266 POLARIZATION OF HEAT. SECT. XXVII. 



mica, placed at the polarizing angle, and so cut that the plane 

 of incidence of the heat corresponded with one of the optic axes 

 of this mineral. The heat transmitted through this apparatus 

 was polarized from a source whose temperature was even as low 

 as 200; heat was also polarized by reflection; but the ex- 

 periments, though perfectly successful, are more difficult to 

 conduct. 



It appears, from the various experiments of M. Melloni and 

 Professor Forbes, that all the calorific rays emanating from the 

 sun and terrestrial sources are equally capable of being polarized 

 by reflection and by refraction, whether double or single, and 

 that they are also capable of circular polarization by all the 

 methods employed in the circular polarization of light. Plates 

 of quartz cut at right angles to the axis of the prism possess the 

 property of turning the calorific rays in one direction, while other 

 plates of the same substance from a differently modified prism 

 cause the rays to rotate in the contrary direction ; and two plates 

 combined, when of different affection, and of equal thickness, 

 counteract each other's effects as in the case of light. Tourma- 

 line separates the heat into two parts, one of which it absorbs, 

 while it transmits the other ; in short, the transmission of radiant 

 heat is precisely similar to that of light. 



Since heat is polarized in the same manner as light, it may be 

 expected that polarized heat transmitted through doubly refract- 

 ing substances should be separated into two pencils, polarized in 

 planes at right angles to each other ; and that when received on 

 an analyzing plate they should interfere and produce invisible 

 phenomena, perfectly analogous to those described in Section 

 XXII. with regard to light (N. 221). 



It was shown, in the same section, that if light polarized by 

 reflection from a pane of glass be viewed through a plate of tour- 

 maline, with its longitudinal section vertical, an obscure cloud, 

 with its centre wholly dark, is seen on the glass. When, how- 

 ever, a plate of mica uniformly about the thirteenth of an inch 

 in thickness is interposed between the tourmaline and the glass, 

 the dark spot vanishes, and a succession of very splendid colours 

 are seen ; and, as the mica is turned round in a plane perpendi- 

 cular to the polarized ray, the light is stopped when the plane 

 containing the optic axis of the mica is parallel or perpendicular 

 to the plane of polarization. Now, instead of light, if heat from 



