L 314 ] 



XXXVIII. r ±he Influence upon the Fluorescence of Iodine and 

 Mercury of Gases -with different Affinities for Electrons. 

 By J. Franck and R. W. Wood *. 



[Plate III.] 



WARBURG t has shown that in nitrogen, helium, argon, 

 and hydrogen, which have been very carefully freed 

 from all traces of oxygen, the current obtained with the 

 negative point discharge is much greater than when traces of 

 oxygen are present. To explain this circumstance he made 

 the hypothesis that in the pure gases the negatively charged 

 electrons move with a higher velocity. Small traces of 

 oxygen, by condensation on the electrons, increase their mass 

 and reduce their velocity. 



Many other phenomena of the discharge of electricity 

 through gases are influenced by traces of oxygen J. 



Finally J. Franck made direct measurements of the mobility 

 of the electrons in argon, nitrogen, and helium (in the latter 

 case in collaboration with Gr. Grehloff — unpublished ), and 

 showed that small traces of more or less electro- negative 

 gases operated in the manner assumed by Warburg. 



The affinities for electrons — i. e., the forces acting between 

 neutral molecules and electrons — decrease as we proceed from 

 strongly electro-negative to the inert gases argon, helium, 

 &c. In the latter the forces appear to be nil. The possible 

 existence of a relation between the affinities for electrons and 

 an effect upon the emission of spectrum lines has been 

 suggested in the paper referred to. 



For the investigation of such a possible effect the 

 fluorescence of iodine and mercury is especially adapted, for 

 the excitation is caused by a single factor only, namely the 

 impact of light- waves, which is uninfluenced by the ad- 

 mixture of other gases, whereas in the case of electrical 

 excitation, potential gradient, current strength, and density 

 are all affected by small traces of other gases. 



The reduction in the intensity and the final destruction of the 

 fluorescence of iodine vapour by the presence of other gases 

 has been investigated by Wood and described in the pre- 

 ceding paper. 



Hydrogen showed the least influence, and taken in 

 increasing degree, air, G0 2 , and ether vapour. 



This sequence is not opposed to the above hypothesis, for 



* Communicated by the Authors. 



f Warburg, Wied. Ami. vol. xl. p. 1 (1896). 



| See Summary by J. Franck, Verh. derUeut. Phys. Ges., July 1910. 



