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XLIII. On the Theory of Exchanges. 

 By Henry Hudson, M.D.* 



PREVOSTS celebrated doctrine, under the above title, was 

 an " Emission Theory of Heat/' and, having regard to the 

 then existing state of knowledge, was undoubtedly a most inge- 

 nious mode of explaining the apparent " radiation of cold/' 



The objections to it, in connexion with Leslie's discovery of 

 the influence of surface on radiation, have not, I think, been 

 sufficiently attended to ; and after briefly considering the matter 

 theoretically, I shall adduce, as I believe, conclusive experimental 

 evidence against it. 



There can be no doubt that the ingenious speculations of 

 Professor Balfour Stewart and others have contributed to the 

 advancement of the doctrine of exchanges in no small degree, 

 as being immediately connected with the well-known parallelism 

 (or equality) of the " radiating and absorbing powers " of the 

 same body for heat. But to my mind these qualities are essen- 

 tially due to the " different capacities for heat " of the surfaces. 

 Thus, if two bodies, in a common medium, are equally above (or 

 below) the temperature of that medium, the tendency of each body 

 to attain the temperature of the medium will be perfectly equal. 



Each therefore tends to radiate (or absorb) the same (say ^th) 



portion of its excess (or deficiency) of temperature, and conse- 

 quently each body will radiate (or absorb) quantities of heat pro- 

 portional to its " capacity for heat." Hence radiation and ab- 

 sorption (being both due to the same quality in bodies) must be 

 proportional to each other. 



Assuming a " wave- theory " of heat, let us suppose a body (A) 

 placed in a medium of the same temperature as itself (i. e. both 

 body and medium being in a state of perfect equilibrium as to 

 molecular and setherial vibrations). How is it possible to conceive 

 that this body can lose any portion of its vibratory motion so 

 long as that of the surrounding medium continues undisturbed ? 

 If a body (B), say of a lower temperature, be introduced into (or 

 placed outside) the medium, the vibrations of A are, of course, 

 liable to be diminished. But this effect (on a wave-theory as 

 distinguished from " emissive power ") can only occur through a 

 disturbance in the medium extending from B to A, whereby the 

 equilibrium which previously existed is subverted. In accord- 

 ance with these views, it appears impossible to admit that a body 

 can show any of the effects termed " radiation " so long as itself 

 and its surrounding medium are in a perfect state of equilibrium- 



* Communicated by the Author. 



