20S: Prof. Forbes on the Refraction and Polarization of Heat, 



I cannot, therefore, entertain any doubt on the polarization 

 of heat by tourmaline, notwithstanding the opposite result 

 which M. Melloni (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 accompa- 

 nies light is excessively small, or else that radiant light during 

 its instantaneous passage through a medium, is capable of 

 being converted into radiant heat. The latter supposition we 

 have no analogies strong enough to warrant us to adopt, 

 though were heat really not polarized 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 circumstances (as refrac- 

 tion and reflection), is capable of almost complete separation 

 from it in 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 investi- 

 gating the influence of light, since the quantity of matter to 

 be traversed is exactly the same, whatever be the direction of 

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

 of absorption. 



27. M. Melloni has proved that the more light that accom- 

 panies heat, the greater power it has to traverse most media, 

 such us clear glass or alum. I made several experiments on 

 the quality of the heat which passed through the tourmalines 

 in their darkest and in their brightest positions, and I always 

 found that the presence of the light materially increased the 

 power of the heat to permeate such screens, though we have 

 seen how little it added to the quantity, 



28. This fact, namely, that by sifting, as it were, heat sepa- 

 rate from light, we give to it the characters of non-luminous 

 heat, or heat of low temperature, and small refrangibility, such 

 as exists beyond the red extremity of the spectrum, seems so 

 far congenial with analogy. But according to Melloni's ex- 

 periments, this does not hold with other degrees of sifting of 

 heat. Thus the absorption of all rays of light, except the blue, 

 the yellow, or the red, by coloured glasses, does not give the 

 peculiar character to the heat which it possesses, when it ac- 

 companies light in the process of refraction, namely, that of 

 permeating screens (in general) more readily as the refrangi- 

 bility is greater. Hence I conceive we must conclude, that 



