the Corundum Stone , and its Varieties , &c. 297 
by Sir Charles Oakley himself. The colour of this is a 
blackish brown ; and its exterior appearance is nearly similar 
to that of mica ; but the lustre of its surface is somewhat less 
bright. Its texture is very distinctly laminated ; the laminae, 
which are very thin, being chiefly evident at the edges ; they ad- 
here, however, more strongly to each other than those of mica. 
These laminae may be bent, without breaking ; but they do not 
show the smallest signs of elasticity. This substance possesses 
but a small degree of transparency, and that only when it is 
brought into the state of very thin laminae ; its colour then ap- 
pears a brownish yellow, not much unlike that of resin. It is 
much more greasy to the touch than mica; it is also less 
hard, so that it may be easily scratched with the nail; and, 
if we scratch it with the point of a penknife, we are not sen- 
sible of that kind of slight shivering which takes place when 
mica is so treated. Mr. Greville, in the Paper upon corun- 
dum which he presented to the Royal Society, in June, 1798, 
was perfectly aware of the difference between this substance 
and that properly called mica. In the collection he received 
of the former, are many crystals, several of which are nearly 
an inch in length, and two or three lines in thickness. Some 
of these are in the form of a rhomboidal prism, of 6 o° and 120°; 
others have the form of a regular hexaedral prism. Upon the 
whole, the characters of this substance may be considered as 
partaking both of those belonging to mica and those belonging 
to talc. 
Its mean specific gravity, taken from three trials, which dif- 
fered very little from each other, is 2709.. 
Garnets. In the matrix here spoken of, and also in the corun- 
dum itself, garnets are sometimes met with.; they are of a- deep.- 
