Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/268

 other way (M on the opposed plane) 62°. 56′. The Count de Bournon however considers the primitive crystal to be a rectangular tetrahedral prism with square bases. This determination arose on his part from having cleaved in directions parallel with the planes of that solid, some crystals in the form of square laminæ, which are often extremely thin, and which always yielded in those directions. These crystals, he adds, are found in Derbyshire, in the Bannat, and in Siberia.

From an amorphous specimen of this substance I succeeded in extracting a solid in the form of an octahedron almost entire, and having more or less of every plane brilliant enough for the use of the reflecting goniometer. The results of measurements taken in the same directions as those given by Haüy were, for the first 71°. 40′, being 1°. 10′ more; and for the second, 62°. 42′, being 14′ less; and not only did this fragment yield each of these measurements twice, but also the complementary numbers of 108°. 20′ (P on P) and 117° 18′. (M on M), each also twice. Several other fragments gave perfectly coinciding results.

This substance may be split with about equal ease by the assistance of a sharp knife, whether it be held in the hand or placed on a table; provided the blade of the knife be carefully placed in the direction of the laminæ, and the back gently struck by a light