the Corundum Stone, and its Varieties, &c. 291 
however add, that among the pieces of this substance, I have 
met with several, which appeared to have more or less tendency 
to the above-mentioned form. 
The analysis of this substance, made by Mr. Chenevix, 
concurs with the whole of its external characters, in warranting 
us to consider it as being different from any of the mineral 
substances hitherto known ; in consequence of which, I have 
thought it right to distinguish it by the name of Jibrolite . 
Thallite, The substance called thallite (the epidote of the 
Abbe Hauy) also sometimes accompanies the corundum from 
the Carnatic, in its matrix. This substance is found in three 
distinct states, hitherto unobserved, in all of which its appearance 
is so different from its usual one, as to have prevented me, for 
some time, from knowing it. 
In one of the above states, this substance is inclosed in the 
matrix, in small detached masses, from the size of a pea to that 
of a hazle nut, and even larger. Its usual colour is either a 
brownish green or a yellowish green ; and it has only a slight 
degree of semi-transparency, even at the edges. 
Its hardness is the same as that of the other known kinds of 
thallite, which I have always found to be rather superior to that 
of quartz ; and, as most of the other characters belonging to this 
kind of thallite are similar to those of the kinds already known, 
I shall, in the following description, mention only such of its 
characters as, on account of their being different, -might lead to 
false ideas respecting it. 
The major part of these small masses present no determined, 
form ; in some of them, however, a perfectly regular crystalli- 
zation may be observed. In this latter state, the greater number 
of crystals appear in the form of rhomboids! tetraedral prisms, 
of 128° and £1° 30', in which the terminal faces are perpen- 
