512 J. ~W. Qibhs — Equilibrium of Heterogeneoits Substances. 



iu mercury,* the energy of the cell is increased by a transfer of zinc 

 to the mercury, when the temperature is maintained constant. Yet 

 in this combination, the electromotive force acts in the direction of 

 the current producing such a transfer. f The couple presents certain 

 anomalies when a considerable quantity of zinc is united with the 

 mercury. The electromotive force changes its direction, so that this 

 case is usually cited as an illustration of the principle that the electro- 

 motive force is in the direction of the current which diminishes the 

 energy of the cell, i. e., which produces or allows those changes which 

 are accompanied by evolution of heat when tliey take place directly. 

 But w^hatever may be the cause of the electromotive force which has 

 been observed acting in the direction from the amalgam through the 

 electrolyte to the zinc (a force which according to the determinations 

 of M. Gaugain is only one twenty-fifth part of that which acts in the 

 reverse direction when pure mercury takes the place of the amalgam), 

 these anomalies can hardly affect the general conclusions with which 

 alone we are here concerned. If the electrodes of a cell are pure 

 zinc and an amalgam containing zinc not in excess of the amount 

 which the mercury will dissolve at the temperature of the ex2:)eriment 

 without losing its fluidity, and if the only change (other than thermal) 

 accompanying a ciirrent is a transfer of zinc from one electrode to 

 the other, — conditions which may not have been satisfied in all the 

 experiments recorded, but which it is allowable to suppose in a 

 theoretical discussion, and which certainly will not be regarded as 

 inconsistent with the fact that heat is absorbed when zinc is dissolved 

 in mercury, — it is impossible that the electromotive force should be 

 in the direction of a current transferring zinc from the amalgam to 

 the electrode of pure zinc. For, since the zinc eliminated from the 

 amalgam by the electrolytic process might be re-dissolved directly, 

 such a direction of the electromotive force would involve the pos- 

 sibility of obtaining an indefinite amount of electromotive work, and 

 therefore of mechanical work, without other expenditure than that of 

 heat at the constant temperature of the cell. 



None of the cases which we have been considering involve com- 

 binations by definite proportions, and, except in the case of the cell 

 with electrodes of mercury and zinc, the electromotive forces are 

 quite small. It may perhaps be thought that with respect to those 

 cells in which combinations take place by definite proportions the 

 electromotive force may be calculated with substantial accui-acy from 



* J. Regnauld, Com]ytes Rendus, t. li, p. 778. 

 •]- Gaugain, Comptes Rendus, t. xlii, p. 430. 



