250 POLARIZATION OF CALOEIC. [SECT. xxv. 



axes of this mineral. The heat transmitted through this 

 apparatus was polarized when, from a source whose tempera- 

 ture was even as low as 200, heat was also polarized by re- 

 flection ; but the experiments, 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 

 simple, 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 

 any 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. Tourmaline separates 

 the caloric 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 

 refracting 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. 212). 



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

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

 tourmaline, with its longitudinal section vertical, an obscure 

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

 however, 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 



