carnot's theory of the motive power of heat. 543 



" In our demonstrations we tacitly assume that after a body has experienced 

 a certain number of transformations, if it be brought identically to its primitive 

 physical state as to density, temperature, and molecular constitution, it must 

 contain the same quantity of heat as that which it initially possessed ; or, in 

 other words, we suppose that the quantities ofheat lost by the body imderone set 

 of operations are precisely compensated by those which are absorbed in the others. 

 This fact has never been doubted ; it has at first been admitted without reflection, 

 and afterwards verified, in many cases, by calorimetrical experiments. To deny 

 it would be to overturn the whole theory of heat, in which it is the fundamental 

 principle. It must be admitted, however, that the chief foundations on which the 

 theor}"^ of heat rests, would require a most attentive examination. Several expe- 

 rimental facts appear nearly inexplicable in the actual state of this theory." 



7. Since the time when Carnot thus expressed himself, the necessity of a 

 most 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, invariable in quantity ; not convertible into any 

 other element, and incapable of being generated by any physical agency ; in fact 

 the acknowledged principles of latent heat ; would require to be tested by a most 

 searching investigation before they ought to be admitted, as they usually have 

 been, by almost every one who has been engaged on the subject, whether in com- 

 bining the results of experimental research, or in general theoretical investigations. 



8. The extremely important discoveries recently made by Mr Joule of Man- 

 chester, that heat is evolved in every part of a closed electric conductor, moving 

 in the neighbourhood of a magnet,* and that heat is generated by the friction of 

 fluids in motion, seem to overturn the opinion commonly held that heat cannot 

 be generated, but only produced from a source, where it has previously existed 

 either in a sensible or in a latent condition. 



* 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 wecannot infer from anypreviously-published experimental researches, the actual generation ofheat 

 when the current originates in electro-magnetic induction ; since the question occurs, is the heat which is 

 evolved in one part of the closed conductor merely 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 operations, it would be 

 necessary to prove that the inducing magnet does not become lower in temperature, and thus com- 

 pensate for the heat evolved in the conductor. I am not aware that any examination with reference 

 to the truth of this conjecture has been instituted ; but, in the case where the inducing body is a 

 pure electro-magnet (without any iron), the experiments actually performed by Mr Joule render 

 the conclusion probable that the heat evolved in the wire of the electro-magnet is not aflfected by 

 the inductive action, otherwise than through the reflected influence which increases the strength of 

 its own current. 



L 



