by doubly refracting crystals. 
*5 
Sect. III. On the 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 mercury, 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 ex- 
posing 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 yel- 
low, and must either form the colour or exist in the colour 
of one of the pencils produced by double refraction. The 
heat does no more than discharge one colour, and leave the 
other almost unimpaired. This result is of considerable 
practical importance to the jeweller, as it enables him to 
determine before hand, whether or not any topaz will receive 
a pink colour from heat ; for if this colour 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 
MDCCCXIX. E 
