144 Professor Forbes on the Refraction 



24. The following are the general results of my experiments 

 on Tourmaline. 



Source of Heat. No. of Comparisons. Proportions of Heat polarized by 



A and B E and F Tourmalines A and B E and F ■ 



Argand lamp, - 3 16 per cent. 



Oil lamp, 7 3 14 per cent. 11 



Incandescent platinum, 4 3 15 12 



Brass at 700°, - 7 (1 negative) 3 



I cannot, therefore, entertain any doubt on the polarization of 

 heat by tourmaline, notwithstanding the opposite result which 

 M. Mellon i (and I also at first) obtained. 



25. Some very curious considerations arise from the study of 

 these facts. Since 84 per cent, of the heating rays of an Argand 

 lamp pass through the second tourmaline in the case where the 

 light is entirely stopped, we must adopt one of two conclusions : 

 either that the heat which necessarily accompanies light is exces- 

 sively small, or else that radiant light during its instantaneous 

 passage through a medium, is capable of being converted into ra- 

 diant heat. The latter supposition we have no analogies strong 

 enough to warrant us to adopt, though were heat really not polar- 

 ized by tourmaline, we must have done so. All our experiments 

 point to the first, namely, that heat, though intimately partaking 

 of the nature of light, and accompanying it under certain circum- 

 stances (as in refraction and reflection), is capable of almost com- 

 plete separation from it under others. Thus, almost all the heat 

 is stopped by a plate of alum, which transmits nearly the whole 

 light, whilst a second plate of tourmaline stops the whole light, 

 but transmits a large share of the heat. 



26. The tourmaline affords a precious method of investigat- 

 ing the influence of light, since the quantity of matter to be tra- 

 versed is exactly the same, whatever be the direction of the axes 

 of the crystal. In this it differs from all other modes of absorp- 

 tion. 



* It appears that the axes of E and F were not precisely crossed in these ex- 

 periments. 3 



