the Electrolytic Sohdion-Pressure. 431 



As a further step in the theory of the electromotive force of 

 contact, Nernst introduced the idea of the electrolytic-solution 

 pressure II such that 



and consequently 



const. = log e II ; 



w RT i n 



This idea has two uses. In the first place, it affords a con- 

 venient mode of expressing the observed facts as to production 

 of electromotive force : we may say that the B.M.F. of a 

 contact is proportional to the logarithm of the ratio between 

 the solution-pressure of the metal and the osmotic pressure of 

 the metallic ions in solution : to this there is no exception to 

 be taken. In the second place, the idea suggests a physical 

 theory of the production of electromotive force, which theory 

 in turn changes the solution-pressure from a merely mathe- 

 matical convenience to the expression of a physical reality. 

 According to Nernst, the mechanism is somewhat as follows : — 

 When a piece of metal is put into a solution of which the ions 

 have an osmotic pressure less than the solution-pressure of 

 the metal, some of the metal dissolves: the ions formed carry 

 their positive charge with them, making the solution positive 

 to the metal. The electrical double layer thus formed at the 

 surface of contact exercises an attraction which balances so 

 much of the solution-pressure as is not already balanced by 

 the osmotic pressure, and equilibrium ensues. But on account 

 of the large charges associated with the atoms, the amount of 

 metal that goes into solution is immeasurably small. 



Now the values of the solution-pressure II are easy to 

 calculate from the observed values of the electromotive force. 

 According to Le Blanc (Elektrochemie, p. 185) we have 





n. 



Zinc . 



. 9-9 x 10 18 atmos 



Nickel . . 



. 1-3 x. 10° „ 



Palladium . 



. 1-5 xlO- 36 „ 



There are certain obvious difficulties in the way of accepting 

 these numbers as representing a physical reality. The first 

 of them is startlingly large ; that, however, may not be a true 

 difficulty. The third is so small as to involve the rejection of 

 the entire molecular theory of fluids. If it is true that fluids 

 consist of molecules with a diameter of the order of mas;- 



