372 DR. MEYER WILDERMAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
prove, however, conclusively that when, as in our case, a weak source of light is 
judiciously employed, the variation in the AgBr salts due to tnis effect is so small 
that the same dehectioils are always obtained. But when after the same illumination 
the two measurements are separated not by short intervals only, but by many hours, 
a Great increase of the E.M.F. obtained is first observed. Similar results were 
obtained with other systems, also with black Cu-CuO plates in a solution of NaOH, 
i.e., where no darkening of the electrodes could come into consideration. On the 
other hand, no such increase of sensitiveness was found with the Ag-AgBr plate of 
the system which was kept the whole time in the dark. It follows from this that we 
meet here with a peculiar phenomenon which we may call “ continuous light sensi- 
bilisation in the darky The light previously absorbed continues to produce some 
variation in the plate in the dark : it either continues first to form Ag 2 Br, Cu 2 0 
(Ag 3 Br gives, according to Becquerel, a greater E.M.F. than AgBr), and then 
continues to form Ag from Ag 2 0, Cu from Cu 2 0 (which give smaller E.M.F.’s), or it 
produces some other physical variations in the crystals of AgBr or CuO (it is known, 
e.g., that the process of “ripening” of emulsions in the dark is accompanied by an 
increase of size of the crystals of BrAg). 
We must give our attention to curves N9 and NlO, obtained on July 31, with 
normal NaBr solution : here, on the top (at b), we notice for the first time the 
formation of a very small 'polarisation, though its presence is not yet to be seen at 
the end of C, when the deduction period begins. This suggests that, as in the case 
of Ag-ClAg in NaCl solution, under the repeated exposure to light the transformation 
of AgBr or Ag 2 Br into Ag has gone far enough towards the formation of the system 
(Ag plate in light, NaBr solution, Ag-BrAg plate in the dark) which is in light an 
inconstant cell. 
V. Influence of Temperature upon the E.M.F. obtained in Cells Reversible in Respect 
of the Anion (Table V.). 
Some modifications had to be made in the method. It was found impossible to 
determine the influence of temperature upon the E.M.F. obtained with the above 
plates up to 40° or 50° C., as before, since the paraffin and the shellac split oh from 
the plates. I covered for this reason the plates on all sides with a (thick) coating 
of AgBr. 
In all the experiments the whole of the front surface of the front plate was always 
exposed to light, while its back and the two surfaces of the back 
plate remained always in the dark. 
Since the quartz vessel, for which Crookes’ cement has to be used, 
could not be well employed for higher temperatures, vessel fig. 1 was 
used instead. ( a ) represents a very thin and very clear round hat side, 
about 5 centims. diameter, battened down with the flame in the thin 
bulb. The front Ag-BrAg plate was about 5 millims. from the hat side. 
