~Thorianite.~[82]--This interesting mineral consists chiefly of thoria, ThO₂ (55-79 per cent.), with oxides of uranium (11-32 per cent.), and ceria oxides (1-8 per cent.); oxides of lead and iron are also present in small quantities, and zirconia with silica, probably due to associated zircon. [82] Dunstan and Blake, _Proc. Roy. Soc._ 1905, A, ~76~, 253; Dunstan and Jones, _ibid._, 1906, A, ~77~, 546. Helium is present, and the mineral is strongly radioactive. A careful analysis by Hahn[83] shows traces of many metals; the same chemist has also separated an extremely active component, 250,000 times as active as thorium nitrate, which he calls Radiothorium. [83] Hahn, _ibid._, 1907, A, ~78~, 385. The composition has been accounted for (Dunstan and Jones, _loc. cit._) on the hypothesis that thoria (ThO₂) and uranous oxide (UO₂) are isomorphous, the mineral being really a solid solution. Whilst, however, the crystal system of the natural body is really rhombohedral (_vide infra_) the two pure oxides appear to be cubic. Thus Troost and Ouvrard[84] obtained artificial thoria in minute octahedra; and, similarly, Hillebrand[85] obtained uranous oxide in octahedra by reduction of uranyl chloride, UO₂Cl₂, though his work seems to be open to objection. On the other hand, the same author[86] found that uranous oxide and thoria, fused together in almost any proportions, gave a homogeneous body crystallising in octahedra (cf. Szilard, _Compt. rend._ 1907, ~145~, 463, quoted under Uraninite). The probability of the isomorphism of the oxides is strengthened by the observation of isomorphism in the sulphates. As early as 1886, Rammelsberg showed that uranous sulphate, U(SO₄)₂, crystallises with nine molecules of water and is isomorphous with the corresponding thorium sulphate, Th(SO₄)₂,9H₂O; and six years later, Hillebrand and Melville[87] obtained mixed crystals of the two sulphates which were exceedingly close in forms and angles to those of pure uranous sulphate. It is then at least probable that the two oxides are isomorphous, though the point cannot be regarded as satisfactorily proved, by reason of the anomalous crystal forms of the naturally occurring mixtures, thorianite and uraninite. The recent results of Kobayashi[88] point to the conclusion that different varieties of thorianite may exist, in each of which the oxides of thorium and uranium bear definite simple ratios to one another. [84] _Compt. rend._ 1882, ~102~, 1422. [85] _Zeitsch. anorg. Chem._ 1893, ~3~, 243. [86] _Bull. U.S. Geol. Surv._ No. 113, 1893. [87] _Ibid._ No. 90, 1892, p. 30. [88] _Abstr. Chem. Soc._ 1912, ~102~, ii. 1181.