Measurement and Cleavage of certain Primitive Crystals. 257 



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 

 free 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 Hauy. 



A prismatic crystal from Sicily, having the primitive planes 

 brilliant at one end, gave the incidence of 104°. and planes ob- 

 tained 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 

 Hauy, a rectangular octohedron, measuring one way over the sum- 

 mit 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 laminae, 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. Hauy considers its primitive form to be a rhomboidal dodeca- 



