218 • Professor Joseiili Larmor [May 1, 



tion of energy is merely the progress towards an equilibrium. As 

 regards the purposes of man, whole regions of available energy may 

 exist, of which he is ignorant, because he does not happen to have 

 learned how to use them. The amount of energy available at a given 

 temperature in a lump of carbon is possibly not yet exactly known : 

 the process of turning it into heat before utilising it of course wastes 

 most of it. Once, however, any slow reversible method of combustion 

 has been discovered, in a voltaic battery for instance, the determina- 

 tion will be possible and may be effected once for all. Or, following 

 a hint thrown out by Lord Rayleigh in 1875, afterwards developed 

 more fully by Gibbs, we may make a rough estimate by applying the 

 Carnot-Ciausius formula to a cycle of which the upper temperature is 

 that of spontaneous dissociation of the materials. We can, in fact, 

 ascertain available energies only for systems which we can reach from 

 a standard one by processes reversible in Carnot's sense. 



Very early in Joule's investigations (1811) on the quantitative 

 equivalence of various kinds of energy, he attacked the problem of the 

 voltaic cell, and found his expectation verified, that in many cases the 

 electromotive force was proportional to the thermal value of the 

 chemical action of one Faraday equivalent of the reagent materials — 

 provided he employed* " galvanic arrangements adapted to allow the 

 combinations to take place without any evolution of heat in their own 

 localities." He concluded that the condition thus laid down must be 

 departed from in certain observed cases of discrepancy, and Thomson, 

 in 1852,t conducted experiments to detect such local reversible heat. 

 This principle of Joule was also stated quantitiitively later, in a general 

 way, by Helmholtz in the 'Erhaltung der Kraft' in 1847. It lies 

 at the foundation of Thomson's memoir of December 1851, ' On the 

 Mechanical Theory of Electrolysis,' whence the restriction above 

 stated, the absence of local reversible heat, is quoted. On this condi- 

 tion the principle is exact ; and the main point of Thomson's paper 

 is the calculation, with a view to comparison with direct experiment^ 

 of the theoretical absolute value of the electromotive force of a 

 Daniell's cell, from Joule's measurements of the heat developed by 

 the combination of an electrochemical equivalent of its materials. 

 The paper also developed the parallel between chemical energy and 

 mechanical energy as sources of electromotive force, including the 

 deduction by the principle of energy of the force induced by motion 

 of a circuit across a permanent magnetic field. The further prosecu- 

 tion of the main subject, into cases where local reversible heat is 

 developed (as evidenced by sensible change in electric conditions with 



* Math, and Phys. Papers, i. p. 477. 



t Loc. cit., p. 503; cf. also p. 496, where, in agreement with Joule, he 

 ascribes the main loss to the work done by evolved gases in expanding against, 

 the atmospheric pressure. 



