Crystals, orthorhombic, holosymmetric; _a_ : _b_ : _c_ = 0·9988 : 1 : 0·8127. Usual forms--the Pinakoids _a_, _b_, and _c_ {100}, {010} and {001}, prisms _m_ {110} and _q_ {130}, domes _u_ {101}, _t_ {301} and _n_ {011}, and some pyramids {hkl}. Angles, _a_ ∧ _m_ = 44° 58´, _u_ ∧ _c_ = 39° 8´, _n_ ∧ _c_ = 39° 6´. The crystals usually occur as short prisms. No cleavage. Optical constants unknown. In flakes the absorption spectrum of didymium can be observed. The mineral is brittle; hardness 5 to 6 on Mohs’ scale; sp. gr. varies a little about 4·9. Fracture splintery; lustre dull, resinous. Colour brown to red and greyish-red, streak greyish-white. The mineral is almost opaque. Cerite is infusible before the blowpipe. It is attacked readily by sulphuric acid, less easily by hydrochloric acid, with which it gives a gelatinous mass. Rammelsberg[16] found that the silica left behind on treatment of the powdered granular variety with the latter acid contained a variable proportion of bases, which he obtained and estimated after fusing the siliceous residue with sodium carbonate. From the different proportions of the earths in the part attacked by the acid and that left in the silica, he remarks, ‘It would almost appear that Cerite is a mixture of silicates which are not all attacked with the same ease by hydrochloric acid.’ Apparently without previous knowledge of this observation, Welsbach[17] noticed the same thing in 1884. He concluded that ordinary granular ‘cerite’ is a mixture of several minerals, among which there are at least two which contain rare earths. Of these, one, the chief constituent of the aggregate, is probably identical with the crystallised mineral, and is characterised by the readiness and completeness with which it is attacked by hydrochloric acid. The other does not react, with hydrochloric, but is readily attacked by sulphuric acid; it contains yttria earths, in addition to the ceria earths. In the extraction of ceria earths from the mineral aggregate, Welsbach used hydrochloric acid, so leaving this second mineral unchanged; but to avoid loss of the rare earths, sulphuric acid is more commonly employed for the decomposition. [16] _Pogg. Ann._, 1859, ~107~, 631. [17] _Monats._, 1884, ~5~, 512.