CONDUCTION AND RADIATION. 45 



of molecular extra-radiation acquires great con- 

 sistency. 



Sect. 8. Discovery of the Polarization of Heat. 



THE laws of which the discovery is stated in the 

 preceding Sections of this Chapter, and the expla- 

 nations given of them by the theories of conduction 

 and radiation, all tended to make the conception 

 of a material heat, or caloric, communicated by an 

 actual flow and emission, familiar to men's minds ; 

 and, till lately, had led the greater part of thermo- 

 tical philosophers to entertain such a view, as the 

 most probable opinion concerning the nature of 

 heat. But some steps have recently been made 

 in thermotics, which appear to be likely to over- 

 turn this belief, and to make the doctrine of emis- 

 sion as untenable with regard to heat, as it had 

 before been found to be with regard to light. I 

 speak of the discovery of the polarization of heat. 

 It being ascertained that rays of heat are polarized 

 in the same manner as rays of light, we cannot 

 retain the doctrine that heat radiates by the ema- 

 nation of material particles, without supposing 

 those particles of caloric to have poles ; an hypo- 

 thesis which probably no one would embrace ; for, 

 besides that its ill fortune in the case of light must 

 deter speculators from it, the intimate connexion 

 of heat and light would hardly allow us to sup- 

 pose polarization in the two cases to be produced 

 by two different kinds of machinery. 



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