360 Professor Forses’s Researches on Heat. 
former and the pile. The quantities of heat reaching the pile 
were thus almost equalized, and the result was, that the heat 
froma dark source, after transmission through glass, became as 
polarizable as that from incandescent platinum, although. before 
NINE per cent. less of it was stopped. 2. If heat from boiling 
water or hot mercury be not really /ess polarizable: than that 
from luminous bodies, it must appear to be so in consequence of 
the surface being larger, or closer to the pile, and therefore seen 
at the pile under a greater angle. To shew that this effect, if it do 
exist, is at least insignificant in relation to the effect due to the 
variable nature of the heat, I placed the brass heated to 700° at 
12 inches from the pile, and caused its rays to be sifted by a 
plate of glass. I found that 73 per cent. of this heat was pola- 
rized by the mica plates I and K. I then removed the glass 
plate, and withdrew the heated brass from the pile, until the 
impression on the pile was nearly the same as before. This was 
at a distance of 26 inches, instead of 12. The source of heat 
was therefore seen under a much smaller angle than before. 
But instead of the polarization being augmented by this cireum- 
stance, the change in the quality of the heat by the removal of 
the interposed glass reduced it to 64 per cent.—an effect which 
must have been owing to that cause, and to that alone. Ano- 
ther experiment similarly conducted with the mica plates G and 
H gave 73 per cent. of heat sifted by glass polarized at a dis- 
tance of 12 inches, and only 68 per cent. of heat from brass at 
700° in its simple state, at a distance of 27 inches. In all these 
experiments it is clear that the result is true, independently of 
any reduction for the degrees of the galvanometer, since in each 
set the deviation is made the same. 
24. The general fact that heat from sources of higher tempe- 
rature is more polarizable by refraction, agrees with the corres- 
ponding case of light. Heat of low temperature is least refrangible, 
and Sir Davip Brewster has found that light of less refrangibi- 
