306 



Prof. R. B. Clifton on the 



[June 14, 



under the same circumstances. The result of my experiments up to the 

 present has, however, led me to conclude that the final difference of 

 potential between amalgamated zinc and copper in water, or dilute sul- 

 phuric acid, is very nearly, if not exactly, the same as that exhibited by 

 zinc and copper under the same conditions, if the immersion of the plates 

 is sufficiently prolonged. 



This rise of the difference of potential between the plates of a voltaic 

 element while the circuit is open seems to me to be due to exactly the 

 same cause as the so-called polarization of the plates when the circuit is 

 closed, but operating in the opposite direction. 



The natural result of this change in the difference of potential of the 

 immersed plates is, that, in a cell composed of zinc and copper plates im- 

 mersed in water, the zinc being furnished with a copper terminal, the 

 difference of potential exhibited by the terminals should increase with 

 the length of time the plates have been immersed, the circuit of course 

 never having been completed. In order to verify this I have endeavoured 

 to compare, by the electrometer, the difference of potential of the ter- 

 minals of such a cell with the difference of potential of the terminals of 

 a Daniell's cell. Several determinations gave nearly the same results ; 

 and the following numbers show the magnitude of the change which 

 takes place : — 



D represents the difference of potential of a Daniell's cell, in which the 

 amalgamated zinc plate is immersed in a liquid composed of 1 part by 

 weight of pure sulphuric acid and 4 parts by weight of distilled water. 

 According to Mr. Latimer Clark, D= 1*079 volt. 



The difference of potential of the copper terminals of a zinc-water- 

 copper cell is, 



immediately after the immersion of the plates, 

 0-760 D, or 0-820 volt ; 



after the plates have been immersed 1-5 hour, 



0-821 D, or 0-886 volt ; 

 after the plates have been immersed 3 hours, 



0-838 D, or 0-905 volt. 



The experiments with the condenser above mentioned, by which the 

 corresponding change in the potentials of the plates themselves was 

 observed, would indicate that the difference of potential of zinc and 

 copper in contact (which I will represent, as usual, by Zn | Cu) is between 

 0*760 D and 0*821 D, and nearer to the former. This indication is 

 justified by the result of direct experiment, as described in a later part of 

 this paper. 



From the observations previously described as to the difference of 

 potential of copper with respect to water and with respect to a solution 

 of copper sulphate, it follows that when copper plates are immersed 



