S^i M- MELLON! ON THE POLARIZATION OF HEAT. 



take a sufficiently tliick plate of tourmaline cut parallel to the axis, and 

 attenuate it obliquely at one side, so that the planes of its two faces 

 may intersect each other exactly on one of the edges, and thus form a 

 certain angle. A very narrow slip of white paper, or any other object 

 equally minute, if viewed across this angle and in the direction of the 

 edge, will give two images neither lying one upon the other nor con- 

 founded together, as they are when the faces are parallel, but separated 

 by the double refraction of the tourmaline. If we achromatize the re- 

 fringent angle by means of a glass prism in order to have a clearer view, 

 we find that these two images, viewed through the thinnest part of the 

 tourmaline, are nearly of the same intensity ; but by passing the thicker 

 parts successively before the eye, we perceive the image formed by the 

 ordinary refraction becoming gradually weaker and weaker until it is 

 finally extinguished. 



Thus we see that it is in consequence of the unequal absorption of 

 the two pencils formed by double refraction that the polarization in a 

 plate of tourmaline becomes perceptible. If the absorbency of the mat- 

 ter of which the plate is composed acted with the same intensity on each 

 of them, the two pencils would emerge intermixed, and exhibit all the 

 properties of ordinary light ; so that a second plate of tourmaline would 

 no longer, by having its axis placed transversely to that of the first, pro- 

 duce any diminution of intensity in the light transmitted. 



Let us now apply these notions to calorific polarization. Let us sup- 

 pose that all the rays of heat, like those of light, undergo a complete 

 polarization as soon as they enter a plate of tourmaline, and that each 

 of them is consequently divided into two pencils or bundles possessing 

 equal intensities and polarized at right angles. Let us admit, besides, 

 that the inequality of absorption effected by the matter of the tourma- 

 line in the two pencils varies with the different calorific rays; that it is 

 very great with respect to some rays, and little or none with respect to 

 others : it is evident that the former will issue from the tourmaline en- 

 tirely polarized in one plane, while the latter will be more or less po- 

 larized in the two planes standing at right angles to one another, and 

 will therefore present little or no appearance of polarization. 



All the facts which we have stated may then be explained on the hy- 

 pothesis of a complete polarization of the calorific rays ; and we shall 

 see, indeed, that this hypothesis is rendered more and more probable, nay, 

 certain, by the following experiments. But before we conclude our 

 observations on this part of the subject, it will not, perhajjs, be useless 

 to set in contrast, by means of an example easily to be comprehended, 

 the two different effects which the tourmalines have on the rays of light 

 and the rays of heat. 



Let us imagine a series of alcohol flames coloured by different salts, 

 and several common flames masked by glasses of different colours. If 



