100 Dr Brewster wi the General Properties of Tahaslieer, 
By the application of a microscope, I observed the air form it- 
self into globules in the interior of the tabasheer, which slowly 
advanced to the edge of it, and at last escaped into the oil 
After a lapse of nearly two hours, the greater part of the air 
was extricated, and the expulsion of the remainder was quickly 
effected by a gentle heat ; but the transparency was of that 
imperfect kind, which results from the union of two bodies of 
different refractive powers. By increasing the heat, the taba- 
slieer became more transparent, and at a certain temperature it 
could scarcely be seen in the oil in, which it was placed. When 
the heat was still farther augmented, it became more and more 
opaque, and a corresponding opacity was induced by cooling it 
down as much below the temperature of maximum transparency. 
When pure tabasheer is boiled for any length of time, or is 
brought to a red or a white heat, it suffers no change either in 
its colour or in its optical and physical properties : if we wrap ity 
however, in a piece of paper, and set the paper on lire, the taba- 
sheer becomes either black, or brownish-black, and the black 
colour increases in depth by the repetition of the experiment. 
If the blackened tabasheei* is brought to a red heat, it is re- 
stored to its primitive whiteness, and resumes all its former 
properties ; but if the heat is considerably below redness, some 
specimens acquire a slight transparency, and a dark slaty blue 
colour, shading in some places into whiteness. When slightly 
wetted in this state, it becomes chalky white ; with a greater 
•portion qf'^ water it becomes black; and with a still greater 
portkm it becomes again transparent. If we break a piece of 
tabasheer a few days after it has received a deep black tint, we 
shall find that it has often a fine ash-grey colour, which be- 
comes deeply black when wetted, and afterwards resumes its 
primitive tint when dry. 
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 qf pure water upon the blue 
