106 Prof. Forbes's Researches on Heat. 



polarized in the plane of incidence, the ratio of the polarized 

 to the total heat transmitted would be greatest in the heat of 

 highest temperature. Unfortunately for this theory, careful 

 experiments assured me that heat from different sources un- 

 derwent the same, or nearly the same, intensity, of reflection 

 under the same circumstances. 



We are, therefore, led to regard this character of unequal 

 polarizability, as probably indicating a difference of character 

 of a fundamental kind between heat and light ; at least a super- 

 added quality or peculiarity of vibration, which becomes more 

 and more sensible as heat is removed in its character from 

 light, or has (as we shall hereafter see), generally speaking, a 

 lower degree of refrangibiiity. A sensible undulation, normal 

 to the surface of the wave, would of course satisfy this condi- 

 tion. I am far from saying that my experiments warrant such 

 a conclusion. I am aware that it is inconsistent with the ideas 

 entertained by some ingenious speculators upon the nature 

 of heat * ; but this very circumstance has led me to bestow 

 the greater pains upon establishing the phaenomenon in an 

 incontrovertible manner. 



§ 2. On the Depolarization of Heat, 



In the first series of these researches, § 4, I entered pretty 

 fully into the subject of depolarization. The establishment 

 of the fact was of the highest importance, since there is little 

 probability of proving in any more direct manner the doubly 

 refractive energy of crystals with respect to heat. But, be- 

 sides the demonstration of the fact, I pointed out in that 

 paper the important numerical determinations to which it 

 might lead ; determinations of the first consequence to the 

 theory of heat, and the discrimination of heat from light. The 

 measure of depolarization in the case of light, or the quantity 

 of light which has become polarized in a new plane by passing 

 through a doubly refracting plate, such as mica, depends, 

 1. upon the length of a wave of light; and, 2. upon the retar- 

 dation which one of the doubly refracted pencils of light suf- 

 fers, upon the other, in passing through the mica, which re- 

 tardation differs with the material of the plate, varies directly 

 as its thickness, and may also vary with the quality of the inci- 

 dent ray. 



Hence, as a little reflection clearly shows, if the quantity of 

 light (or, by analogy, of heat) depolarized by a plate of given 

 thickness be numerically estimated, we may, if the length of 

 the wave be given, determine the retardation, or energy of 



« Kelland on Heat, art. 166. 



