MR. TALBOT ON THE OPTICAL PHENOMENA OF CRYSTALS. 
35 
When nitre is made to crystallize in gum, it often shoots into prismatic crystals, 
which are very interesting objects, the more so, that they are of a permanent nature, 
and not liable to spontaneous change. When examined by polarized light, these 
prisms, in one position of their axis, frequently disappear completely. This arises 
from the refractive power of the gum being equal to that of one of the two rays in 
the crystal. Reverse the polarization of the ray, and the crystal appears, as it were, 
to start into existence, acquiring great strength and blackness of outline, and, not 
unfrequently, entire opacity. Again, when the sulphate of lime is interposed, this 
opacity disappears, and the crystal becomes brightly coloured. 
Since it is probable that many better methods may be found of obtaining this kind 
of crystals than have hitherto presented themselves, I have hopes that it will be pos- 
sible to obtain large and permanent artificial crystals, which may possess the advan- 
tages of the tourmaline without the inconvenience resulting from its dark colour. 
