250 Count de Bournon’s Description of 
colour, and was nearly opaque. The heaviest was as high as 
4161 ; this was of a beautiful deep blue colour, and was very 
transparent. Three of the iff were above 4100. 
The inferences which I think myself warranted to draw from 
the results of the above-mentioned trials, are, 
1. That the specific gravity of the imperfect corundum is 
always less considerable than that of the perfect kind. 
2. That this gravity varies according to the degree of per- 
fection of the crystallization ; and, consequently, according as 
the stone is more or less transparent. 
3. That, in general, the corundum of a blue colour, whether 
of the perfect or the imperfect kind, is of a greater specific 
gravity than that of any other colour. 
What is here stated respecting the specific gravity of the 
different kinds of corundum, is exactly analogous to what has 
been already mentioned respecting their various degrees of 
hardness. 
CRYSTALLINE FORMS. 
The primitive form of corundum, whatever may be its degree 
of perfection, is a rhomboid slightly acute ; the obtuse angles of 
the planes measuring 94 0 , and the acute ones 86°. (See Plate 
VI. Fig. 1.) The description of the crystalline forms will be 
more easily and more clearly understood, by considering (as I 
shall constantly do in what follows) this rhomboid as being 
formed by the union of two triedral pyramids, united at their 
bases ; the solid angle of the summit will then be formed by 
the meeting of three of the more acute angles ; and its measure, 
taken upon one of its edges, and in the middle of the opposite 
face, will be very nearly 95 0 30'. 
