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SCIENCE PROGRESS. 



centrations or between solutions of two different electrolytes 

 can be calculated. This is of great interest, for primary 

 voltaic cells can be constructed, with a plate of the same 

 metal for both electrodes, by placing the electrodes in solu- 

 tions of different substances, or even in solutions ot the 

 same substance at different concentrations. 



In the following table, 1 C, and C 2 denote the concentra- 

 tions of the solutions round the opposite electrodes, and a 

 comparison is made between the observed and the calculated 

 values of the electromotive force. 



The difference of potential between metals and electro- 

 lytes is explained in a similar manner. Nernst supposes 

 that each metal in contact with an electrolyte has a definite 

 solution pressure, analogous to the vapour pressure of a 

 liquid, in consequence of which ions are detached from it, 

 and go into solution, carrying their charges with them, and 

 leaving the metal oppositely electrified. The electromotive 

 force of an ordinary voltaic cell will depend on the difference 

 between the solution pressures of the two electrodes, and 

 could be calculated from Nernst's theory if these pressures 

 were known. When a current passes, the ions ot the metal 

 with the smaller solution pressure are forced out of solution 

 by the others, and deposited on the electrode. 



The electromotive force of the cell 



1 W. Nernst, Zeits. fur physikal. C/ie/nie, iv., p. 161, 1889. 



