ELECTRICAL PROPERTIES OF FLUORESCENT SOLUTIONS. 



151 



while screening the electrodes. In the latter case the effect was observed, 

 as in the experiments already described. In the former ease only a very 

 small change could be detected. 



The other experiments with eosin that bear upon the question of polari- 

 zation were of an entirely different character, and represent one of our 

 many early attempts to devise a satisfactory method of attacking the 

 general problem. Fig. 150 shows both the form of tube used to contain 

 the solution and a diagram of the connections. The platinum-wire elec- 

 trodes a, b were connected to the terminals of the secondary of an induction 

 coil, /, and to the two pairs of quadrants of an electrometer, E. The elec- 

 trode C at the center of the tube was connected to the needle. The 

 primary of the induction coil was excited by an alternating current of 120 

 cycles. When the solution was not illuminated the electrometer needle 

 stood nearly at its zero. Under these circumstances illumination of the 

 upper half of the tube pro- 

 duced a deflection in one 

 direction, while the illumina- 

 tion of the lower half gave an 

 approximately equal deflec- 

 tion in the opposite direction. 

 The direction of the deflection 

 in each case was such as to 

 indicate a decrease of resist- 

 ance. The spectrum of the 

 arc was used in these experi- 

 ments as in those made by the 

 bridge method; and the evi- 

 dence that the effect was due 

 to fluorescence, and not to rise 

 of temperature, was as con- 

 clusive as in the experiments 

 previously described. The 

 method w r as less sensitive, 

 however, and was therefore used only in the case of eosin. 



Shortly after the announcement of the results just described, 1 Camichel, 2 

 in a paper in the Journal de Physique, reported a series of measurements 

 with entirely negative results, and suggested that our precautions against 

 heating were insufficient and that the apparent change of conductivity 

 might be thus explained. It seemed desirable, therefore, particularly as 

 our experiments had been merely preliminary and prospective, to investi- 

 gate further. Dr. Percy Hodge, at our suggestion, took the matter in hand 

 and it was found, as will appear from the summary of his work given in the 

 second part of this chapter and from a later paper by Goldman, 3 that the 

 effects of light were much more complicated than had been suspected. 



Fig. 150. 



Nichols and Merritt. Physical Review, xix, p. 415, 1904. 

 2 Camichel, Journal de Physique, IV, 1905. 

 'Goldman, Annalen der Physik, vn, p. 322, 1908. 



