378 On Galvanic Cells produced ~by the Action of Light. 



If PT and PT,, are the solution pressures of the electrodes, they must 

 follow a law analogous to Clausius' law for evaporation in a vacuum, i.e., 



d JL = ^ KT and therefore log, P T/ = ^ + log, T + K 



and log* PT,, = j^r- + log e T,, -r K. 



Putting these values into equation I and II' we get : 



2E = [0-860 (K - log e ajn)10- 4 + E'] (T, - T,,), instead of (!') ; 

 2E = [0-860 (loge W - K') 10~ 4 + E'] (T - T,,), instead of (IT), 



i.e. y the E.M.F. of constant thermogalvanic cells reversible in respect 

 of either cation or anion, must be directly proportional to the differ- 

 ences of temperature of the two plates in contact with the liquid. 

 From the observations of Nernst and myself, it can be shown that this 

 is actually the case. Hence we prove that : 



(1) The thermogalvanic E.M.F. is directly proportional to the difference of 

 temperature of the two places of contact of the plates with the liquid (deduced in 

 two ways). 



(2) The solution pressure of the electrodes follows a law similar to Clausius' law 

 of evaporation. 



(3) The law of Gribbs and Helmholtz giving the connection between the E.M.F. 

 of a galvanic combination, the heat of reaction, and the temperature coefficient of 

 the E.M.F., holds good for each of the two potentials separately, giving thus the 

 connection between the electric potential differences of the given plate and solution, 

 the heat of ionisation of the given plate, and the temperature coefficient of the 

 value of this potential difference. 



(7) A very detailed investigation of the E.M.F. in the dark 

 showed that it is due to the formation of "gas batteries" 

 through adhesion of gases in different concentrations to the 

 plates, the degree of adhesion depending upon the state of the 

 surface of the plate. The current polishes the plates, the amount 

 of the gases adhering to them becomes thus more equal, and 



0*0002T P 



according to the equation TT = _ loge 1 the E.M.F. of the gas 



n e P 2 



battery becomes smaller. From these we must distinguish gas batteries 

 created under the action of light as the result of the chemical processes 

 going on in the system under the action of the current the separation 

 of gas upon one electrode only, or of different gases on the two 

 electrodes. 



