224 Mr. R. H, Kent : A Note on Radiation. 



Accordingly conduction and radiation do not refer to 

 mutually exclusive classes of phenomena, and hence from the 

 absence of energy transfer by conduction and from the fact 

 that the total net energy transfer is zero, the absence of net 

 energy transfer by radiation cannot be deduced. 



It is easy to see how such confusion could have arisen. 

 Most of our ideas of conduction depend on the observation 

 of phenomena at temperatures at which no radiation is 

 visible. Hence, conduction has come to connote absence of 

 energy transfer by radiation, although no such absence is 

 implied in the definition of conduction here and commonly 

 used. Formerly the conduction of heat was conceived as a 

 flow, and as such was regarded as entirely distinct from the 

 transfer of heat by radiation. But since the rise of the 

 modern molecular and electronic theories such a conception 

 can no longer be considered adequate. The conduction of 

 heat is effected not only by what may be loosely termed 

 molecular and electronic impacts, but also by radiation from 

 molecule to molecule. 



This note has been written with no quibbling intent, but 

 with a view seriously to contest the thermodynamic foun- 

 dations of KirchhofPs law. A consideration of the state of 

 affairs when two abutting media are at the same temperature 

 may make the situation clearer. Energy may be transmitted 

 across the boundary in two ways, by molecular and elec- 

 tronic impacts and by radiation. From thermodynamic 

 considerations alone it is impossible to show that the net 

 energy transfers in these two ways are separately zero ; all 

 that can be thus legitimately deduced is that the rate of 

 transfer by one of these means is equal and opposite to the 

 rate of transfer by the other. An attempt might be made to 

 demonstrate the absence of net impact transfer by kinetic 

 theory. But in view of the well-known failure of the law of 

 the equipartition of energy— in particular, the fact that the 

 molecules of a metal at a low temperature do not possess 

 the same mean kinetic energy of translation as the molecules 

 of a gas in contact with it and at the same temperature 

 might be given as a pertinent instance of this — such reasoning 

 would lack cogency. That Kirchhoff's law as an empirical 

 fact may be true is, of course, not questioned. Tf such is the 

 case, it is likely that it is the consequence of a fundamental 

 similarity in the radiating mechanisms of all substances. 



My views on this subject owe such clearness as they 

 possess to several conversations with Dr. H. M.Trueblood, of 

 the Department of Electrical Engineering of the University 

 of Pennsylvania. 



