354 DR. MEYER WILDERMAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
(2) The gram-atom of Cl ions passes from the solution in light to the solution in 
the dark, falling from the higher chemical potential to the lower, through a deduction 
period. 
(3) The gram-atom of Cl, ions of the lower chemical potential separates from the 
solution in the dark upon the Ag-ClAg plate in the dark. 
And, conversely, if the current is sent in the opposite direction, the opposite reaction 
takes place, the system returning to its initial state. The mechanism of the reaction 
is here similar to that observed for the first system, with the only difference that the 
anions play here the same part as the cation in the first. 
We have next to see whether such reversible heterogeneous systems are also con¬ 
stant cells. In a paper “ On Chemical Statics and Dynamics of Galvanic Cells, ’ which 
will soon be sent for publication, the author puts forward the following generalisation 
as a condition for a galvanic cell to be “ truly reversible and constant ” : “ That a 
galvanic cell should be reversibly constant, the chemical composition of the hetero¬ 
geneous system constituting the same must, while the current is passing through it, 
remain constant; for this all substances in solution taking part in the chemical 
reaction under the action of the current (whether passing in the normal direction or 
in the opposite) and those which result from this reaction must not be allowed either 
to increase or decrease while the current is passing.” This, it is shown, “ can be 
achieved in ordinary galvanic cells if the substances taking part in and resulting 
from the reaction are all taken in a saturated solution in contact with their solids so 
as to keep the concentrations of the substances in solution each at the constant 
point of saturation, preventing thus the formation under the action of the current 
of an over-saturated or unsaturated solution of any of them. The nature and 
composition of an heterogeneous system being independent of the amount of the 
solid present in the same, all we require is evidently to keep the composition of 
the solution constant.” The same principle, which is more of an axiomatic nature, 
must evidently be applied here : “ That a galvanic cell should under the action of 
light be reversibly constant, the composition of the heterogeneous system, while 
the current is passing through it or is sent in the opposite direction, must remain 
constant.” 
In the first combination, the Ag plate in light and the Agj plate in dark remain 
under the action of the current the same, whether more or less Ag ions pass from a 
plate into the solution or separate on it from the solution. When one gram-atom 
of Ag passes from one electrode into the solution, a gram-atom of Ag separates on 
the other electrode. Thus the total amount of nitrate of silver in solution, whether 
saturated or not, also remains the same. A little reflection shows also that the above 
system must soon reach in light a state when the ratio of Ag x ions to Ag and of 
N0 31 to N0 3 ions will remain constant. The composition of the homogeneous liquid 
part, and with it of the whole of the heterogeneous systems, will thus remain under 
