340 DR. MEYER WILDERMAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
polarisation ; Ag-AgCl plates in water form inconstant cells which exhibit polarisa¬ 
tion and will give a current in one direction or the other. An Ag-AgCl plate placed 
in water containing S0 4 H 2 is, in the first instance, an inconstant cell, showing 
polarisation ; subsequently Ag 2 S0 4 is formed in solution by the acid acting upon the 
silver plates, the system transforming into Ag plates in an Ag 2 S0 4 solution, which is a 
constant cell reversible in respect of the cation, and its current must flow, as will be 
seen later on, from the plate in light to the plate in the dark. This, however, will 
not be the last possible transformation of such a system (Ag-AgCl in S0 4 H 2 solution). 
Since Ag 2 S0 4 is not easily soluble in water, and under the action of light the current 
will pass first from the plate in light to the plate in the dark, S0 4 ions will separate 
on the Ag-AgCl plate in light, forming Ag 2 S0 4 on the same ; the system (Ag-Ag 2 S0 4 
in light, Ag 2 S0 4 solution, Ag-AgCl in the dark) will thus gradually be formed, which 
is reversible in respect of the anion in which the current must flow from the plate in 
the dark to the plate in the light. This now is exactly the combination Becquerel 
used for his actinometer, employing AgCl, AgBr, and AgT, deposited upon Ag plates 
immersed in a solution of “ 2 grammes of sulphuric acid monohydrate in 100 grammes 
of water,” by no means a weak acid. 
Again, in his experiments with AgCl, AgBr, Agl precipitated on platinum or gold 
plates, he puts them into pure water. Such systems must form inconstant cells, 
showing polarisation. 
§ 2. What is the Nature of the E.M.F. Created by Light ? 
Before entering upon a study of the effect of light upon different combinations, the 
very first question to be clearly settled was: Are the phenomena produced by light 
new phenomena, or are they ordinary heat phenomena (since one plate is heated by 
light, the other not) ? 
A calculation on the basis of the quantitative measurements made for the purpose 
showed that if light should heat up the electrodes of different systems to the same 
degree as it heats up the Rubens thermopile, the total deflections which ought to be 
obtained under the action of light with different systems ought to be even greater 
than those actually observed, and the conclusion would become inevitable that the 
E.M.F. created by light may be nothing else but a thermo E.M.F. We take the two 
principal systems: Ag plates in AgN0 3 solution (June 6, 1903) and Ag-BrAg plates 
in NaBr solution (July 29, 1903). 
The first system gave, with the arc at a distance = 27 centims., 5'5 centims. 
deflection ; at the same distance the Rubens thermopile gave 101’6 centims. deflection 
= 1 0, 466 C. The thermo E.M.F. of the system in the dark being O’OOll volt per 
1° C., we ought to have got 16’35 centims. deflection, if the system were also heated 
by the light 1 0- 46G C. Similarly with the second system : the deflection observed 
with acetylene light at a distance = 116 centims. was 8T6 centims. ; the same light 
heated Rubens thermopile at 116 centims. distance 0°T208 C. The thermo E.M.F. 
