the Corundum Stone , and its Varieties, &c. 283 
crystals confusedly aggregated. In this matrix, the crystals of 
imperfect corundum are dispersed, in the same manner as 
those of felspar are dispersed in porphyry, or rather in certain 
granites which, besides the aggregated constituent parts be- 
longing to that kind of rock, also contain crystals of felspar 
which are of a more or less considerable size, and of a perfectly 
determined form. 
When this substance is of that texture in which the grains 
are closely connected together, it is of a pearly gray colour, 
sometimes slightly tinged with green, and has a degree of semi- 
transparency, not unlike that of calcedony. If a piece of this 
kind is moved about in a strong light, its surface shows a con- 
siderable number of small brilliant particles. This appearance 
arises from the reflection of the light, by the small laminae that 
are exposed, in consequence of the fracture of the grains of 
which the stone consists ; and this circumstance proves that it 
has a laminated texture. 
In the last mentioned state, (the most perfect one in which I 
have observed this stone,) its hardness, although sufficient to 
scratch glass very easily, is rather inferior to that of felspar. 
It gives sparks when struck with steel ; and, by means of strong 
collision, emits a phosphorescent light, of a bluish white colour. 
Friction does not produce any signs of electricity. When put 
into nitric acid, no effervescence was perceptible. 
The specific gravity of this stone, as determined by four trials, 
which scarcely differed from each other, was 274,2 ; but it is 
difficult to procure pieces of a tolerable size, which are not 
mixed, either with hornblende, or with particles of corundum. 
It is fusible by means of the blowpipe. 
This substance is more usually met with in pieces of a coarser 
