The element forms a rose-coloured oxide, and rose-coloured salts, which give to the compounds of the mixed erbia earths their characteristic colour. The oxide gives a very definite and characteristic reflection spectrum, but the salts do not possess this property;[375] the reflection spectrum remains unchanged in the presence of foreign oxides, provided no combination occurs. From the atomic weight determinations, it seems clear that the salts described by Cleve and his pupils[376] were not pure erbium compounds; a few salts only appear to have been recently obtained in the pure state for the atomic weight determination (_q.v._). [375] See Kruss and Bugge, _Ber._ 1908, _41_, 3783. [376] See _Compt. rend._ 1880, ~91~, 381. The _sulphate_ separates from aqueous solutions at ordinary temperatures as the octohydrate, Er₂(SO₄)₃,8H₂O, which forms rose-coloured monoclinic crystals isomorphous with the corresponding sulphates of the whole group. The anhydrous sulphate is formed by long heating at 400°, more quickly at 475°, and can be heated to 630° without decomposition. At 845° a basic salt, Er₂O₃,SO₃, is formed, which begins to decompose at 950°; at 1055° the transformation to the oxide is complete. The ammonium and potassium double sulphates are easily soluble in cold water. The _oxalate_ is thrown down in rosettes of bright rosy plates, which according to Hofmann[377] have the formula Er₂(C₂O₄)₃,10H₂O, even when dried in the air. Cleve believed the salt to be thrown down as the enneahydrate. When kept _in vacuo_ over phosphoric anhydride, the decahydrate passes into the trihydrate, which when heated decomposes, passing into the oxide at a temperature of 575°. The _nitrate_, Er(NO₃)₃,5H₂O, separates from aqueous solution as the pentahydrate, in large stable red crystals. The _platinocyanide_, Er₂[Pt(CN)₄]₃,21H₂O, has the characteristic red colour with green fluorescence. The _formate_, Er(HCOO)₃--Cleve, _loc. cit._--is a red powder, obtained by dissolving the oxide in formic acid; it crystallises from water as the dihydrate. [377] _Loc. cit._ ~Atomic Weight.~--The determinations of the earlier workers, being carried out with impure material, gave results which differ very widely, and are quite unreliable. Cleve’s value of 1880, for material free from ytterbia, but not apparently free from earths of lower equivalent, was 166·25; Brauner,[378] using the same material in 1905, obtained the much higher value 167·14. The determinations of Hofmann and Burger[379] in 1908 gave the mean value 167·38; with purer material, Hofmann in 1910[380] obtained the mean value 167·68, on which is based the value accepted by the International Committee, 167·7. [378] Abegg, III. i. 318. [379] _Loc. cit._ [380] _Loc. cit._