prism, but have in vain attempted it in a direction perpendicular to its planes.
In pursuing this subject, it occurred to me that the exposure of the crystals of this substance to the action of heat, might possibly lead to some further discoveries. Accordingly, some were placed in the centre of a common fire during an hour or two, and being afterwards left to cool I found that a slight touch with a hammer immediately reduced them into small pieces: a research among these afforded very many of the above cited cleavages, which I had previously obtained from crystals that had not been subjected to the action of heat.
Let fig. 1. Pl. 15. represent the cleavages, which are easily obtained parallel with the faces of the prism, and fig. 2 its diagonal cleavages. By a combination of all these in fig. 3, it will be seen that the prism is divisible into right-angled triangular prisms, of which I have numerous instances.
In pursuing a research among the fractures, I found several quadrangular prisms with oblique terminal faces, parallel with each other, as represented by fig. 4, and others similar to fig. 5; which it will be obvious differ only from each other in these respects, that the edges f g and b c are replaced by the planes a and b, and that the two other edges, a d and e b, are also replaced by similar planes, all which planes are parallel with one or other of the diagonals of fig. 4.
I have other fractures described by fig. 7, which are the result of a mechanical division of fig. 4 in the direction of its diagonal a b and c d, and along the edges b c and a d. It follows that fig. 7 is a right-angled triangular prism with oblique terminal faces, which in some of these fragments are perfectly brilliant.
If also a section of fig. 4 be made in the direction of its other