The _phthalates_ of the yttrium group have been found to be very valuable for purposes of separation by Meyer and Wuorinen.[172] The salts are readily obtained in solution by shaking together cold aqueous suspensions of the rare earth hydroxides, and phthalic acid; the clear solutions when warmed become cloudy, the organic salts hydrolysing very easily, with separation of the hydroxides. The most positive elements naturally remain longest in the solution, the weakly basic oxides accumulating in the first precipitates. [172] _Zeitsch. anorg. Chem._ 1913, ~80~, 7. An organic compound which has proved very useful in the treatment of the rare earths is acetylacetone, CH₃.CO.CH₂.CO.CH₃.[173] In its enolic form, this substance forms salts with metals, which in the case of the rare earth elements are especially characterised by the ease with which they may be obtained, and their high crystallising power. They may be prepared by double decomposition of neutral solutions of rare earth salts with ammonium acetylacetone, and crystallise readily from dilute alcohol. They have been used by Urbain in the fractionation of the yttrium group, and for determination of molecular weights by the boiling point method; Biltz[174] has shown that in solution they generally have the double formula R₂(C₅H₇O₂)₆. [173] Urbain, _Bull. Soc. chim._ 1897, [iii.], ~17~, 98; Urbain and Budischofsky, _Compt. rend._ 1897, ~124~, 618; Biltz and Clinch, _Zeitsch. anorg. Chem._ 1904, ~40~, 218. [174] _Annalen_, 1904, ~331~, 334. THE RARE EARTH ELEMENTS, AND THE PERIODIC CLASSIFICATION At the time of the introduction of the periodic classification the rare earth elements were generally believed to be divalent. This belief, which has persisted until quite recently,[175] was based chiefly on the electropositive character of the metals, and their general chemical resemblance to the elements of the alkaline earths; the isomorphism of the tungstates of calcium and the cerium elements, and of the molybdates of lead and the cerium elements, also supports this view. The physical evidence in favour of Mendelejeff’s view, however, is quite overwhelming; the specific heats of the metals, the equivalent conductivities of the chlorides, and molecular weight determinations by means of vapour densities and the boiling point method, prove beyond doubt that the elements are in fact trivalent. [175] See Wyrouboff, _Bull. Soc. franc. Min._ 1896, ~19~, 219; Wyrouboff and Verneuil, _Compt. rend._ 1897, ~124~, 1230 and 1300; _ibid._, 1899, ~128~, 1573; etc. In deciding in favour of the trivalent nature of the rare earth metals, Mendelejeff was influenced chiefly by the fact that there was no room in the table for divalent elements with the equivalent weights then assigned to the cerium and yttrium elements. At that time, only the six oxides obtained by Mosander were known; of these the accepted equivalents and atomic weights were as follows: