156 STUDIES IN LUMINESCENCE. 



sibility of the bridge as used in these tests was at least 0.0 1 per cent. There- 

 fore it seems fair to conclude that there could not have been any appreciable 

 effect due to electrons set free at the instant fluorescence began in any part 

 of the liquid, unless this effect required a considerable time to make itself 

 known. 



Attention was next given to the effects at the two electrodes separately 

 and to the liquid between them, to find out, if possible, just where the phe- 

 nomena took place which caused the changes in conductivity. 



As nothing but negative results were to be obtained from a circulation 

 cell the original type of cell was again adopted, and in the front of the 

 box D of Fig. 151 was placed a screen with a vertical slit cut in it about 4 

 mm. wide. By placing the cell behind this screen in such a manner that 

 only one electrode could be seen from the front of the box through the slit, 

 it was possible to observe the effect of illuminating one electrode at a time, 

 the other being in darkness behind the screen. With this arrangement and 

 two gravity cells the effect was tried on anode and kathode in turn. When 

 the anode was exposed the effect was very small, while on the contrary 

 when the kathode was exposed the effect was found to be about as large as 

 when both were exposed. As some of the liquid between the electrodes 

 was illuminated in either case it was not certain whether the effect was pro- 

 duced only at the electrode or whether the liquid at a distance from the 

 electrode also played its part in it. Therefore the question as to what 

 would happen when the electrodes were both covered and the liquid between 

 them exposed had next to be settled. 



To this end several different arrangements of the cell were tried. The 

 first tests were with the same electrodes which had been used in the earlier 

 type of cell, but in front of each of them was placed a thin strip of hard 

 rubber 0.15 mm. thick, projecting just beyond the platinum strip so as to 

 completely screen it from the light, but so as to leave about 80 per cent of 

 the liquid between the electrodes exposed to the light. 



With this arrangement no effect could be obtained by illumination. How- 

 ever, it was felt that the test was not conclusive, for the total thickness of 

 the film of liquid acting as conductor between the electrodes was that of 

 both the electrode and the rubber strip, and since the absorption coefficient 

 of a saturated solution of eosin is very large it was not at all certain that 

 the whole thickness of the layer was illuminated. 



To overcome this difficulty another type of cell was constructed after the 

 following manner: 



A block of hard rubber was sawed of such shape and size as nearly to fit 

 the rectangular cell used in most of the experiments. The block was then 

 clamped to a block of ebonite and with a f -inch drill two holes were bored 

 between the blocks in such a manner as to leave in each block a pair of 

 grooves of semicircular cross-section, and separated from each other by a 

 narrow strip approximately 2 mm. wide. The block that had been sawed 

 to fit the cell was then placed in the cell and two L-shaped electrodes were 

 placed in the grooves in the positions shown in Fig. 155, which illustrates 

 a section of the cell with block and electrodes in place. It will be seen 

 from the figure that the electrodes hold the ebonite block back from the 

 front wall of the cell, so as to leave a film of liquid of the thickness of the 



