446 MEASUREMENT OF ELECTROMOTIVE FORCES. 



acid on the one hand, and the reformed potass on the other, diffuse 

 through the liquid and reproduce the original potassium sulphate. 



Are we to attribute solely to the effect of the current the liberation 

 of an equivalent of potassium, the other actions being considered 

 as purely chemical and independent ; or is there a simultaneous 

 decomposition of an equivalent of potassic sulphate into acid and 

 base and an equivalent of water into hydrogen and oxygen ; or 

 again, given that the acid and base are constantly recombining, 

 and that the final result reduces to the disengagement of oxygen 

 and hydrogen, the mere decomposition of an equivalent of water ? 

 The work corresponding to these different cases would be 98,000, 

 50,200, or 34,500 thermal units. Berthelot has shown that it is 

 the second which is realised. He associates constant elements, and 

 of graduated electromotive forces, such as Daniell's or Regnauld's 

 element, and investigates the least electromotive force which pro- 

 duces decomposition. The electrolysis of potassium sulphate 

 requires an electromotive force corresponding to at least 50,000 

 thermal units. This electromotive force is insufficient when mercury 

 is used as negative electrode, as the potassium amalgamates instead 

 of decomposing the water. The electromotive force necessary in 

 that case approaches 98,000 thermal units, though still inferior; 

 for we must allow for the heat of amalgamation of potassium. 



An interesting case is that in which electrolysis may take place 

 in several different ways. Experiment shows that if the electro- 

 motive force is progressively increased, the reaction which absorbs 

 least heat shows itself first. Each kind of decomposition appears 

 in turn when the electromotive force has attained the desired 

 value, although the preceding ones continue to act. 



The same takes place for a mixture of salts, and a method of 

 analytical separation of certain metals has been based on the 

 action of gradually increasing electromotive forces. 



Berthelot concludes from his experiments that the sum of the 

 energies necessary to produce electrolysis must comprise all re- 

 actions which take place in the passage of the current, without 

 distinguishing between what are called primitive and what are 

 considered as secondary reactions; at any rate, as regards those 

 which take place in the immediate vicinity of the electrode, the 

 experiment of potassium sulphate showing that we need not take 

 into account the recomposition of the potassium sulphate which 

 is due to the diffusion of acid and base across the electrolyte. 



1039. If we compare electromotive forces of different couples, 

 calculated according to these rules, with the electromotive forces 



