284 Dr. Brewster on the optical 
About two years ago my friend, Dr. Kennedy, received 
from India, a considerable quantity of tabasheer, a portion of 
which he presented to the Royal Society of Edinburgh. From 
this portion I took two or three fragments, with the view of as- 
certaining if it possessed any crystalline structure, but having 
found that it had no particular action upon polarised light, I 
was not led to any farther examination of its physical pro- 
perties. In the course, however, of a series of experiments 
upon the phosphorescence of minerals, I was surprised to 
observe, that the tabasheer emitted a light when placed upon 
a hot iron, more intense than most of the phosphorescent 
minerals. This unexpected property induced me to resume 
the investigation, and having received, through the kindness 
of Dr. Kennedy, an additional supply of tabasheer, I was 
enabled to examine it with peculiar care in all its optical and 
physical relations. 
Among the pieces of tabasheer which I have examined, 
there appear to be three different kinds. The first has a 
milky transparency, transmitting a yellowish, and reflecting a 
bluish white light. It is easily broken between the fingers, 
and has a sort of aerial and unsubstantial texture, entirely 
different from any other solid substance. The second kind 
is more opaque, and harder than the first, having a slight 
degree of translucency at the edges ; and the third kind is 
perfectly opaque, resembling a piece of stucco or chalk, or 
the opaque subsulphate of alumine. 
If we form two parallel faces upon a piece of transparent 
tabasheer, by grinding it on a plate of smooth but unpolished 
glass, we shall be able to see objects through it with perfect 
distinctness, although no polish is induced upon the surfaces. 
