534 Prof. J. J. Thomson on the Relation between the 



solution, we have 



w 5 + ??? 6 = N, (7) 



m± + 2m 2 + m 6 = lX (8) 



Thus to determine the 6 quantities m 2 . . . m 6 , V, we have only 

 the five equations (3), (4), (6), (7), (8). The charge on 

 the condenser Q = 2w 2 , and V = Q/C; hence if we know C the 

 capacity of the condenser we have another equation connecting 

 the six quantities, so that they can he determined. To calcu- 

 late C, however, we should require to know the distance between 

 the plates of the condenser, that is the distance between the 

 positively and negatively charged zinc atoms. We do not 

 know this distance, but if we had a series of measurements of 

 V we should by the preceding equations be able to calculate 

 m 2 , and hence the capacity of the condenser, and from this 

 capacity the distance between the two layers. 



The preceding expression gives the potential difference 

 between the zinc and the electrolyte on the supposition that 

 these are not connected by an external conducting circuit; the 

 presence of such a circuit might, however, so modify the con- 

 ditions as to render the preceding considerations quite in- 

 applicable. Thus, take the preceding case of zinc immersed 

 in dilute HC1, if the solution is exceedingly dilute, we have 

 on the surface of the zinc a number of positive hydrogen 

 atoms. If the Yolta effect could come into play there would 

 be a diminution in the potential energy of the whole system, 

 if the —zinc and the +H interchanged their charges: in 

 the absence of the external circuit this on our hypothesis is 

 impossible, but it would take place if such a circuit were 

 present. In this case the negative charges on the zinc 

 atoms would be replaced by positive, the potential of the zinc 

 instead of being lower than that of the electrolyte would be 

 higher. This illustrates the principle that the two causes 

 which tend to produce potential differences between sub- 

 stances in contact, viz. direct chemical combination and the 

 interchange of charges on the atoms, tend to establish potential 

 differences of opposite sign. Chemical combination tends to 

 make the electropositive substance negative to the electro- 

 negative one, since the positive atoms combine while the 

 negative ones are left, while the interchange of charges tends 

 to make the electropositive substance positive to the electro- 

 negative. Thus, when two substances are placed in contact 

 under varying physical conditions, the sign of the potential 

 difference will depend upon the relative intensity of these 

 two effects. Observation seems to prove that a difference in 





