[49] This isomorphous series has recently been extended by Zambonini, and also by Schaller, by the inclusion of minerals containing Columbium and Tantalum; see under Ilmenorutile and Strüverite, end of Ch. IV., p. 71. It will be seen that if the numbers for cassiterite and rutile be doubled, four out of the five show very fair approximation to the constant value 40. The number 60 for thorite is quite irreconcilable with the values obtained from the other members; of course pure silicate of thorium, ThSiO₄, is not known as a mineral, but it is most unlikely that the relatively small amount of impurity in the densest specimens of orangite should have depressed the specific gravity by over two units, as would be required if the molecular volume of thorite were to show even the most approximate semblance of agreement with the others. It cannot be too often remarked, however, that very little indeed is known of the molecular formulas of minerals, and that very little reliance can be placed on such figures as the above. On the contrary, it is hardly conceivable that amphoteric oxides like those of tin and titanium, occurring in the form of heavy crystalline minerals, should have molecular formulæ only double the empirical formulæ. Where agreements of the kind do occur, they must be taken as indicating approximately equal degrees of molecular complexity in the minerals concerned, rather than as affording any real insight into the molecular condition. ~Zircon.~--Zircon is a silicate of zirconium, ZrSiO₄, with small quantities of other elements. Most varieties contain ferric oxide and thoria; more rarely small proportions of the yttria earths may be present. All varieties contain traces of a large number of the common metals. Traces of radium are usually present, with helium and neon,[50] and the mineral is strongly radioactive. [50] Strutt, _Nature_, 1906, 102. System tetragonal, holosymmetric sub-class. _c_ = 0·6404; (001) ∧ (101) = 32° 38´. Usual forms--Prisms _a_ {100} and _m_ {110}; pyramids _e_ {101}, _p_ {111}, _u_ {221} and _x_ {311}, etc. The basal pinakoid _c_ {001} is rare. The usual combination is one or both of the prisms _a_, _m_, with one or two pyramids. Twinning is rare, the twin plane being _e_ (101), giving knee-shaped twins similar to those so characteristic of cassiterite and rutile. Cleavage ∥ _m_ imperfect, ∥ _p_ bad. Brittle; conchoidal fracture. Hardness 7¹⁄₂; sp. gr. usually 4·68-4·70, but varying from 4·2 to 4·86. Adamantine lustre. Clear and colourless to yellow-, red- or greenish-brown. Transparent to opaque. Refraction and double refraction strong, double refraction positive (ω = 1·924, ε = 1·968, for sodium light); on heating it becomes biaxial, and occasionally is found biaxial in nature. By alteration it becomes isotropic.