The oxides of this group, as contrasted with the ytterbia oxides, give rise to coloured salts, which in solution show definite absorption spectra in the optical region; the spectrum of erbium salts is particularly definite and characteristic. Erbium has among the yttrium elements the place of neodymium among the cerium elements; after yttria, erbia is the commonest oxide of the yttria group, though on account of the difficulties of separation the chemistry of erbium is by no means so complete and definite as that of neodymium. The oxides in order of decreasing basicity, as shown by the order in which they are thrown down by ammonia, are: dysprosia, holmia, erbia, thulia; the electropositive character becomes weaker, therefore--as generally in the rare earth series--as the atomic weight of the elements increases. ~Dysprosium~, Dy = 162·5 Compounds of this element were probably prepared in the pure state for the first time by Urbain[362] in 1906, by the fractional crystallisation of the ethylsulphate. He showed that after fourteen recrystallisations, the absorption spectrum of the salts and the mean atomic weight of the element remain unaltered, and that after removal of terbium by the very efficient ethylsulphate method, all remaining traces of yttrium could be rapidly removed by crystallisation of the nitrate. The salts have generally a more or less pronounced yellow colour. [362] _Compt. rend._ 1906, ~142~, 785. The _oxide_, Dy₂O₃, is a white powder which does not alter in composition when strongly heated in reducing or oxidising atmospheres. It is remarkable in that it is the most strongly paramagnetic oxide known, having a coefficient of susceptibility much greater than that of ferric oxide.[363] The _chloride_ crystallises with 6, the _sulphate_ with 8, and the _nitrate_ with 5 molecules of water of crystallisation. The _bromate_, Dy(BrO₃)₃,9H₂O,[364] obtained by double decomposition, melts at 78°. The _platinocyanide_, Dy₂[Pt(CN)₄]₃,21H₂O, forms bright red cubic crystals, with greenish fluorescence. [363] _Compt. rend._ 1908, ~146~, 922. [364] Jantsch and Ohl, _Ber._ 1911, ~44~, 1274. Several other salts are described by Urbain, and by Jantsch and Ohl (_loc. cit._). ~Atomic Weight.~--Urbain and Demenitroux[365] determined this constant from the ratio Dy₂(SO₄)₃,8H₂O : Dy₂O₃. The mean value of six determinations carried out with material obtained by fractional crystallisation of the nitrate was 162·52; with material purified by the ethylsulphate crystallisation, the mean of six determinations gave the value 162·54. The International Atomic Weight is 162·5. [365] _Compt. rend._ 1906, ~143~, 598. ~Detection.~--Lecoq de Boisbaudran[366] and Urbain[367] give the position of the following absorption maxima in the visible and ultraviolet regions respectively: ┌──────┴──────┐ 753 368·5 338 475 379·5 332·5 451·5 365 427·5 351 [366] _Ibid._ 1886, ~102~, 1003.