REVERSIBLE AND IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 371 
All the above results show that the E.M.F. of galvanic combinations reversible 
in respect of the anion is independent of the concentration of the solution. No 
correction for the thermo E.M.F.’s was considered necessary to employ, in view of 
experimental errors here. 
IV. The Effect of Previous Illumination upon the Increase of Sensitiveness of the 
System in the Dark. 
On July 28 we exposed the Ag-BrAg plate 3 times to the light of the acetylene 
at the great distance of 116 centims. Remaining then about 16 hours in the 
dark, the sensitiveness of the plates increased from 61 to 69'2 millims., i.e., by 
about 0'5 millim. (07 per cent.) per hour. It was then exposed 9 times on the 29th 
(Plate a) to very weak acetylene light at 116 centims. distance. After this it 
increased in about 4 hours by about 3 millims., i.e., 07 millim. (0’9 per cent.) per 
hour. Then we exposed the plates 7 times on the same day (Plate b) to acetylene, 
and 8 times to the more powerful arc; during the next 16 hours in the dark the 
sensitiveness also increased more rapidly, namely, from 83 to 114 millims., i.e., by 
about 1*5 per cent, per hour (July 30). 
The system was then exposed to light 6 times. After the system remained 
further 22 hours in the dark we found on further exposure to acetylene (on July 31) 
only a small increase, if any; the maximum of this increase could be from about 
113-8 to 
123x 885 
878 
- = 124 millims., i.e., 
10’2 millims. (or 0 - 47 per cent.) per hour (com¬ 
pare a with a'). The system was then exposed to acetylene 6 times, again at a 
distance of 116 centims., and left in the dark about 18 hours, till August 1. On 
exposure to light only a very small increase, if any, was found ; the maximum increase 
was from 96 to 105, i.e., 0'5 millim. (or 0 - 5 per cent.) per hour (compare c' and c"). 
If we assume that the E.M.F. is independent of the concentration, {c") will mean 
105 x 977 3 
8777 
= 117 millims. for the same total resistance in the circuit of 877'7 ohms, 
i.e., there is a decrease from 124 to 117 millims., i.e., a decrease of 0’4 millim., or 
0*4 per cent., per hour. I conclude from this, as well as from numerous other 
observations to be dealt with later on, that the influence of previous illumination upon 
the subsequent increase of sensitiveness in the dark is the greatest at the beginning, 
and is gradually diminishing to small values, when the sensitiveness of the system 
seems to become constant. It is possible even that after this a decrease in the 
sensitiveness to light takes place. 
The increase of sensitiveness of Ag salts, till they reach a constant maximum, 
when kept in the dark, after previous illumination, must not be mixed up with the 
ordinary effect of light upon Ag salts, in darkening them. The last naturally increases 
to some extent their capacity of absorption of light. The numerous photographic 
curves obtained on repetition of the same experiment after short time intervals 
3 b 2 
