loB Dr. Brewster on new properties oj heat, 
The radiant heat would find a quicker passage through the 
transparent screen, and therefore, the difference of effect was 
not due to the transmitted heat, but to the heat radiating 
from the anterior surface. The truth contained in M. Dela- 
roche’s fifth Proposition is almost a- demonstration of the 
fallacy of all those that precede it. He found that M a thick 
plate of glass, though as much, or more permeable to light 
than a thin glass of worse quality, allowed a much smaller 
quantity of radiant heat to pass/' If he had employed very thick 
plates of the purest flint glass, or thick masses of fluid that 
have the power of transmitting light copiously, he would have 
found that not a single particle of heat was capable of passing 
directly through transparent media. 
Proposition XLL 
To construct a chromatic thermometer for measuring differences of 
temperature below that of find glass, by the optical effects which 
they produce. 
Differences of temperature have hitherto been measured by 
the expansions and contractions which they produce in solid, 
fluid, or gaseous bodies, and all the various thermometrical 
instruments that have been constructed, differ from each other 
only in the' method by which these mechanical effects are 
rendered visible. The experiments contained in the first 
Section of this Paper, present us with an entirely new princi- 
ple for the construction of a thermometer. We. have there 
seen, that the tints polarised by a plate of glass, increase with 
the temperature by which they are produced, and therefore 
these tints may be used as a measure of the temperature, after 
