Thorianite occurs in jet-black crystals with a bright resinous lustre. They are pseudocubic, and the twinning resembles that of the cubic mineral fluorspar--interpenetrant cubes, twin axis a cube diagonal. Close examination shows, however, that twinning can only take place about one of the four diagonals, and an optical examination makes it clear that the symmetry is really rhombohedral. The case is exactly analogous to that of the mineral chabazite, a zeolite which occurs in rhombohedra of which the angles differ but little from those of the cube, and which also forms the interpenetrant twins. In view of the fact that both uranous oxide and thoria have been obtained as octahedra, whilst a fused mixture of the two on cooling forms cubic crystals, it seems not unlikely that at high temperatures the pseudocubic thorianite would become truly cubic; but no experiments in this direction seem to have been tried. The crystals are brittle; hardness 7; sp. gr. 8·0-9·7. Thorianite is infusible, incandescing before the blowpipe. When powdered, it dissolves readily in nitric and sulphuric acids, with evolution of helium. Gray[89] has shown that the helium content can be reduced by 28 per cent. by fine grinding, thus showing that part at least of the gas must be mechanically held. [89] _Proc. Roy. Soc._ 1908, A, ~82~, 306. Thorianite was found in Ceylon, being originally mistaken for pitchblende. A sample was supplied by the discoverer, Mr. Holland, to the officers of the Mineral Survey, by whom it was sent to London for examination. Its composition was determined by Dunstan, who named it. It was found in the river gravels (gem-gravels), the matrix being a pegmatite granite. It is a valuable source of thorium nitrate for incandescent mantles, one ton of the mineral (with thoria content of 70 per cent.) having been sold for £1500; but the supply is small and unreliable. ~Baddeleyite.~[90]-- Baddeleyite consists of almost pure zirconia (ZrO₂ = 96·5 per cent.) with small quantities of ferric oxide, alumina, lime, magnesia, alkalies and silica. Thoria and rare earths are present in traces, uranium is absent; the mineral is not radioactive, and contains only traces of helium. [90] _Vide_ Fletcher, _Min. Mag._ 1893, 46, ~10~, 148; Hussak, _Zeitsch. Kryst. Min._ 1895, ~24~, 164, and ~25~, 298. Monoclinic--_a_ : _b_ : _c_ = 0·9871 : 1 : 0·5114. β = 98° 45¹⁄₂´. Common forms--all three pinakoids, _a_ {100}, _b_ {010}, and _c_ {001}, with the hemi-prisms _m_ {110}, _k_ {120}, and _l_ {230}, and various pyramids and domes. Angles--(100) ∧ (110) = 44° 17¹⁄₂´; (100) ∧ (001) = 81° 14¹⁄₂´; (100) ∧ (101) = 55° 33¹⁄₂´. Cleavage ∥ _c_ and ∥ _b_, parting ∥ _m_ due to repeated twinning. Twinning is exceedingly common; of many hundred crystals examined by Hussak, only three were found untwinned. Twin planes _m_ (110), _a_ (100), and _x_ (201).