240 REPORT — 1897. 



one metal precipitated from its solution by another. In each case an 

 electrically equivalent amount of the metal is dissolved. 



When hydrogen is evolved, it is first dissolved by the metal, from 

 which it separates in an electrically neutral form as soon as its concentra- 

 tion is high enough to give it a vapour pressure exceeding one atmosphere. 

 The process can be arrested by the application of an atmospheric pressure 

 sufficiently great, and this gives a measure of the solution pressure of the 

 metal used. Experiments are difficult, but Beketoff ' and Brunner - have 

 shown that hydrogen at a high pressure can precipitate silver, platinum, 

 and palladium ; Cailletet ^ arrested its evolution from zinc and sulphuric 

 acid, while Nernst and Tammann * have examined the action of other 

 metals. 



The electromotive force developed at the contact of a metal and a 

 solution of one of its salts has been deduced by Nernst by considering the 

 work done when one unit of electricity passes,^ but it seems that the same 

 result can be obtained from the equation giving the potential difference 

 between the solutions of an electrolyte of different concentration, which 

 we have already developed in the form 



If we suppose that in the case of a metal we are concerned with one 

 ion only — the positive one — we can put V = O, since no negative ions 

 enter or leave the metal, and the equation becomes 



E= -RTW^^ 

 Pi 

 where f.^ denotes something corresponding to the osmotic pressure of 

 the kations in the substance of the electrode, which gives its solution 

 pressure P. Thus, neglecting the negative sign, we get 



E = ET%,? 

 P 

 p denoting the osmotic pressure of the ions of the metal in the solution. 



In a simple galvanic cell of any ordinary form there are two metals, 

 say zinc and copper, in contact with the same solution of electrolyte. The 

 equation then becomes 



E = RT (log, ?.' - log, ??) 



where P, and^, refer to the zinc, and Pj and p^, to the copper. In two- 

 fluid cells, such as the Daniell, since the electromotive forces at the 

 electrodes are much greater than at the contact of the liquids, the same 

 equation may still be applied. 



Galvanic cells can be constructed with two electrodes of the same metal 

 placed in solutions of different substances, or even of the same substance 

 at different concentrations. In this case, since the unknown solution 

 pressure of the metal is the same at the opposite electrodes, we can calcu- 

 late the total electromotive force of the cell. 



' Corrfpt. Rend,, vol. xlviii. p. 442 (1859). 

 "^ Pogg. Ann. vol. cxxii. p. 153 (1864). 

 ' Com^pt. Rend. vol. Ixviii. p. 395 (1869). 

 * Zeitg. physiltal. Chem. vol, ix. p. 1 (1892). 

 ' Zeits. physlkal. Chem. vol. iv. p. 148 (1889). 



