[158] The position of yttrium in this series is not known with certainty; it is probably as positive as neodymium. It is usually stated (see Meyer and Hauser, pp. 32-33) that the terbia oxides are intermediate in basic strength between the ceria and yttria earths, though the arrangement into two series, consisting of the cerium and yttrium groups respectively, is generally adopted; the electropositive character of the elements in each series then weakens as the atomic weight rises, scandium being of course exceptional. It will be seen that, with the exception of scandium and yttrium, the metals of the cerium and yttrium groups become less electropositive as the atomic weight increases. This arrangement is obtained by ascertaining the order in which the various hydroxides are precipitated from a solution by gradual addition of a dilute solution of a strong base. The weakest base is precipitated first, and the strongest last; those intermediate in strength are thrown down in ascending order of strength. Similar results may be obtained by the fractional decomposition of the nitrates by heat; in this case the nitrate of the weakest base is decomposed at the lowest temperature. This order is also confirmed, as far as the data are available, by measurements of the equivalent conductivities of solutions of the salts (see, for example, p. 122). Quite recently, a very different order has been obtained from a consideration of the dissociation tensions, and of the heats of dissociation of the anhydrous sulphates.[159] In the following table the elements are arranged in the order of the increase of the dissociation tension (T) measured at 900°, which is the same as the order of decrease of the heats of dissociation (Q): Element At. Wt. T. (Mm. Hg.) Q. La 139·0 2 59·8 Yt 89·0 3 58·9 Lu 174·0 3·5 58·5 Yb 172·0 4 58·2 Er 167·7 5 57·6 Pr 140·6 5·5 57·4 Nd 144·3 6 57·2 Gd 157·3 7 56·9 Sa 150·4 8 56·5 Sc 44·1 11 54·5 Ce 140·25 52·4 [159] Wöhler and Grünzweig, _Ber._ 1913, ~46~, 1726. It will be observed that the order is very different from the order of increase of atomic weight, the positions of lutecium and ytterbium being especially surprising; these elements are generally considered to be among the least electropositive of the whole series. The anomalous position of cerium is probably due to the fact that the sulphate on decomposition leaves the dioxide, and not the sesquioxide, as with the other elements; this would undoubtedly affect the values. The heats of dissociation are the greatest yet observed for the sulphates of trivalent metals, a further evidence of the strongly basic nature of the oxides.