382 DR. MEYER WILDERMAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
region. Only where the solution pressure of the plate increases in light essentially, 
so that a comparatively great E.M. F. is created, can we realise, under proper 
conditions, constant reversible cells, e.g., in case of Ag-ClAg, Ag-BrAg in CINa, BrNa 
solution, or Cu-CuO in NaOH solution. As, however, the E.M.F. created by light 
is always very small, special care will always have to be taken for a corresponding 
reduction of the other interfering E.M.F.’s, to an extent not known for ordinary 
galvanic cells : (1) the E.M.F. in the dark must first be reduced by special polishing 
of the plates, by mechanical means and with the current, to a few hundred 
thousandths of a volt. On such plates usually, but not always, also very little 
gas separates while the current is passing between the plates under the action of 
light. Only very few metals allow such a polishing. The rougher the grain of the 
metal or the thicker the deposited compound is, the more difficult is the realisation 
of the same. (2) Only chemically pure metals or compounds should be used as 
electrodes. This can be realised in case of Ag, Pt, Au, Hg, Ag-ClAg, AgBrAg, 
Ag-l-Ag, Hg with insoluble Hg salts, and perhaps in one or two more cases, but it is a 
matter of quite extraordinary difficulty in case of other metals. (3) The currents 
used should be made, as corresponds to this region, very small by inserting, when 
possible, resistances into the circuit, and using a very sensitive galvanometer. 
In addition to the above conditions, which are common for all galvanic cells, a 
series of further conditions and possibilities is never to be lost sight of, which are 
important for this region only, such as the formation of thermo E.M.F.’s in the 
circuit, the variation of the sensitiveness of the plates to light in the dark, the effect 
of previous illumination, the character of the induction and deduction periods, the 
transformation of one system into another, &c., &c., which can be mastered only after 
a detailed and careful study of each system separately is made. 
XV. On the E.M.F.’s of Constant Reversible Cells and the Intensity of Light. 
Becquerel found with Ag-Ag 2 Cl plates in a solution of 2 grammes of S0 4 H 2 in 
100 grammes water (‘La Lumiere,’ vol. II., p. 145), that the E.M.F. is not directly 
proportional to the intensity of light, since the product I.H. is not constant. The 
result obtained by Minchin with his sensitised cell, consisting of Sn plates covered 
(evidently) with Sn0 2 in methyl-alcohol, is different. The E.M.F. was “ with fair 
accuracy found to be inversely proportional to the distance ” of the candle from the 
cell, i.e., the intensity is directly proportional to the square root of the intensity 
of light. 
Contrary to these observations, I found that with constant reversible cells (i.e., cells 
the E.M.F. of which gives the true measure of the maximum work performed under 
the action of light) the E.M.F. is directly proportional to the intensity of light. 
The reasons of this discrepancy are : 
(1) The system Ag-Ag 2 Cl in a solution of S0 4 H 2 and the system Sn-Sn0 2 in 
