Chemical Affinity in terms of Electromotive Force. 249 



consumption of zinc and precipitation of copper) ; whilst with 

 a Grove's battery a considerably larger heat-absorption took 

 place, amounting to about 11*9 per cent., of the heat due to 

 solution of zinc and reduction of nitric acid. 



16. This result with a Daniell cell is borne out by expe- 

 riments of a different kind. The E.M.F. which a Daniell 

 cell is practically capable of giving has been variously esti- 

 mated by different observers, at from 1*05 to 1*14 volt, 1'10 

 to 1*12 being a usual average. The E.M.F. corresponding 

 to the heat evolved in the reaction 



CuS0 4 + Zn=Cu + ZnS0 4 



(which expresses the ultimate change in a Daniell cell, apart 

 from local action) is several per cent, higher. Thus Favre 

 estimated the heat thus evolved per gramme-equivalent to be 

 27,388 (Comptes Rendus, lxiii. p. 369), and subsequently as 

 25,060 {ibid. Ixix. p. 35) : the mean of these two numbers is 

 26,224, corresponding to an E.M.F. of 26,224 x 0*000105 x 

 42 x 10 6 = 1-156 x 10 8 C.G.S. units, or 1-156 volt. Julius 

 Thomsen has calculated (Berichte deut. chem. Ges. 1872, 

 vol. v. p. 170) from his own determinations, together with 

 those of Andrews, Dulong, Hess, and Favre and Silbermann, 

 that the heat-evolutions taking place during the formation of 

 zinc sulphate and copper sulphate are respectively 108,460 and 

 56,216 per " gramme-molecule " (these formations being ex- 

 pressed in Thomsen's symbols thus — Zn, O, S0 3 , aq, and Cu, 

 O, S0 3 , aq). Hence the heats are 54,230 and 28,108 per 

 gramme-equivalent ; whence the displacement of copper from 

 copper sulphate by metallic zinc, forming copper and zinc 

 sulphate, must give rise to an evolution of 54,230—28,108 = 

 26,122 gramme-degrees per gramme-equivalent, corresponding 

 to 26,122 x%J= 1-152 volt. 



17. Results analogous to those of Favre have also been ob- 

 tained by Raoult (Ann. de Chim. et de Phys. 4th series, ii. 

 p. 317, and iv. p. 392) — some forms of voltaic combinations 

 giving rise to the production of more heat outside the battery 

 than corresponds to the heat evolved by the ultimate chemical 

 and physical changes taking place therein, and others to less. 

 Raoult found that the current from the Daniell cell used by 

 him was capable of doing total work equal to 23,900 gramme- 

 degrees per gramme-equivalent of zinc dissolved in each cell : 

 this represents only 23,900 x%J = l'054 volt, a quantity less 

 by 8*5 per cent, than the value calculated above from Thom- 

 sen's figures as the E.M.F. corresponding to the sum of the 

 chemical and physical actions taking place in the cell. 



To express the difference between the heat actually produ- 



