360 Professor Forbes'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 

 from a 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 less 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 circum- 

 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 David Brewster has found that light of less refrangibi- 



