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.[1]
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
- ↑ Many other substances also possessing the characters of brittleness and softness at the same time, may be likewise split while held in the hand, with the greatest success. Other substances yield best to the same mode, for other reasons. Blende is one of these. It may be cleaved in so many directions, that if attempted to be split by means of a blow on the back of a knife whose edge is placed parallel with the natural joints, it is most probable that a fracture will ensue, which, though in the direction of the laminæ, is not in the desired direction. A specimen of no particular external form, but internally laminated with great regularity, and about an inch and a half square, and half an inch thick, lately yielded me, I believe, all the forms into which blende can be cleaved, and even duplicates of them. Haüy considers its primitive form to be s rhomboidal dodecahedron, its substractive crystal an obtuse rhomboid, and its integrand molecule au irregular tetrahedron. I obtained solids not only in these forms, but also others in the form of an octahedron of 90° over the summit, and of a plane of one pyramid on the adjacent plane of the other, and of 120° of one plane of either pyramid, on the adjacent plane of the same pyramid; I procured also others in the form of an acute rhomboid of 60° and 120° These measurements were obtained by means of the reflecting goniometer, which also gave there of the obtuse rhomboid 60° and 120°, which by Haüy, are said to be 70°, 31′. 44″ and 109°. 28′. 16″. Hence blende may be split into five different solids.
