LS2 HISTOKY OF THEEMOTICS. 



duction, in their ultimate analytical form, were almost identical with 

 the laws of motion of fluids. Fourier's principle also, that the radia- 

 tion of heat takes place from points below the surface, and is intercept- 

 ed by the superficial panicles, appears to favor the notion of material 

 emission. 



Accordingly, some of the most eminent modern French mathemati- 

 cians have accepted and extended. the hypothesis of a material caloric. 

 In addition to Fourier's doctrine of molecular extra-radiation, Laplace 

 and Poisson have maintained the hypothesis of molecular intm-radia- 

 tion, as the mode in which conduction takes place ; that is, they say 

 that the particles of bodies are to be considered as discrete, or as points 

 separated from each other, and acting on each other at a distance ; 

 and the conduction of heat from one part to another, is performed by 

 radiation between all neighboring particles. They hold that, without 

 this hypothesis, the differential equations expressing the conditions of 

 conduction cannot be made homogeneous : but this assertion rests, I 



o 



conceive, on an error, as Fourier has shown, by dispensing with the 

 hypothesis. The necessity of the hypothesis of discrete molecular 

 action in bodies, is maintained in all cases by M. Poisson ; and he has 

 asserted Laplace's theory of capillary attraction to be defective on this 

 ground, as Laplace asserted Fourier's reasoning respecting heat to be 

 so. In reality, however, this hypothesis of discrete molecules cannot 

 be maintained as a physical truth ; for the law of molecular action, 

 which is assumed in the reasoning, after answering its purpose in the 

 progress of calculation, vanishes in the result; the conclusion is the 

 same, whatever law of the intervals of the molecules be assumed. The 

 definite integral, which expresses the whole action, no more proves 

 that this action is actually made of the differential parts by means of 

 which it was found, than the processes of finding the weight of a body 

 by integration, prove it to be made up of differential weights. And 

 therefore, even if we were to adopt the emission theory of heat, we are 

 by no means bound to take along with it the hypothesis of discrete 

 molecules. 



But the recent discovery of the refraction, polarization, and depolari- 

 zation of heat, has quite altered the theoretical aspect of the subject, 

 and, almost at a single blow, ruined the emission theory. Since heat 

 is reflected and refracted like light, analogy would lead us to conclude 

 that the mechanism of the processes is the same in the two cases. 

 And when we add to these properties the property of polarization, it is 

 scarcely possible to believe otherwise than that heat consists in trans 



