7 6 -Dr. Brewster on new properties of heat, 
Proposition XX. 
When heat is propagated from Izvo different sources , in contact 
with the opposite edges of a plate of glass , the different Sets of 
fringes preserve the same character , the only effect of the addi- 
tional heat being to polarise higher tints in the different sets of 
fringes. 
I placed 12 plates of window glass upon a hot iron, and 
when the different sets of fringes were distinctly visible, I held 
another bar of hot iron in contact with their upper edges, and 
observed higher tints polarised in all the four sets of fringes. 
Many of the plates, however, burst with great violence, so 
that I could not perceive the phenomena that took place when 
the diffusion of the heat became more uniform. 
Proposition XXI. 
TVhen heat is propagated through calcareous spar , rock crystal , 
topaz , beryl , the agate and other minerals that have the pro- 
perty of double refraction, no optical change is produced in their 
structure. 
The greatest heat which I could conveniently apply to dou- 
bly refracting crystals, produced no change whatever in their 
action upon light, whether the heat was propagated in the 
direction of their neutral, or of their depolarising axes. These 
crystals appear to be in the state of steel bars saturated with 
magnetism, which cannot acquire any additional impregnation. 
Being already in a state of perfect crystallization, they are not 
capable of receiving from heat any addition to their crystalline 
structure. 
