A5 2 Count de Bournon's Description of 
whole. I have likewise observed the two last, among some small 
crystals of imperfect corundum from China, which were pretty 
transparent. 
Second Modification. At other times, the edges of the base of 
the primitive rhomboid are each of them replaced by a single 
plane, which is parallel to the axis, and which, when its extent 
is rather considerable, separates the two pyramids by a hexae- 
dral prism with rhombic planes. I have never seen this modifi- 
cation with complete pyramids, as it is represented in Fig. 5, 
but I have often observed it combined with the preceding 
modification. This combination is not unfrequently met with 
in the oriental ruby, in which, the two varieties represented in 
Figs. 3 and 4 are found with a small beginning of a prism, &s 
is shewn in Figs. 6 and 7. There are also in the collection of 
Sir John St. Aubyn, two crystals of sapphire, belonging to the 
same variety, one of which is tolerably regular in its form ; but 
it is much more common to find these crystals with prisms 
of rather greater length, as is represented in Figs. 8 and 9. In 
Mr. Greville's collection also, there is contained a crystal of 
a pretty large size, and very perfect, in which the plane that . 
has replaced the solid angle of the summit of the pyramid is 
very small, as in Fig. 10. All these varieties, but particularly 
those represented in Figs. 8 and 9, are likewise found among 
the small transparent crystals of imperfect corundum brought 
from China. 
When the decrease produced by the plane which has replaced 
the solid angle of the summit of the rhomboid, has begun to 
take place nearly at the same time with, or even previous 
to, that which gives rise to the planes which replace the edges 
of the base, (as is indicated by the length of the sides of the 
