The _hydroxide_, Gd(OH)₃, is a gelatinous precipitate with strongly basic properties, rapidly absorbing carbon dioxide from the air. The _oxide_, Gd₂O₃, also absorbs carbonic anhydride from the air, and is easily soluble in acids, even after strong ignition. The element is therefore strongly electropositive. Its position among the yttrium elements, however, is justified by the properties of the _platinocyanide_, 2Gd(CN)₃,3Pt(CN)₂,18H₂O, which forms long, pointed red crystals, with a green metallic lustre, belonging to the rhombic system, and isomorphous with the corresponding yttrium and erbium salts; the cerium elements, on the other hand, give yellow platinocyanides, with a blue metallic lustre, which crystallise in the monoclinic system. The _nitrate_, Gd(NO₃)₃,6H₂O, separates from aqueous solutions at the ordinary temperatures in large crystals belonging to the anorthic system, and is isomorphous with the corresponding compounds of praseodymium and neodymium.[330] From solutions in strong nitric acid, a pentahydrate is obtained, which melts at 92°; the hexahydrate melts at 91°. The _sulphate_ separates from aqueous solution as the octohydrate, Gd₂(SO₄)₃,8H₂O, isomorphous with the corresponding salts of both groups. The anhydrous sulphate is much less soluble in water at 0° than the corresponding compounds of the cerium elements. The _selenate_ forms hydrates with 10 and 8 molecules of water of crystallisation respectively; these are isomorphous with the corresponding selenates of yttrium and the erbium metals. [330] Lang and Haitinger, _Annalen_, 1907, ~351~, 450. ~Atomic Weight.~--The determinations of this constant made by the earlier workers were all carried out with impure material and gave results which were considerably too low. The International Committee have adopted the value 157·3, which is based on the work of Urbain.[331] In employing the analytical sulphate method, that author observed that the anhydrous sulphate did not remain constant in weight when allowed to remain in a desiccator, and that it could not be accurately weighed. He therefore determined the ratio Gd₂(SO₄)₃,8H₂O : Gd₂O₃, by converting the octohydrate directly to oxide, and obtained the mean value 157·24. [331] _Compt. rend._ 1905, ~140~, 583. ~Detection.~--Pure gadolinium compounds show no absorption in the visible spectrum, but there are four strong bands[332] in the ultraviolet, viz. 311·6-310·5; 306·0-305·7; 305·6-305·5; and 305·4-305·0. The arc spectrum[333] is very rich in lines, of which the most intense are the following: [332] Urbain, _ibid._ 1905, ~140~, 1233. [333] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 21.