374 Dr. M. Wilderman. On Galvanic Cells [Nov. 8, 



0-860 T 2v log, & . 10~ 4 volt 

 u + v pd 



and the electrical potential difference between the illuminated and the non-illumi- 

 nated solution can be neglected, in comparison with the electrical potentials of the 

 plates and solution. Thus Equation (I) can be written 



2E = 0-860 T (log e ^-\ 10- 4 volt ................ (IV). 



(3) Since P/ is >P</, 2E is positive, i.e., the current must flow from the plate in 

 light to the plate in dark, as was actually found. 



(4) From 2E = 0'860 T log, . 10~ 4 volt or 2E = RT log, -?- , we get on differ- 



entiating and putting ** = ~ P * RT , ^ = "^^ (where p and p' are the 

 dT. 10 1 Lv 



+ 



heat of transformation of Ag into Ag ions in light and in dark), a law, analogous 

 to that of Gibbs and Helmholtz for ordinary galvanic cells : 



(VI). 



(5) Equation (VI) allows us to calculate the heat of reaction (p") from the 



+ 



E.M.F.s observed; p and p', or the heat of transformation of Ag into Ag cations 

 must evidently be different in light and in dark. 



(6) By Equation (IV) we can calculate both the ratio of the two solution 

 pressures of the same electrode in light and in dark, from the E.M.F. observed, 

 and also the absolute value of P/ if P,/ is known otherwise. 



(7) It was found experimentally that the E.M.F. created by light, or the work 

 done by the system under the action of light, is directly proportional to the 

 intensity of light.* 



(5) The physico-mathematical theory of " constant cells reversible in 

 respect of the anion" (e.g., Ag-BrAg plate in light, KBr solution in 

 light, KBr solution in dark, Ag-BrAg plate in dark) is the following. 



The result of the process under the action of light consists of : 



(1) One gramme-atom of the anion (Br) of a higher chemical potential passes 

 from the electrode Ag-BrAg in light into the solution containing a salt of the 

 same acid (BrK) as the salt (BrAg) covering the metal plate Ag. 



(2) The gramme-atom of the Br ions passes from the solution in light to the 

 solution in dark, transforming into anions of a lower chemical potential ; and 



(3) The gramme-atom of the Br ions in the dark of the lower chemical potential 

 separates from the solution upon the electrode in the dark. 



If we now calculate the maximum work done by such a system, when 



under the action of the current 1 gramme of anion (Br) passes from 

 the plate in light to the plate in dark, putting P z , P d , for the solution 



* See 'Zeitsch. phyeik. Ohemie,' 1902, pp. 319 and 333; 'Phil. Mag.,' 1903, 

 vol. 5, pp. 211, 224. 



