the Corundum Stone , and its Varieties , &c. 257 
has been hitherto found, which form part of Mr. Greville's 
collection, and are sufficiently perfect to admit of being mea- 
sured with accuracy, there is one, of which the solid angle at the 
summit, taken in the middle of two of the opposite pyramidal 
faces, is 50° ; two of 40° ; two of 35 0 ; nine of 24 0 ; and seven 
of 12 0 . Among the pyramidal crystals of oriental ruby are, one 
of^o 0 ; one of 40°; four of 30°; one of 24 0 ; and four of 12 0 . 
In the sapphire there are, one of 50°; two of 40°; one of 35 0 ; 
two of 30°; one of 24 0 ; and two of 12 0 . If to these measures we 
add those of two sapphires, and of two oriental rubies, in the col- 
lection of Sir John St. Aubyn, we shall also have 58° and 20° ; 
and we may consequently state, from our present knowledge 
respecting this substance, that it admits no less than eight dif- 
ferent decrements of the laminae, at the same angle of the base ; 
each of which produces a pyramidal modification. And the 
measure of the solid angle of their summits, (considering the 
pyramids as complete, and supposing at the same time that 
the very great care I have taken has prevented me from com- 
mitting any error,) are 58°, 50°, 40°, 35 0 , 30°, 24 0 , 20°, and 
12 ° * 
This difference in the inclination of the faces of the pyramids, 
in the corundum of a pyramidal form, often appears in a very 
striking manner in the same crystal, I have frequently met 
'With oriental rubies, and also with sapphires, in which the faces 
of the pyramids, after having for some time preserved a certain 
degree of inclination, evidently appeared to have changed it, in 
* Ifl Figs. 1 8 A, 18B, and 18C, are represented this simple pyramidal modification, 
having 58°, 35°, and 12 0 , for the measures of the solid angle of the summit of the 
pyramid j from these figures, it will be easy to form an idea of the appearance of those 
grystals which have the other measures above enumerated. 
