REVERSIBLE ANI) IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 355 
tliG fiction of the current constant. Similar is evidently the case with the second type 
of cell, reversible in respect of the anion. 
The cells produced by the action of light differ from ordinary constant reversible 
cells as follows :— 
(1) The cell is a galvanic combination only so long as it is exposed to light, so long 
as the chemical potentials at the two electrodes are kept different by light, while an 
ordinary galvanic cell is a permanent galvanic combination depending upon the 
permanent differences of the chemical potential at the two electrodes. 
(2) Y\ e have here periods of induction before the constant maximum E.M.F. is 
reached, and also a period of deduction, on removal of light. 
(3) The reactions under the action of the current are always accompanied by the 
phenomena of induction and deduction also when the maximum E.M F. is reached. 
(4) The “sensitiveness” of the electrodes to light does not always remain the 
same (see § 7). 
(5) The E.M.F. of such a combination, and the value of the solution tension and 
of the chemical potential, are functions of the intensity, composition of light, of the 
amount of light absorbed under different physical conditions by the electrode, &c. 
It remains now to consider whether there is a connection between the constancy of 
the above reversible systems in light and Gibbs’ rule of phases. If we take the 
system reversible in respect of the anion, p. 353, 
N = Ag, Cl, Na, FLO = 4, m — Ag, ClAg, and solution = 3, 
if AgCl and solution in light and in the dark are regarded each as one phase, but 
m — 5 if AgCl and the solution in light and in the dark are regarded as in two 
different phases. In this case m = n+\, the system is one of complete equilibrium, 
and such combinations ought to be constant. 
Similar is the case with the cells reversible in respect of the cation (p. 352) 
n = (Ag, N0 3 , H 2 0) = 3, while m = 2 if the same substance in light and in the dark 
is regarded as one phase ; but m = 4 = n +1 if it is regarded as two phases. 
Before passing to the account of the experiments made, I should like to state that 
all curves obtained during the first year and a-half were rejected, and that when it 
was thought that all possible errors in the methods, &c., were removed to justify 
complete confidence in the results obtained, all previous experiments were repeated 
again. Some of the more important results were investigated, after this a second 
time, after an interval of a year or more, for greater security. 
§ 6 . Experimental Results obtained with Constant Cells reversible in respect 
of the Cation. 
(a) The system Ag plate in light , AgN0 3 solution in light , AgN0 3 solution in the 
the dark , Ag plate in dark.— Plates of the 3rd and 4th, 5th, 6th, 8th, 11th, (18th), 
19th (a), 19th (b), 19th (c) of June, 1903, under Nl, 2, 3, 4, 5, 6, 7, 8, 9, and Table I., 
9 7 Q 
Li Li Li 
