(iv.) In about 9 per cent. of the cases, the thoria present was insufficient to combine with the silicate silica, from which it follows that some foreign silicate must be at least occasionally present. (v.) A careful microscopic examination showed conclusively that no thorite (ThSiO₄) was present, the silicate being biaxial; quartz is present as such. [111] _J. Amer. Chem. Soc._ 1909, ~31~, 640. They conclude that thorium is present as phosphate, and is an essential constituent, but that there is always some admixed silicate, most probably a felspar. ~Xenotime.~--Chemically this mineral is closely allied to monazite, being an orthophosphate of rare earths, containing silica and thoria; whereas, however, in monazite the content of yttria earths does not rise above 4 per cent., in xenotime these constitute by far the greater part of the bases, the content of ceria earths ranging from 8·2 to 11 per cent. The yttria earths, chiefly oxides of yttrium and the erbium group, vary from 54·1 to 64·7 per cent. There are traces of zirconia; Ramsay, Collie and Travers detected helium, whilst Boltwood, and also Strutt, found uranium and radium. It also appears to contain traces of sulphuric anhydride. The crystals are tetragonal, holosymmetric. _c_ = 0·6187; (001) ∧ (101) = 31° 45´. Common forms are the prisms _a_ {100} and _m_ {110}, the basal pinakoid _c_ {001}, the pyramids _e_ {101}, _f_ {201}, _z_ {111}, etc. Cleavage ∥ _m_, perfect. Uniaxial, double refraction strong, positive. Transparent to opaque. Colour, brown to reddish-brown and yellow. Hardness 4-5; sp. gr. 4·45-4·56. It is insoluble in acids, and infusible before the blowpipe; when moistened with sulphuric acid, however, it turns the flame bluish-green, like most mineral phosphates (_vide_ monazite). It is not so widely distributed as monazite, but is not uncommon. It often occurs with zircon--to which it is very closely allied in crystal form, if the two are not actually isomorphous--in parallel growth, in granitic rocks. The diamond sands of Diamantina, Brazil, form the richest source of the mineral, but it is also found in Scandinavia, at Hitterö, Åro, etc. The mineral is of considerable importance, chemically, on account of the high percentage of erbia earths.