9,6 Dr. Brewster on the absorption of polarised light 
from the fire, it is at first perfectly colourless, and acquires 
the pink colour gradually in the act of cooling. By ex- 
posing it repeatedly to the action of a very intense heat, I 
have never been able either to remove or to modify this per- 
manent tint. 
With the view of ascertaining if the absorbing structure 
could be induced by heat, I exposed to a white heat several 
crystals of yellowish calcareous spar. After the action of 
the fire had been continued for some time, a sort of opales- 
cence, or milky opacity, was induced ; and the light, which 
went to the formation of the ordinary image, was much 
redder than that which formed the extraordinary ray. This 
effect I naturally ascribed to some change in the state of the 
carbonic acid ; and upon continuing the action of the heat, 
and watching the process of decomposition, I found that 
when the carbonic acid was expelled from a film about the 
200th part of an inch thick, its surface was covered with 
vesicles arranged in straight lines parallel to the short diagonal 
of the rhomboid. These vesicles had, in general, an ellip- 
tical form, with a cut or opening in the direction of their 
transverse axis through which the gas had escaped. After 
the spar was taken from the fire, a great number of the 
vesicles burst with a noise similar to that which accompanies 
the bursting of the indusium of the fern, and carried off a 
portion of the thin calcareous pellicle. When this pellicle 
was removed, the subjacent surface was covered with a series 
of minute parallel grooves inclined about 20° gf to the 
short diagonal. In repeating this experiment, and seizing 
the proper time for withdrawing the spar from the fire, I 
