552 Prof. Forbes's Researches on Heat, Second Series. 



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 show that this effect, if it do exist, is at least in- 

 significant 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 polarized 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 di- 

 stance 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. 

 Another experiment similarly conducted with the mica plates 

 G and H gave 73 per cent, of heat sifted by glass polarized 

 at a distance 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, in- 

 dependently of any reduction for the degrees of the galvano- 

 meter, since in each set the deviation is made the same. 



The general fact that heat from sources of higher tempera- 

 ture is more polarizable by refraction, agrees with the cor- 

 responding case of light. Heat of low temperature is least 

 refrangible, and Sir David Brewster has found that light of 

 less refrangibility is less completely polarized by a bundle of 

 plates placed at a given angle, than the more refrangible 

 rays*. 



Mica is preeminently adapted for the purpose of polarizing 

 by its considerable diathermancy^ and by the extreme thinness 

 of its laminae. I have, however, succeeded in polarizing heat 

 by transmission through a bundle of rock-salt plates with pa- 

 rallel surlacest. Two bundles consisting of three plates, or 

 six surfaces each, polarized about one-seventh of the heat 

 which passed in the parallel position, the angle of inclination 



* The question of the unequal polarizability of heat from different sources 

 is resumed and very fully discussed in the Third Series of these Researches^ 

 an abstract of which will immediately follow the present paper. 



t For a supply of this valuable substance I have been greatly indebted 

 both to Sir Philip Grey Egerton, Baronet, of Oulton Park, Cheshire, and to 

 Dr. Traill. . 



