ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 1 49 



cent or more. But the sensitiveness of the arrangement to changes in the 

 resistance of the test cell was high. In order to avoid disturbances due 

 to variations in the polarization E.M.F. it was found necessary to keep both 

 the battery circuit and the galvanometer circuit closed. Even the small 

 change made in N during the adjustment of the balance produced a con- 

 siderable alteration in the polarization, so that the adjustment often had to 

 be continued for an hour or more before the needle of the galvanometer 

 was sufficiently steady for observations to be begun. During this time 

 the cell was protected from the action of light by an opaque screen. When 

 the conditions had become steady, or more frequently when the motion 

 of the galvanometer needle was reduced to a slow uniform drift, the screen 

 was removed and light from an arc was allowed to fall on the cell. After 

 the effect of illumination had been noted, and when the needle had again 

 settled down to a steady drift, the screen was replaced, and the throw of the 

 needle in the opposite direction was observed. 



In the measurement of so small an effect as that here considered it is 

 clear that the heating effect of the rays from an arc is likely to produce 

 serious errors, for the change in conductivity due to rise in temperature is 

 in the same direction as the change that we are attempting to detect. For 

 this reason the use of the direct rays of the arc, even at the distance of a 

 meter, was entirely out of the question. Even when a water cell was inter- 

 posed in the path of the rays the needle was displaced through several 

 hundred divisions. That this movement of the needle was due to rise in 

 temperature, and not to the effect sought, was indicated by the fact that the 

 change persisted after the rays were cut off; it was necessary to wait at 

 least 15 minutes before the original balance was approximately restored. 



In order to avoid the disturbances due to rise in temperature we dispersed 

 the rays from the arc by a prism and used only those portions of the spec- 

 trum that were most effective in producing fluorescence. With this arrange- 

 ment the effects observed were much smaller than before, but they were 

 entirely free from any indication of temperature changes. Upon removing 

 the screen, so as to illuminate that part of the fluorescent solution lying 

 between the electrodes, a throw of the galvanometer needle was observed, 

 and if originally free from drift the needle vibrated about a new position and 

 finally came to rest. Continued illumination of the solution produced no 

 increase in the deflection. Upon replacing the screen a throw in the oppo- 

 site direction occurred, and the needle finally returned to its original reading. 

 If observations were made while the needle was in motion, these throws 

 were simply superposed upon the steady drift. In this case the effect of 

 illumination was measured by the average of the two throws. 



vSo far as we were able to judge, the effect produced by light reached its 

 full value at once and ceased as soon as the light was cut off. The dis- 

 turbance of the balance of the bridge was always such as to indicate an 

 increase in the conductivity of the fluorescent solution. Upon repeating 

 the experiments with rays from different portions of the spectrum it was 

 found that a change in conductivity was produced only by those rays which 

 were able to excite fluorescence in the solution tested. And so far as we 

 could estimate the intensity of fluorescence by the eye, the rays which gave 

 the most intense fluorescence also produced the greatest change in con- 



