the Corundum Stone, and its Varieties, & c. 245 
surface always rough, and being usually more or less impreg- 
nated with fine particles of the various substances which compose 
their matrix, very seldom possess any degree of transparency ; 
but, when these crystals are broken, their fragments generally 
have a degree of semi-transparency, but most commonly a very 
slight one, unless the fragments happen to be very thin ; even 
then, I have never found them perfectly transparent. 
If such of these fragments as have the greatest degree of 
semi-transparency, are held between the eye and the light, there 
may be observed, within their substance, a great number of lines 
or fissures, which cross each other, and prevent the free passage 
of the light, the greater part of which is reflected. These fis- 
sures, which arise from there not being a complete adherence 
between all the parts of the crystalline laminae, are the principal 
cause of the slight degree of transparency commonly met with 
in the kind of corundum here spoken of ; which kind may truly 
be said not to have attained, in its crystallization, all the perfec- 
tion it is capable of acquiring, and which may be observed in the 
perfect corundum of Ceylon. 
I think it also right to observe, that the corundum of the 
Carnatic, when of a red or a blue colour, has always a greater 
degree of transparency, and is more pure, than that which is of 
any other colour ; and, in these respects, the corundum of a 
blue colour is much superior to that which is red. 
In the imperfect corundum of China and of Malabar, although 
the surface of the crystals is also generally rough, yet, as they 
are less impregnated with foreign substances, it is not uncom- 
mon to observe in them a greater or less degree of transparency 
at their edges. Some crystals have, indeed, been sent to us from 
China, (very small ones, I confess, but very perfect,) which 
mdcccii. K k 
