Prof. Forbes on the Refraction and Polarization of Heat, 209 



heat in the spectrum accompanies the hght, and has corre- 

 sponding properties, but that in genera] these properties are 

 independent of the nature of the accompanying light. 



29. The only fact which appeared to militate against this 

 view, so far as coloured media were concerned, was the case 

 of green light. It appeared probable that this arose from some 

 peculiarity in the absorptive nature of the material, not from its 

 colour. To investigate this point, I tried the relative trans- 

 parency (or diathermancy, to borrow a word from M.Melloni,) 

 of screens for the heat of various coloured flames. I did not 

 find that marked peculiarity in the green, which M. Melloni 

 observed in the absorptive action of green glass. The fol- 

 lowing results are not pretended to be numerically accurate, 

 but they are probably nearly comparable. The flames were 

 obtained from alcohol, » combined with the following sub- 

 stances: for the red, nitrate of strontia (the muriate is better); 

 the yellow, with muriate of soda ; the green, boracic acid ; the 

 blue, pure alcohol. The unsteadiness of intensity of an al- 

 cohol flame prevents great numerical accuracy. 



Number of Rays of Heat out of 100 transmitted by 

 Colour of Flame. Alum. Glass. Rock Salt. 



Red, 11 26 85 



Yellow, U\ 28 87 



Green, 11 26 84- 



Blue, 10 30 83 



The differences are certainly within the limits of errors of 

 observation. 



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

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

 the simple presence of light of a particular quality, though its 

 heating 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 ab- 

 sorption (and especially that peculiar absorptive power which 

 permits the tourmahne only to transmit one polarized pencil,) 

 are completely overcome. 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 lH.eat by Refraction and Refection, 



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 



Third Series, Vol. 6. No. 33. Ma7ch 1 835. 2 E 



