i 5 o 



STUDIES IN LUMINESCENCE. 



ductivity. In the case of eosin we were able to follow both effects to the 

 extreme edge of the violet, while illumination by the red of the spectrum 

 produced no effect. 



In order to form an estimate of the magnitude of the change produced 

 we observed in each case the throw caused by increasing or decreasing the 

 resistance A 7 by one ohm. By comparing this with the throw due to 

 illumination, it was possible to express the observed change as a fraction 

 of the normal (apparent) resistance. The results are given in Table 19. 

 The light used to illuminate the solution, in the case of the measurements 

 included in the table, was that which produced the brightest fluorescence. 



We have also tested in the same way one solution that was not fluorescent, 

 namely, an alcoholic solution of fuchsin. The result was entirely negative. 

 No change in resistance due to illumination could be found in any part of 

 the spectrum, although the sensitiveness of the bridge was such that a 

 change of 0.008 per cent could have been detected. The solution was an 

 old one and had probably been made with commercial alcohol, since the 

 resistance, 90,000 ohms, was much lower than that of the other solutions. 

 For this reason we are not inclined to look upon this single experiment with 

 a non-fluorescent solution as possessing much significance. 



Table 19. 

 Increase in electrical conductivity due to fluorescence. 



In the preceding description of the experiments upon fluorescent solu- 

 tions we have ascribed the observed movements of the galvanometer needle 

 to a change in the resistance of the solution. The galvanometer deflections 

 undoubtedly indicate a disturbance in the balance of the bridge; but this 

 disturbance might equally well result from a diminution in the E.M.F. 

 of polarization. Without special modifications the method does not permit 

 a separation of these two effects. In the case of eosin, however, special 

 experiments were made bearing upon this point. 



The experiments referred to were of two kinds. In the first of these a bridge 

 method was employed as before. The solution was contained in a glass tube 

 originally about 5 mm. in diameter. A portion of this tube had been drawn 

 down to a diameter of about 1 mm. and the current passed through the 

 liquid contained in this contracted part. The platinum electrodes were 

 placed in the larger part of the tube at each end. We were thus able (1) 

 to illuminate the solution about either electrode while the remainder of the 

 tube remained dark; or (2) to illuminate the contracted part of the tube 



