2g2 Dr. Brewster on the optical 
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 tabasheer 
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 maxi- 
mum 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 it, however, in a piece of paper, and set the paper 
on fire, the tabasheer becomes either black, or brownish black, 
and the black colour increases in depth by the repetition of 
the experiment. When immersed in water, it disengages 
the included air, but with less rapidity than before; and when 
it is broken and pounded, its fracture and its powder are 
black. 
If the blackened tabasheer is brought to a red heat, it is 
restored 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 of water it becomes black ; and with a still 
greater portion, it becomes again transparent. If we break 
a piece of tabasheer a few days after it has received a deep 
