836 Dr. S. W. J. Smith on the 



The relation necessary for equilibrium is 



s n s n /• x-x 



p-k-f 1 * =■/*.- a*. M 



The amount of M which enters solution (and of H which 

 escapes) before equilibrium is attained will be determined 

 by the value of 



where c is the concentration of the salt MX near the surface 



which is necessary in order that // may acquire the value 



required to satisfy (i.). If no such value of c can arise equi- 

 librium is impossible (unless owing to secondary effects other 

 conditions supervene), and interaction will continue until 

 the supply of acid or of metal is exhausted. 



§ 5. Possible effect of Surface Tension. — The above state- 

 ment of the conditions of equilibrium neglects possible 

 variation of the surface energy during the interaction. In 

 many cases there may be relatively little variation ; but in 

 one at least, where the metal is mercury and the interface 

 consequently separates two liquids, changes of surface tension 

 are easy to detect. In this case the surface energy can be 

 seen to diminish with increase in the concentration of the 

 mercury salt in solution. 



Consequently there is now a greater diminution of avail- 

 able energy than is represented by the left-hand member 

 of (i.), when an equivalent o£ H is replaced in solution by 



an equivalent of M. Hence fi must attain a greater value 



in the solution before equilibrium is reached than would be 

 required to satisfy (i.). 



Thus if — 7 is the decrease of surface tension which w r ould 

 accompany the exchange of equivalents of H and M across 

 unit surface when equilibrium is attained, we should have 



s n . s n /• \ 



and the equilibrium concentration of the salt of M in solution 

 would now be 



* In the strictly reversible system M : MX : HX : H pt , the corre- 

 sponding* equation of equilibrium (no electromotive force) is approximately 

 of the form 



aTlog c^ - E h = bT log c^-i: m , 



where E h and B m are ' electromotive constants ' of hydrogen (at given 

 pressure) and the metal, at the temperature of equilibrium T. 



