SECT. XXV. POLARIZATION OF CALORIC. 217 



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 diffi- 

 cult to conduct. 



It appears from the various experiments of M. Mel- 

 loni and Professor Forbes, that all the calorific rays ema- 

 nating from the sun and terrestrial sources are equally 

 capable of being polarized by reflection and by refrac- 

 tion, whether double or simple, and that they are also 

 capable of circular polarization by all the methods em- 

 ployed in the circular polarization of light. Plates of 

 quartz cut at right angles to the axis of the prism, pos- 

 sess 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 ab- 

 sorbs, while it transmits the other ; in short, the trans- 

 mission 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 polar- 

 ized by reflection from a pane of glass be viewed through 

 a plate of tourmaline, with its longitudinal section verti- 

 cal, an obscure cloud, with its center 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 

 colors is seen; and as the mica is turned round in a 

 plane perpendicular 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 polar- 

 ization. Now instead of light, if heat from a non-lumi~ 



