid; addition of alkali acetate to a solution of a rare earth salt gives no precipitate, even on boiling, behaviour which is in marked contrast to the ease with which the salts of other trivalent metals are hydrolysed under these conditions. In this respect the rare earth elements differ also from the tetravalent elements zirconium and thorium (and from cerium in the tetravalent state); soluble salts of the latter, on boiling with sodium acetate, give insoluble basic acetates. Even sparingly soluble compounds of the rare earth elements are as a rule taken into solution by digestion with ammonium acetate. _Tartrates._--Addition of ammonium tartrate to a neutral solution of rare earth salts throws down an amorphous precipitate, which dissolves easily in acids, and in excess of the precipitant. In the presence of tartaric acid, precipitation of the earths by addition of sodium hydroxide is completely inhibited. Potassium hydroxide under these conditions gives a precipitate in the case of the yttrium elements, though only on boiling; ammonia gives a crystalline precipitate even in the cold with this group. These precipitates are alkali double tartrates of the yttrium metals; the cerium elements give no precipitate at all. In all cases, therefore, the precipitation of the hydroxides is inhibited by the presence of tartaric acid. A very large number of organic salts of the rare earth elements has been prepared and examined during the past two decades, in the endeavour to find some class of compounds which will allow of an easy separation of the group. The _benzoates_, _succinates_, _hippurates_, _citrates_ and similar relatively simple salts first received attention, but less common acids, as _e.g._ the hydroxynaphthalenesulphonic acids, have also been employed.[169] The use of various organic acids for the separation and estimation of thorium in presence of the rare earths is outlined in that connection (see p. 288). More recently, the glycollates and cacodylates have been prepared. The _glycollates_[170] of the cerium elements have the general formula R(C₂H₃O₃)₃, and crystallise in crusts; they are more soluble than the yttrium compounds, which have the formula R(C₂H₃O₃)₃,2H₂O, and crystallise in needles. The _cacodylates_,[171] R₂[As(CH₃)₂O₂]₆, crystallise with 16 or 18 molecules of water, and have similar solubility relations. [169] Erdmann and Wirth, _Annalen_, 1908, ~361~, 190; see also Pratt and James, _J. Amer. Chem. Soc._ 1911, ~33~, 1330; Baskerville and Turrentine, _ibid._, 1904, ~26~, 46; James, Hoben and Robinson, _ibid._, 1912, ~34~, 276, etc. [170] Jantsch and Grünkraut, _Zeitsch. anorg. Chem._ 1913, ~79~, 305. [171] Whittlemore and James, J. _Amer. Chem. Soc._ 1913, ~35~, 627.