Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/267

 time the nail of the right thumb in the opposite direction. But if the blade of the knife be not held perfectly level with the direction of the natural joints, it is apt to injure the brilliancy of the plane it produces. The terminations of the crystals being in general most tree from crevices, and most transparent, are therefore best adapted to the purpose I have been describing.

Five very small fragments procured in this manner, yielded by the reflecting goniometer, co-incidences on the obtuse angle of 104°, and one of 76° on the acute angle of the prism; the one being 48 less, the other 48′ more than the measurements obtained by Haüy. A prismatic crystal from Sicily, having the primitive planes brilliant at one end, gave the incidence of 104°. and planes obtained by fracture at the other end, gave the same result.

Carbonate of Lead.

Fig. 8.

The primitive crystal of the carbonate of lead is, according to Haüy, a rectangular octahedron, measuring one way over the summit of the same pyramid (P on the opposed plane) 70°. 30′; the