120 THOMSON SEES JOULE'S DIFFICULTIES. 



(this ignores the fact that the latent heat of the battery was 

 absorbed in Joule's electro-magnetic engine, without pro- 

 ducing any change in the condition of the substance of the 

 engine) " and the fundamental axiom adopted by Carnot 

 may be considered as still the most probable basis for an 

 investigation of the motive power of heat ; although this, 

 and with it every other branch of the theory of heat, may 

 ultimately require to be reconstructed upon another founda- 

 tion, when our experimental data are more complete. On 

 this understanding and to avoid a repetition of doubts, I shall 

 refer to Carnot's fundamental principle in all that follows 

 as if its truth were thoroughly established." 



Besides this general qualification, the paper contains two 

 long notes in which the writer discusses and argues some 

 of Joule's results. In the first of these, Joule's experiments, 

 on the effect of magneto-electricity in producing heat, are 

 shortly described, and subjected to the criticism that Joule 

 had not explicitly discussed any possible change of heat in 

 the inducing magnets, the writer coming to the conclusion 

 that in all probability this was not much. This criticism is 

 a mistake, as there were no means for the conduction of heat 

 or electricity between the two systems except radiation, and 

 that Joule had discussed. The second note has reference to 

 the difficulty which arises in explaining what becomes of 

 the vis viva that would have been produced in an engine 

 when the temperature of the heat is similarly lowered by 

 simply causing it to pass through the solid plate of a boiler 

 from the fire to the water. This was the very point that 

 Joule took hold of in the passage quoted, page 88, from 

 Joule's paper "On the Changes of Temperature produced by 

 the Rarefaction and Condensation of Air ;" and now Sir 

 William Thomson is struck by it, his attention apparently 



