Employing this method in the case of titanite, Bruckmoser claims to have obtained the acids H₂Si₂O₅ and H₂Ti₂O₅. He therefore concludes that the constitution of the mineral is represented by the formula Si₂O₅,Ti₂O₅Ca, which presumably may be written Ca(Ti,Si)₂O₅. Crystal system--monoclinic; _a_ : _b_ : _c_ = 0·7547 : 1 : 0·8543. β = 60° 17´. Common forms (Des Cloizeaux’s orientation)--the pinakoids _a_ {100} and _c_ {001}, with _m_ {110}, _s_ {021}, _x_ {102}, _n_ {111}, and many others. (100) ∧ (110) = 38° 14¹⁄₂´; (001) ∧ (1̅01) = 65° 57´; (001) ∧ (011) = 36° 34´. The habit is very varied, the commonest being the wedge form, elongated ∥ _c_. Twinning is fairly common, especially on the law--Twin plane ∥ _a_, which gives both contact and interpenetrant twins. Cleavage ∥ _m_, fairly distinct. Hardness 5 to 5¹⁄₂; sp. gr. 3·40 to 3·56. Lustre adamantine to resinous. The colour varies very much, doubtless with the content of iron and manganese; it is commonly yellow, green, or brown. Pleochroism is very distinct. The refraction and dispersion are very high, giving the facetted stone a ‘fire’ inferior only to that of diamond. Birefringence positive, strong; the axial angles vary very widely in different specimens. It is fusible with difficulty before the blowpipe. Hot concentrated hydrochloric acid decomposes it partially, with separation of silica; boiling sulphuric acid, or, better, fused potassium hydrogen sulphate, decomposes it completely. On account of the high dispersion and refractive index, clear specimens of sphene make very beautiful gems, but the stone is not sufficiently hard to stand much wear. The mineral was discovered in Chamouni by Pictet in 1787, and was named Pictite by Delamètherie (1797). In 1795 Klaproth analysed a specimen from Passau, and, observing the presence of titanium (which he had just discovered in rutile), proposed the name Titanite. The mineral described by de Saussure (1796) as ‘Schorl rayonnante,’ and afterwards by Hauy (1801) as Sphene (σφήν = a wedge), was shown to be identical in composition with titanite by Cordier, and also by Klaproth (1810); the crystallographic identity was proved by G. Rose (1820).