ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 57 



platinum strips between the two grooves when the cell is filled with liquid. 

 The film of liquid between the grooves could be perfectly illuminated, was 

 isolated from the electrodes, and at the same time included a large part of 

 the total resistance. 



Careful tests were made with this cell and the results were entirely nega- 

 tive, though the resistance of the cell was quite low compared with others 

 that had been tried, averaging about 40,000 ohms with saturated solution, 

 and the sensibility of the bridge was as high as 0.005 P er cent. The con- 

 ditions within a few minutes after it was set up were very steady indeed, 

 and a slight change of conductivity would have been readily detected. 



In addition to the saturated solution a solution of five parts alcohol to 

 one of saturated eosin solution was tried with both 2 and 4 volts E. M. F., 

 but likewise with entirely negative results. From the results of these last 

 experiments it was evident that a change of conductivity in the liquid itself, 

 except at or very near the electrodes, did not. take place when fluorescence 

 was produced. 



It had frequently been observed that, however smooth the strips of 

 platinum were when they were placed in the cell, and however tightly they 

 were clamped against the front wall, when the solution was poured in a 

 thin film of liquid immediately crept up between the electrodes and the 

 front wall of the cell. There seemed, therefore, to be two regions in which 

 the effects might be produced, either at the inner edges of the electrodes or 

 in the region in front of the electrodes, or in both places at once. 



From a variety of minor indications which had been noticed from time 

 to time it seemed probable that the main effect was to be looked for in 

 the thin film and not at the edges of the electrodes. In order to test the 

 matter the effect was first tried of placing thin strips of ebonite in front of 

 the electrodes as in one of the previous experiments, but instead of cover- 

 ing the electrodes the ebonite strips were only allowed to cover the extreme 

 outer edges of the platinum strips, thus leaving nearly the whole of the 

 electrodes exposed to the light, but behind a layer of liquid a number of 

 times as thick as the capillary film formed when the electrodes were against 

 the front wall of the cell. The change of conductivity, though still present, 

 was found to be very greatly diminished. 



As mentioned in connection with a previous experiment, the reduction 

 of the effect might easily have been due to the absorption of the light by 

 the thick layer of liquid in front of the electrodes, so that fluorescence did 

 not occur at the surface of the platinum strips. 



A second arrangement was then tried in which the platinum strips were 

 separated from the front wall of the cell by strips of lantern-slide cover- 

 glass a millimeter in thickness. The inner edge of each electrode (see Fig. 

 I 55) was allowed to project slightly beyond the edge of the glass. Thus 

 the capillary film between the electrode and strip of cover-glass could be 

 illuminated while the inner edge of the electrode was completely screened 

 from the light by a thick layer of liquid. When the light was turned on 

 this cell the effects were found to be present in as great a degree as when 

 the whole of the electrode was illuminated. 



Lastly, the platinum strips were again placed against the front wall of the 

 cell and the inner edges covered, while the rest of the electrodes was illumi- 

 nated. Again the effect was present, as was to be expected. 



