RESPECTING CARNOT'S AXIOM. 121 



having been called to it by Joule's remark, to which he 

 refers in the same note. The note is — " When thermal 

 agency" is thus spent in conducting heat through a solid, 

 what becomes of the mechanical effect which it might 

 produce? Nothing can be lost in the operations of nature — 

 "no energy can be destroyed!" (The sense is identical with 

 Joule's — " Believing that the power of destroying belongs to 

 the Creator alone, I entirely coincide with Roget and 

 Faraday in the opinion that any theory which, when carried 

 out, demands the annihilation of force, is necessarily errone- 

 ous." " The principles, however," Joule then continues, 

 " which I have advanced are free from this difficulty.") Sir 

 William Thomson then continues : — " What effect, then, is 

 produced in place of the mechanical effect which is lost ? 

 A perfect theory of heat demands an answer to this ques- 

 tion ; yet no answer can be given in the present state of 

 science. A few years ago a similar confession must have 

 been made with reference to the mechanical effect lost in a 

 fluid set in motion in the interior of a rigid closed vessel 

 and allowed to come to rest by its own internal friction ; 

 but in this case the foundation of the solution of the 

 difficulty has been actually found in Mr. Joule's discovery 

 of the generation of heat, by the internal friction of a fluid 

 in motion. Encouraged by the example, we may hope that 

 the very perplexing question in the theory of heat, by 

 which we are at present arrested, will before long be 

 cleared up." 



" It might appear that the difficulty would be entirely 

 avoided by abandoning Carnot's fundamental axiom ; a 

 view which is strongly urged by Mr. Joule (at the conclusion 

 of his paper ' On the Changes of Temperature produced by 

 -the Rarefaction and Condensation of Air'). If we do so 



