11 



26 



85 



1H 



28 



87 



11 



26 



84 



JO 



30 



83 



146 Professor Forbes on the Refraction 



strontia (the muriate is better) ; the yellow, with muriate of 

 soda ; the green, boracic acid ; the blue, pure alcohol. The un- 

 steadiness of intensity of an alcohol flame prevents great numeri- 

 cal accuracy. 



Number of Rays of Heat out of 100 transmitted by 

 Colour of Flame. Alum. Glass. Rock Salt. 



Red, 



Yellow, ..... 



Green, . ... 



Blue, 



The differences are certainly within the limits of errors of obser- 

 vation. 



30. I am disposed to believe, however, that in these experi- 

 ments, as well as Melloni's, some effect is probably due to the 

 simple presence of light of a particular quality, though its heat- 

 ing power may be small. This my experiments with tourmalines 

 countenance. We can hardly, however, look for a solution of 

 these difficulties, until some of the most stubborn difficulties in 

 the theory of light, the laws of dispersion and absorption (and es- 

 pecially that peculiar absorptive power which permits the tour- 

 maline only to transmit one polarized pencil) are completely over- 

 come. Meanwhile, we pass with pleasure to the consideration of 

 some of those properties of heat which serve to connect it with 

 the best determined and best explained departments of optics. 



§ 3. On the Polarization of Heat by Refraction and Reflection. 



31. Soon after the discoveries connected with the polariza- 

 tion of light, which illustrated the earlier part of this century, 

 the question of the polarization of heat was taken up by Malus 

 and Berard. * In the case of heat accompanying solar light, it 

 was decisively proved, as might have been anticipated ; but in the 

 case of heat from terrestrial, and especially non-luminous sources, 

 though M. Berard considered that he had proved it, he gives 



* Memoires cTArceuil, torn. iii. 



