437 
Corundum Stone from Asia. 
of fig. 13. in the line ADB. These strise are not to be con- 
founded with those in numberless substances, as in tourmalines, 
schorls, &c. which arise from the longitudinal union of num- 
berless distinct crystals. The crystal resulting from this new 
mode of decrease in the crystalline laminae, will represent one 
or other of the varieties shewn in fig. 11, 12, and 13, according 
to the period when such decrease has begun in the process of 
the crystallization ; and, if it has begun very late, the new 
faces will only be small, nay almost imperceptible, isosceles 
triangles, forming solid angles of ibo° 42', with the planes of 
the extremities of the prism, as in fig. 5. ; the measure of the 
angles however must be excepted. 
If this irregular mode of decrease had begun with the first 
crystalline laminae which were deposited on the primitive 
rhomboidal parallelopiped, the hexaedral prism resulting there- 
from would have been terminated by two very obtuse triedral 
pyramids, whose planes would have been rhombs ; and they 
would have been placed in a contrary direction to each other, 
as may be seen in fig. 12, by the dotted lines. I have not met 
with this variety, but its existence may be supposed. 
It happens sometimes, that the crystallization has not been 
so perfect as to destroy every appearance of the faces of the 
primitive rhomboidal parallelopiped; in this case, there remains, 
on the solid angle of 112 0 , formed by the junction of the new 
faces with the edges of the prism, a small isosceles triangle, 
as in fig. 13, which corresponds to those in fig. 5. of the pre- 
ceding modification. 
The crystals which explained the second modification, form 
also a part of Mr. Greville's collection : one, in particular, is 
highly worthy of notice ; it is the most perfect crystal I have 
