and physical properties of tabasheer. 293 
black tint, we shall find that it has often a fine ash grey 
colour, which becomes deeply black when wetted, and after- 
wards resumes its primitive tint when dry. 
As the blackness communicated to the tabasheer is not 
produced by heat alone, or by any particular method of 
cooling, it cannot arise from any mechanical change, similar 
to that which Thenard observed in the cooling of melted 
phosphorus ; and there is reason to believe that it does not 
arise from the absorption of any sooty matter thrown off dur- 
ing the combustion of the paper, as the blackened tabasheer 
almost always recovers a certain degree of whiteness, and 
imbibes water almost as freely as in its original state. With 
the view of discovering if the blackness was owing to any 
vegetable matter in the tabasheer, I repeated the operation 
of blackening it and restoring its colour by heat about 50 
times ; but after all these operations, it became black as 
readily as at first. The specimen that had undergone these 
changes had increased in hardness and lustre, and had the 
appearance of the finest Indian ink. Upon breaking it in 
two, the fracture was perfectly black, but assumed a dark 
blue colour by exposure to the air ; and upon putting a drop 
of pure water upon the blue fracture, it was instantly con- 
verted into a deep and glossy black. 
When the pure tabasheer was exposed to a white heat 
for several hours, and was then burned in paper, it received 
its black colour as before. When it was held in the flame 
of alcohol or of carburetted hydrogen, it was merely stained 
in a slight degree, which was probably owing to the inten- 
sity of the heat, which may have discharged the black tint as 
soon as it was formed. 
