154 STUDIES IN LUMINESCENCE. 



between zero and 20 degrees of not more than 1 .5 per cent per degree. The 

 effect of the light was also tried while the cell was in ice water and found 

 to be apparently about as large as at room temperature. These tests 

 seemed to preclude the possibility of any considerable portion of the effect 

 being attributable to heat. An additional reason why the effect should 

 not be attributed to heat was mentioned on page 149 of this chapter, namely, 

 the fact that the change of conductivity took place within an exceedingly 

 short time after the light was thrown on the cell, and after the light was 

 shut off the cell immediately resumed its former state. This fact was con- 

 firmed in the course of these experiments, the time required for the change 

 to take place being apparently considerably less than a second, and the 

 decay of the effect taking about the same time. 



A series of tests was then made with the same cell and solution, but using 

 four gravity cells in series as the source of E.M.F. A difference of effect 

 from that obtained with two cells was expected, as with two cells only a 

 very small current could possibly have passed through the cell, since the 

 applied E.M.F. was not equal to the E.M.F. of polarization, while with 

 four volts the current flowing was of considerable magnitude. The effect 

 anticipated was one similar to the first but of smaller magnitude. When 

 the light was thrown on the cell, however, the surprising result was obtained 

 of a large increase of resistance instead of a decrease as in the former experi- 

 ments. The effects were produced as promptly as those of the preceding 

 experiments and died away nearly as quickly. 



Here was something quite unexpected. If the remarkable decrease of 

 resistance observed in the first experiments was due to electrons set free by 

 ionization of the solution accompanying the phenomenon of fluorescence 

 it seemed highly improbable that an increase of resistance would be pro- 

 duced by light under any conditions whatever. Further, both effects were 

 entirely too large to be satisfactorily accounted for by any theory anal- 

 ogous to that applied to the phenomena of ionization in gases ; phenomena 

 which it might be supposed would very probably accompany fluorescence. 



Again, if the decrease of resistance in the first case were due to ionization 

 of the nature of that produced in gases, it should be greater in dilute solu- 

 tions than in concentrated, since, owing to the greater penetration of the 

 light into a dilute solution the number of molecules affected would be as 

 great as in a concentrated solution, while, owing to the greater distance 

 between the molecules, the mean free path of the ions would be greater and 

 therefore recombination less rapid. A number of experiments were made 

 by diluting the saturated solution with two, four, and six parts by volume of 

 alcohol, and the effects were found to be greatly diminished, so that even in 

 the solution with two parts of alcohol there was a change of not more than 

 1 per cent in resistance. 



In order to vary the conditions it was thought worth while to try next 

 a cell in which, as in Regner's experiments, the liquid could be kept moving 

 past the electrodes while being illuminated. All possible heat effects 

 would be in this way eliminated, and some light might be thrown upon the 

 nature of the phenomena which would not be brought out when the liquid 

 was at rest. A cell was therefore prepared in the following manner, again 

 copy the designing of one described on page 150. 



