248 Count de Bournon's Description of 
to all the other varieties of the latter kind. In the perfect corun- 
dum of other colours, the hardness is nearly the same as in the 
red ; that which is of a blue colour, or sapphire, and only that, 
rather exceeds the others in hardness. We have just seen, that 
in the imperfect corundum also, the blue colour was accompa- 
nied by a degree of hardness greater than that of the other 
colours. 
This substance emits pretty bright sparks, when struck with 
a piece of steel ; but they are by no means proportioned to its 
hardness. If a piece of flint be struck with the same force, the 
sparks it produces are more numerous, as well as more bright ; 
and it is possible to obtain sparks from flint, by a very slight 
blow’, such as would not be sufficient to produce them from 
perfect corundum. It is also necessary, in order to obtain sparks 
from corundum, that the stone should have pretty sharp edges : 
if the part that is struck is obtuse,, it is with some difficulty that 
any sparks can be obtained. The imperfect corundum, however* 
has, in this respect, some advantage over the perfect kind, 
PHOSPHORESCENCE. 
The substance here treated of becomes, like quartz, phos- 
phorescent by collision ; it requires only, in order to exhibit this 
property, a somewhat stronger degree of friction. The light 
which it emits has also less intensity ; and does not appear to be 
accompanied by the smell which is peculiar to that obtained 
from quartz. A very remarkable circumstance may likewise be 
observed respecting this light. In all the varieties of this stone 
which are of a red colour, whether of the imperfect or of the 
perfect kind, or oriental ruby, the light here spoken of is of a 
very deep fire colour, similar to that of red hot iron, when 
