344 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



the phenomena exhibited in the direct combination of the substances, 

 is corroborated by the experiments of M. Favre, who has observed an 

 absorption of heat in the cell in which this acid was electrolyzed.* 

 The electromotive work expended must therefore have been less than 

 the increase of energy in the cell. 



In both cases of composition in definite proportions which we have 

 considered, the compound has more entropy than its elements, and 

 the difference is by no means inconsiderable. This appears to be the 

 rule rather than the exception with respect to compounds which have 

 less energy than their elements. Yet it would be rash to assert that 

 it is an invariable rule. And when one substance is substituted for 

 another in a compound, we may expect great diversity in the relations 

 of energy and entropy. 



In some cases there is a striking correspondence between the electro- 

 motive force of a cell and the rate of diminution of its energy per unit 

 of electricity transmitted, the temperature remaining constant. A 

 Daniell's cell is a notable example of this correspondence. It may 

 perhaps be regarded as a very significant case, since of all cells in 

 common use, it has the most constant electromotive force, and most 

 nearly approaches the condition of reversibility. If we apply our 

 previous notation (compare (691)) with the substitution of finite for 

 infinitesimal differences to the determinations of M. Favre, t estimating 

 energy in calories, we have for each equivalent (32*6 kilogrammes) of 

 zinc dissolved 



(V- 7')Ae = 24327 caL , Ae = -25394 ca1 -, AQ = -1067^-. 



It will be observed that the electromotive work performed by the cell 

 is about four per cent, less than the diminution of energy in the cell4 

 The value of AQ, which, when negative, represents the heat evolved 

 in the cell when the external resistance of the circuit is very great, 

 was determined by direct measurement, and does not appear to have 

 been corrected for the resistance of the cell. This correction would 

 diminish the value of AQ, and increase that of ( V" F') Ae, which 

 was obtained by subtracting AQ from Ae. 



It appears that under certain conditions neither heat nor cold 

 is produced in a Grove's cell. For M. Favre has found that with 

 different degrees of concentration of the nitric acid sometimes heat 



* See M6moire8 des Savants Etrangers, se'r. 2, t. xxv, no. 1, p. 142 ; or Comptes Rendus, 

 t. Ixxiii, p. 973. The figures obtained by M. Favre will be given hereafter, in connec- 

 tion with others of the same nature. 



t See M6m. Savants Etrang. , loc. cit. , p. 90 ; or Comptes Rendus, t. Ixix, p. 35, where 

 the numbers are slightly different. 



A comparison of the experiments of different physicists has in some cases given a 

 much closer correspondence. See Wiedemann's Galvanismus, etc., 2 te Auflage, Bd. ii, 

 1117, 1118. 



