199 



niittod without reflection, and afterwards verified in many cases by 

 cAlorimetrical experiments. To deny it would bo to overturn the 

 whole theory of heat, of which it is a fundamental principle. It 

 must be admitted, however, that the chief foundations on which the 

 theory of heat rests would require a most attentive examination. 

 Several experimental facts appear nearly inexplicable in the actual 

 state of this theory." 



Since the time when Carnot thus expressed himself, the necessity 

 of a niobt careful examination of the entire experimental basis of the 

 theory of heat has become more and more urgent. Especially all 

 those assumptions depending on the idea that heat is a substance in- 

 variable in quantity; not convertible into any other element, and in- 

 capable of being ijenerated by any physical agency ; in fact, the ac- 

 knowledged principles of latent heat, would require to be tested by a 

 most searching investigation before they ought to be admitted, as they 

 'sually are, by almost every one who has worked on the subject, 

 whether in combining the results of experimental researches or in 

 reasoning d priori. 



The extremely important discoveries recently made by Mr Joule, 

 of Manchester, that heat is evolved in every part of a closed electric 

 conductor, moving in the neighbourhood of a magnet ;* and that heat 



* I cannot omit this opportunity of correcting an expression which I made 

 use of in a note published in the Philosophical Magazine (vol. xxxiii., p. 315), 

 in alluding to the generation of heat by such operations, which I inadvert- 

 ently asserted to have been proved by " known experiments, adduced by Mr 

 .loule." It is true that the evolution of heat in a fixed conductor, through 

 which a galvanic current is sent from any source whatever, has long been 

 known to the scientific world ; but it was pointed out by Mr Joule that we 

 cannot infer, from any previously published experimental researches, the actual 

 generation of heat when the current originates in electro-magnetic induction, 

 since the question occurs, Is the heat which is evolved in one part of the closed con- 

 ductor merelij transferred from those parts which are subject to the inducing influence ? 

 Mr Joule, after a most careful experimental investigation, with reference to 

 this question, finds that it must be answered in the negative. (See a paper 

 " On the Calorific Effects of Magneto-Electricity, and on the Mechanical Value 

 of Heat; by J. P. Joule, Esq. ;" read before the British Association at Cork, 

 in 1843, and subsequently communicated by the author to the Philosophical 

 Magazine, vol. xxiii., pp. 263, 347, 435.) 



Before we can finally conclude that heat is absolutely generated in such ope- 

 rations, it would be necessary to prove that the inducing magnet does not become 

 lower in temperature, and thus give compensation for the heat evolved in the 



