the Corundum Stone , and its Varieties, &c. 249 
heated to the degree known by the term cherry red. The sparks 
which are obtained from this stone by means of a piece of steel, 
have also some appearance of the above colour. These phe- 
nomena may perhaps serve to assist us in acquiring further 
knowledge respecting the cause of the phosphorescence of stones, 
of which we have hitherto had no very satisfactory explanation. 
GRAVITY. 
The specific gravity of corundum, in its different varieties, 
presents a series of interesting facts, particularly when they are 
compared with what has been already observed with respect to 
its different degrees of hardness. The great interest I have felt 
in the study of this substance, has caused me to take particular 
care in the examination of such of its properties as might lead 
to a perfect knowledge of it. I will now state the results of 
the observations with which the character now treated of has 
furnished me. 
1 
Of 33 specimens of the different varieties of imperfect corun- 
dum, the mean specific gravity was 3931. The lightest was 
3875; and the heaviest 3981. Six of the 33 were above 3900. 
Eleven were between 3900 and 3931 ; and the remaining sixteen 
were above 3931, which, as I have already stated, was the mean 
proportion. 
The mean specific gravity of the perfect red corundum, as 
determined by 20 specimens of oriental ruby, was 3977. The 
lightest of these was 3933. Five of the specimens were above 
4000. One alone was as high as 4087 ; it was of a deep red 
colour, was perfectly transparent, and had been cut. 
Sixteen different specimens of sapphire, gave a mean specific 
gravity of 4016. The lightest was 3907; it had scarcely any 
