112 M. MKLLONI OX THE POLARIZATION OF HEAT. 



at right angles, and sensibly superposed, it might be expected 

 that if they have undergone the same degree of absorption, no 

 index of polarization would be presented by them when issuing 

 from the plates ; but if one of them has lost, in its passage, a 

 greater portion of its intensity, the other will necessarily give 

 signs of polarization on its emergence ; and the appearances of 

 this phaenomenon will become exactly similar to those presented 

 by light when one of the two refracted pencils is entirely absorbed 

 in the interior of the plates. 



According to this manner of considering the subject, the 

 more or less absorbent action of the tourmalines upon one of 

 the doubly refracted pencils of heat would enter, so to speak, 

 into the class of facts that have been observed in our examina- 

 tion of simple calorific transmission by sohd and liquid bodies, 

 and all the rays of heat, like Ught of every colour, would be sus- 

 ceptible of complete polarization by the forces which produce 

 reflection and refraction. This latter conclusion will appear, 

 with the clearest evidence, from the numerous experiments that 

 we proceed to describe. 



We know that a pencil of common light, traversing a series 

 of parallel plates of glass, or other diaphanous substance, at a 

 certain inclination, becomes polarized perpendicularly to the 

 plane of refraction ; so that if a second series of plates be pre- 

 sented to the emergent rays at the same inclination, the light 

 either passes through, or is in great part intercepted, accordingly 

 as the second plane of refraction is disposed in a direction parallel 

 or perpendicular to the first. 



In order to see whether analogous effects are produced rela- 

 tively to radiant heat, we have only to submit these two oblique 

 piles of glass to trial by the thermomultiplier, disposing the planes 

 of refraction successively in a parallel or in a perpendicular 

 direction. But if the plates be sufficiently numerous, the quan- 

 tity of emergent heat is veiy small, and scai'cely appreciable by 

 the most delicate instruments, especially for sources at a low^ 

 temperature, the rays of which undergo an almost complete 

 absorption in penetrating the first vitreous layers. Rock salt 

 may be substituted for glass with the gi'eatest success ; but it is 

 difficult to procure several plates of that substance sufficiently 

 lai'ge and pure. To obviate, in a great degree, these various in- 

 conveniences, Mr. Forbes suggested the employment, for the 

 polarization of heat, of mica reduced into very thin laminae, 



