Corundum Stone from Asia. 441 
rhomboid ; and he seems to attribute the same formation to the 
pyramidal variety with double pyramid, which he supposes may 
exist. 
The primitive crystals, and the first and second modifications 
of Corundum, are from the Peninsula of India. The third 
modification, or the pyramidal variety, is from China ; nothing 
approaching this form being among the specimens which Mr. 
Greville received from the Peninsula of India. 
The preceding observations, and particularly the last men- 
tioned modification of Corundum, compared with the best de- 
scriptions of the sapphire, suggest the further examination of 
the degree of connection, if not of identity, of these Oriental 
stones. 
In both, the hexaedral pyramids are usually incomplete in 
their apex, and they vary in acuteness. I have stated the de- 
gree in which the solid angles of the pyramid (taken as com- 
plete) vary, in Corundum, to be from 20° to 40°. 
Rome' de l’Isle states, that the sapphire varies from 20’ 
to 30°. The Abb6 Hauy ( Journal de Physique, Aug. 1793,) 
mentions two varieties of the sapphire, one measuring at the so- 
lid angle of the pyramid 40° 6 ', the other 57 0 24'. I never saw 
a sapphire with so obtuse an angle as the last; but many, 
whose angle at the top, if the pyramid had been complete, 
would have been the same as that of the Corundum. Besides 
the analogy between the crystals of Corundum and the sap- 
phire, by the union of two hexaedral pyramids at their base, it 
also exists by the measure of their angles ; and both substances 
are subject to the same irregularity, sometimes appearing as 
a single hexaedral pyramid, and sometimes as an hexaedral 
mdccxcviii. 3 L 
