ELECTROCHEMICAL EQUIVALENTS. 247 



equilibrium in a circuit may be modified, if the cells and the 

 electrolytes are at the same temperature. 



It may happen, on the other hand, that certain of the elements 

 decomposed, experience secondary reactions which are independent 

 of the action of the current, and give rise to an absorption or a 

 disengagement of heat. The final result of the electrolysis would 

 no longer be in a simple ratio with the electromotive force, and 

 this latter could no longer be calculated from the heat of com- 

 bination of the elements, taken in the condition in which they 

 appear after the electrical operation. 



257. ELECTROCHEMICAL EQUIVALENTS. Let A, A', A", . . . 

 be various electrolytes, /, /', /", . . . the weight of each decomposed 

 by unit electricity. These numbers are called the electrochemical 

 equivalents of the various bodies, and experiment shows that they 

 are proportional to their ordinary chemical equivalents. 



If a, a', a", . . . are the heats of combination for unit weight of 

 each of the compounds, the elements of the combination being in 

 the condition due to the passage of the current (that is to say, 

 without taking into account the secondary reactions), the products 

 ap, a'p', a"p", . . . will be the heats of combination of the equivalents. 

 By analogous reasoning to that in the case of water, we see that the 

 electromotive forces relative to these various electrolytes are deter- 

 mined by the ratio 



H =Jaf, 

 H' -* 



which give 



H H' H" 



ap dp 1 d'p" 



It follows from this that the electromotive force of an electrolyte is 

 equal to the mechanical equivalent of the heat of combination of its 

 electrochemical equivalent. 



258. E. BECQUEREL'S LAW. The application of this law of 

 Faraday presents no ambiguity in the case of analogous chemical 

 compounds. If, by one and the same current, we effect the electro- 

 lysis of water, and of a series of neutral sulphates of the protoxides, 

 for instance, the electrochemical equivalent of each metal is the 

 weight which is deposited for the disengagement of a gramme of 

 hydrogen ; but there may be some doubt when the compounds have 

 not the same formula. With two neutral sulphates, one of the pro- 

 toxide, and the other of the sesquioxide of iron, decomposed by the 



