EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 341 



attended with evolution or absorption of heat, the electromotive force 

 must vanish in a cell of the kind considered, if it is determined 

 simply by the diminution of energy in the cell. And when the 

 mixture produces cold, the same rule would make any electromotive 

 force impossible except in the direction which would tend to increase 

 the difference of concentration. Such conclusions would be quite 

 irreconcilable with the theory of the phenomena given by Professor 

 Helmholtz. 



A more striking example of the necessity of taking account of the 

 variations of entropy in the cell in a priori determinations of electro- 

 motive force is afforded by electrodes of zinc and mercury in a 

 solution of sulphate of zinc. Since heat is absorbed when zinc is 

 dissolved in 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.! 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 they take place directly. 

 But whatever 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 experiment 

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

 accompanying a current 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, 



* J. Regnauld, Comptes Rendus, t. li, p. 778. 

 t Gaugain, Comptes Rendus, t. xlii, p. 430. 



