352 DR. MEYER WILDER MAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
§ 5. On the Nature of the Chemical Processes in Galvanic Cells created by Light.. 
Light, Heat, and Pressure—Galvanic Cells. 
Becquerel and others assume that “ a current is created, because a chemical 
reaction of decomposition goes on on the plate exposed to light,” but he at once falls 
into difficulties when he has to explain the nature of the action of light upon 
metallic plates, where no “ chemical reaction ” can be assumed. Under the action of 
light in a galvanic combination a reaction takes place, not on the exposed plate only, 
but on the plate in the dark as well, that is, a reaction takes place between both 
plates, and the kind of reaction depends upon the composition of the system. A 
reaction in open circuit is not necessarily the same in closed circuit. 
The general cause and condition for the creation of a galvanic cell is, that the 
chemical potential of the two electrodes should be different. Since under the action 
of light, temperature, or pressure, the chemical potential of a substance becomes 
different, we must therefore get galvanic cells created by light, thermogalvanic cells, 
galvanic cells created by pressure, when two plates are exposed to light of different 
intensity, or different temperatures or pressures. Direct observations of the direction 
of the current, as far as they go to the present, show that under the action of light, 
and with the rise of temperature, the chemical potential changes in the same manner. 
By this an important analogy between the action of light and heat is established. 
Since the point of equilibrium of a system depends upon the values of the chemical 
potentials of its components, it is evident that a variation in the potentials under the 
action of light makes the system strive towards a new point of equilibrium, i.e., a 
reaction takes place in it tending to bring it to this point. As to the direction in 
which the equilibrium will be shifted by light, this is given by my generalisation : 
“ Each kind of equilibrium between two states of matter is, at a constant volume, on 
exposure to light, shifted in the direction accompanied by greater absorption of light,” 
a principle analogous to that put up by van’t Hoff for heat (see ‘ Zeitschr. fur 
Physik. Cbeniie/ 1902, p. 332). 
Velocity of Reaction and Equilibrium. 
Let us take the system : M in light, solution MA (a salt of metal M, where A is 
the acid radical), M in the dark, e.g., Ag plate in light, AgN0 3 in light and in the 
dark, Ag plate in the dark. Here the two electrodes consist of the same metal, one 
is exposed to light, the other is in the dark. Both are immersed in a solution of 
a salt of the metal forming the electrodes. This system is reversible in light in 
respect of the cation and is constant. 
The chemical system taken in all details consists of:— 
— 4 - 
Ag electrode in light Ag electrode in dark 
-P — + — + + 
Ag ions sat. sol. + N0 3 , Ag sol. in light . . . + N0 3 , Ag sol. in dark + Ag ions sat. sol. in dark. 
