S46 Dr Brewster mi the Absorption of Polarised Light 
through the laminae, in the same manner as if it had been 
transmitted through a bundle of glass plates 
Influence of heat in modifying the absorbent power of crystals. 
‘‘ Having selected several crystals of Brazilian topaz, which 
displayed no change of colour by exposure to polarised light, 
I found, that after bringing them to a red heat, or even boil- 
ing them in olive oil or in mercurj^, they experienced such a 
change in their structure, as to exhibit distinctly the power of 
absorbing polarised light. I next took a topaz, which had 
one of its two pencils yellow, and the other pink. By expo- 
sing it to a red heat, the heat acted more powerfully upon the 
extraordinary than upon the ordinary pencil, discharging the 
yellow colour entirely from the one, and producing but a slight 
change upon the pink colour of the other. 
In the formation of pink topazes by heat, it has always been 
supposed, that the yellow colour is actually changed into pink ; 
but this is quite a mistake, for the pink colour must previously 
exist in a state of combination with the yellow, and must either 
form the colour, or exist in the colour of one of the pencils pro- 
duced by double refraction. The heat does no more than dis- 
charge one colour, and leave the other almost unimpaired. 
This result is of considerable practical importance to the jewel- 
ler, as it enables him to determine beforehand, whether or not 
any topaz will receive a pink colour from heat ; for if this co- 
lour exists in one of its images, which will in general be seen by 
exposing it to a polarised ray, he may predict with certainty the 
success of his experiment. 
When a topaz possessing a pink tint has been taken from 
the firie, it is at first perfectly colourless, and acquires the pink 
colour gradually in the act of cooling. By exposing it repeat- 
edly to the action of a very intense heat, I have never been able 
either to remove or to modify this permanent 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 
• The oxidated surfaces of steel, and other metals, absorb polarised light in a 
manner analogous to crystals, as will be minutely detailed in a subsequent memoir 
nn Metallic Polarisation. 
