lj^4 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



26. Since, then, the first four kinds of heat are transmitted without any great 

 difference of proportion, by the piles I and K, and since, especially, the heat from 

 a lamp sifted by glass and that from dark brass possess almost exactly similar 

 characters in this respect, it is very clear that we have a new ground for reject- 

 ing as untenable M. Melloni's supposition (mentioned in art. 10), that the appa- 

 rent differences of polarization in my experiments, arose from the unequal pro- 

 portions of heat absorbed by the mica piles when the source varied. 



27. Admitting, then, the fact of the variable index of polarization exhibited 

 by heat of different qualities similarly treated, we are tempted to inquire what 

 explanation can be offered of it. This question, inferring for its answer a know- 

 ledge of the nature of heat, we are not prepared to answer with confidence. My 

 former suspicion of its being due solely to the difference of the refractive index 

 of mica for heat of different kinds (II. 24), I am disposed to retract as inadequate, 

 or at least to suspend my judgment respecting it. I at one time thought, that, 

 supposing the mica bundles unequally permeable to heat from different sources, 

 a difference of ratio in the total heat reaching the pile with the plates I and K 

 parallel and c7'ossed might be accounted for. But a careful analysis of the cir- 

 cumstances convinced me, that the absorptive action, if assumed the same for 

 common and polarized heat, could produce no such effect. One of the most 

 plausible suppositions which occurred to me was this, — that, supposing the re- 

 flection of luminous heat to take place more copiously at the mica surface than 

 that of dark heat, and supposing the angle of incidence to be that of total polariza- 

 tion, since the refracted ray contains as much heat (if heat be like light) polarized 

 perpendicular to the plane of incidence, as is reflected and 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, care- 

 ful experiments assured me that heat from different sources underwent the same, 

 or nearly the same, intensity of reflection under the same circumstances. 



28. 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 superadded 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 refrangi- 

 bility. A sensible undulation, normal to the surface of the wave, would of course 

 satisfy this condition. 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 phenomenon in an 

 incontrovertible manner. 



* Kellanp on Heat, Art. 166. 



