the Corundum Stone, and its Varieties, &c. 303 
of an indeterminate shape, which very often are nearly conti- 
guous to each other. It is very rare to find among them any 
crystals perfectly formed ; yet I have sometimes observed oc- 
taedrons, dodecaedrons, and segments of the first of these two 
forms, or octaedrons, which had in each pyramid, and exactly 
opposite, one of the faces much larger than the three others. 
This last form, appeared to me to be the most common one. 
This oxide sometimes exists also in masses of a much larger 
size; but they are almost always of an irregular shape. I 
have often observed pieces as large as a hazel nut ; and some- 
times, though much less frequently, of a still more considerable 
size. 
The mean specific gravity of this oxide of iron, taken from 
four trials, was 5073. This is rather superior to what has been 
considered as the specific gravity of this ore of iron, it having 
been always estimated at less than 5000. I know nothing to 
which I can attribute this difference, except to the peculiar 
texture of the oxide here described; which, as far as I have 
been able to observe, has always appeared to me to be much 
more compact than is usual in this species of iron ore. In other 
respects, it has, when perfectly pure, all the other characters 
belonging to this species. 
There are some pieces of the matrix now treated of, in which 
the small masses of the above oxide, by being mixed with 
fibrolite and mica, exhibit an appearance that might cause them 
to be considered as pieces of a true granite; in others, it is 
mixed, in different proportions, with the substance of the co- 
rundum itself, in such a manner, that it is impossible, by the 
eye, to distinguish this mixture from the pure metallic oxide. 
Mr, Chenevix analyzed one of these pieces; and found that 
Rr s 
