REVERSIBLE AND IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 353 
When the current passes in the normal direction the reaction in such a system 
evidently consists of the following :—- 
Ag electrode in light 
J 
+ — + 
Ag ions sat. sol. in light + NO 31 , Agx sol. in dark 
+ 
Agj electrode in dark 
A 
+ — + 
Ag, NO 31 sol. in dark + Ag! ions sat. sol. in dark, 
i.e., under the action of light there is a movement of the Ag ions from the plate 
in light to the plate in the dark. This disturbs the equilibrium, and a reaction 
takes place under the action of light, consisting of: (1) the transformation of 
+ 
1 gram-atom of Ag in light into Ag ions of the higher chemical potential in light ; 
+ 
(2) the separation of 1 gram-atom of Ag 3 ions of the lower chemical potential in the 
dark upon the Ag, plate in the dark; (3) the transformation in the dark of 1 gram- 
+ # + 
atom of Ag ions of higher potential into 1 gram-atom of Ag x ions of lower potential 
passing a deduction period. The first two reactions take place between the different 
parts of the heterogeneous systems, i.e., between the electrodes and their ions in 
solution, as in all galvanic cells. The last takes place in the homogeneous liquid part. 
If now we send a current through the system in the direction opposite to that of 
the current produced by light, while one of the plates is exposed to light and the 
other is kept in the dark as before, the same reaction in the opposite direction takes 
place. If the opposite E.M.F. is equal to the E.M.F. produced by light, no current 
will pass through the system ; if it is greater we shall be able to reverse the reaction 
which previously went on in the light. 
The system is therefore a reversible one in respect of the cation. 
Similar is the case with the system M (metal) covered with the solid salt MA in 
light, M'A solution (salt of the same anion as MA) in light and in the dark, M covered 
with the solid MA in the dark, e.g., Ag-AgCl in light, NaCl solution in light, NaCl 
solution in the dark, Ag-ClAg in the dark, &c., &c. These are reversible in light in 
respect of the anion and are constant. 
The chemical system taken in all detail consists of— 
Ag-ClAg electrode in light 
Ag-ClAg electrode in light 
+ ~ 
(Ag ions sat. sol. -1- Ag, Cl sat. sol.) in light + Na, Cl solution in light 
*k — + — + 
Na, Cl sol. in dark + (Ag, Cl sat. sol. + Ag ions sat. sol.) in dark. 
Such a system is reversible in light. 
When the current is passing in the normal direction :—- 
(1) 1 gram-atom of Cl passes from the Ag-ClAg plate in light into the solution in 
light as Cl ions of the higher chemical potential. 
VOL. CCYI. —A. 2 Z 
