the Corundum Stone, and its Varieties, &c. 261. 
from those belonging to the primitive rhomboid : first, by their 
inclination, which is very different, as they make, at their meet- 
ing with the edges of the prism, an angle of 110 0 , whereas the 
others make an angle of 14,7° 30'. Secondly, they are usually 
very deeply striated; a circumstance which rarely occurs in the 
others. Of the two crystals I have just described, one is nine 
lines in diameter, and six lines in height; it is also slightly 
transparent at the edges. The other is much smaller, more 
transparent, and of a purplish red colour, but rather pale. It is 
one of the purest specimens of imperfect corundum, particularly 
of that from the Carnatic, I have ever seen. 
There are frequently observed, in the small prisms of imper- 
fect corundum, some traces of the planes above described ; they 
may in general be easily known by their striae. I have also 
seen crystals in which were united, at the same time, traces of 
the two secondary rhomboids of the fourth and fifth modifica- 
tions, in the manner represented in Fig. 30. 
Sixth Modification. There also appears to exist, in this sub- 
stance, a third rhomboid, which is much more obtuse than 
either of the two preceding ones; at least it is only. to such a 
modification that I can refer several crystals, both prismatic and 
pyramidal, of imperfect corundum, which made part of a 
parcel lately sent to Mr. Greville, from the district of Ellore, 
in the northern part of the government of Madras. Among 
these crystals are many hexaedral prisms, of a perfectly regular 
form, which have their terminal faces inclined in a contrary 
direction, so as always to make, upon the edges of the prism on 
which they incline, angles of ioo° and 8o°. (Fig. 31. Plate VIII.) 
These terminal faces appear to me to belong to a very obtuse 
rhomboid, of which, the acute angles of the rhombic planes would 
MDcccii. M m 
