~The Magnetic Susceptibility.~--The fact that the rare earths differ very considerably from one another in their magnetic properties has been known for several years,[189] and has recently been employed by Urbain and Jantsch[190] as a means of identification, and a test of purity, and for following processes of fractionation. The magnetic susceptibility reaches a minimum at samarium, and rises very sharply on either side of that element, so that the presence of the closely related elements, neodymium on the one side, and europium and gadolinium on the other, which differ only very slightly from samarium in atomic weight and solubility, can easily be detected by this means. The property is highly additive, and can be used, therefore, to estimate the relative proportions of two oxides in a mixture; the determinations are said to be easily and quickly carried out. [189] See Meyer, _Monats._ 1898, ~20~, 369 and 793. [190] _Compt. rend._ 1908, ~147~, 1286; see also Urbain, _ibid._, 1910, ~150~, 913. When the elements are considered in order of atomic weight, the coefficient reaches a maximum at neodymium in the cerium group, and again at dysprosium (or holmium) in the yttrium group:--[191] Coefficient of magnetisation Element. Atomic Weight. for the oxide. _x_ × 10⁻⁶ Scandium 44·1 -0·05 Yttrium 89·0 -0·14 Lanthanum 139·0 -0·18 Neodymium 144·3 33·5 Samarium 150·4 6·5 Europium 152·0 33·5 Gadolinium 157·3 161 Terbium 159·2 237 Dysprosium 162·5 290 [191] See Urbain and Jantsch, _loc. cit._; the values for lanthana, scandia, and yttria were determined by Wedekind (see Meyer and Wuorinen, _Zeitsch. anorg. Chem._ 1913, ~80~, 7). Erbium, thulium, ytterbium, and lutecium appear in descending order at the end of the series, but no figures are given. The most interesting application of the property has been Urbain’s discovery of the new element Celtium (see p. 207). THE EQUIVALENT WEIGHT DETERMINATION The determination of the mean equivalent weight, which was for the earlier chemists the only reliable method of controlling their fractionations, is still of considerable importance for this purpose, especially in the yttrium group, in which the differences in atomic weights are more considerable than among the cerium metals. Great importance, moreover, still attaches to these determinations, since they serve to fix the atomic weights; save that the methods used in an atomic weight determination are somewhat more elaborate and refined than those used when it is desired merely to test a fractionation, the same processes apply in both cases.