ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 53 



rays of longer wave-length which would be likely to produce heat disturb- 

 ances. In the case of the eosin cell the light used was at the infra edge of 

 the green. 



When the cell was first set up great difficulty was experienced in obtaining 

 a balance, the drift toward higher values of the resistance being quite rapid 

 and continuing for several hours. This was at first thought to be due 

 entirely to polarization, but it was found that if the top of the cell was 

 covered so tightly that evaporation was prevented the drift lasted for only 

 a short time, after which a comparatively steady condition was reached, 

 and accurate measurements were possible. At no time was the cell abso- 

 lutely free from a very slow drift, always toward higher resistance, and 

 probably due partly to polarization and partly to a very gradual decom- 

 position of the electrolyte. Evidence of the latter was seen in a very narrow 

 strip of colorless liquid which was observed along the inner edge of the 

 anode after several hours of application of the current. On account of 

 this decomposition most of the tests were made with cells which had been 

 freshly set up, the electrodes being removed and carefully cleaned between 

 tests. 



The method of the experiment was as follows. After the apparatus had 

 been set up and the cell placed in its dark box the slide was raised and the 

 cell so adjusted in the spectrum that the liquid between the electrodes 

 became brilliantly fluorescent. The slide was then dropped, shutting off 

 the light, and a balance was obtained with the bridge. The resistance 

 of R\ (see Fig. 151) was in nearly every case 10,000 ohms, that of Ri was 

 50,000 ohms, and Ri was adjusted to suit the resistance to be measured. 

 As it was impossible to set the electrodes at exactly the same distance from 

 each other when the cell was set up at various times the apparent resistances 

 differed from each other greatly during different sets of readings. Also the 

 polarization E.M.F. in the case of eosin was found to be over two volts. 

 Therefore when two gravity cells were used the apparent resistance was 

 much higher than when four were used. 



After the cell had stood in darkness long enough to obtain a fairly steady 

 condition of the bridge, the slide was suddenly removed and the cell 

 illuminated. 



With two volts as the applied E-M.F. the result was an immediate and 

 very large decrease of resistance, the amount of resistance change required 

 in the variable arm to restore a balance indicating as high as 10 or 15 per 

 cent increase of conductivity. When the light was shut off the cell returned 

 immediately to nearly the same resistance that it had before illumination. 

 The test was repeated many times and on different days, and always with 

 about the same results. As the effect was much larger than any that had 

 been anticipated it seemed advisable to see if it could in any way be due to 

 heating of the liquid. With this end in view the temperature coefficient 

 of the solution was determined in the following manner: A balance was 

 obtained at the temperature of the room. Then the cell was surrounded 

 with ice water and a balance again obtained. Lastly the ice water was 

 drawn off and the cell allowed to regain the temperature of the room. The 

 first and last values of the apparent resistance were in turn subtracted from 

 the second and then averaged. The result showed a temperature coefficient 



