1904] produced ~by the Action of Light. 375 



pressures of the electrodes in respect of the anion in light and in dark 

 and pi and p d for the osmotic pressures of the anions in light and in 

 dark, we get 



for (1) Ei = 0-860T log, * . 10~ 4 volt : 



for (2) E 2 = 0860T 5_I log e ^ . 10~ 4 volt (approximately) ; 

 for (3) E 3 = 0-860T log, |* . 10~ 4 volt ; 



d 



and 



2E = E! + E 2 - E 3 = 0-860T - log e j + -H- log e | 10' 4 volt, (I). 



This is the general equation. It follows : 



(1) The E.M.F. must be independent of concentration ; and this was actually 

 found by direct experiment. 



(2) An experimental inTestigation of the value 0'860 T log^ 10~ 4 volt and 



u + v pd 



of the electrical potential difference between the illuminated and not illuminated 

 solution showed in two different ways that it can be neglected, so that we can put 



5E = 0-860 T.log^.10- 4 volt ................ (IV). 



ri 



(3) Equation (IV) shows that, since P/ > P^, the current must flow from the 

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



(4) The connection is deduced, from Equation (IV), between the E.M.F. of a 

 constant galvanic combination reversible in respect of the anion under the action of 

 light and the heat of chemical reaction in the same, or the chemical energy created 

 by light : 



We have 



2E = 0-860 AogJ^ + Jl-log,^ lO- 4 volt = 0'860 T log, Jf . !<)-< volt, 

 \ P/ u + v pa/ *i 



or TVoSE = RT log, ?/ , and for n e = 1, f 2E = RT log, ??. 



fi "i 



From this equation we get 

 = RTd lo e 



Now the solution pressures must follow in light and in dark a law similar to 



Clausius' law of evaporation, and - . ( log,-^ = -=* ; therefore 



al \ P// Jtvl 



= ^, and ^e 5E = p"+ ^. T.. .. (VI), 



i.e., we get for constant galvanic cells reversible in respect of the anion in light, a 

 law analogous to that of Gibbs and Helmholtz and according as *^ T is >, - 



or < p", the system will take up or not, or give up heat to the surrounding 

 medium. 



