Fluorescence of Iodine and Mercury, 315 



the affinity for electrons increases for these gases in the order 

 mentioned. It is not a proof of the theory, however, for the 

 molecular weights increase in the same order. If we accept 

 Lorentz's hypothesis that a damping results from collisions, 

 we can assume that the damping factor is a function of the 

 molecular weight, and we have no means of knowing whether 

 a second factor, the affinity for electrons, is superposed on it. 

 The results previously obtained appeared to point towards 

 the existence of some other factor however. 



In the present paper we shall show that gases which 

 interfere with the motions of the free electrons, by their 

 affinity for them, interfere as well with the motions of the 

 bound electrons, the vibrations of which give rise to the 

 spectral lines. 



It may here be pointed out that Pohl and Pringsheim 

 have come to the same conclusion with regard to the natural 

 frequency and damping of the resonance electrons in the 

 selective photo-electric effect. 



To settle the question in the case of the emission of 

 fluorescent light we must compare a heavy gas of small or 

 zero affinity for electrons with a light gas of strong affinity. 

 We have therefore examined the influence of helium, argon, 

 nitrogen, oxygen, and chlorine upon the fluorescence. 



The method was identical with that described in the 

 preceding paper, except that the light from the crater of a 

 right-angle arc lamp was used in place of sunlight, which 

 was not available on account of unfavourable weather 

 conditions. An image of the arc was projected upon the 

 wall of the room by means of a lens, the position of the 

 carbons giving maximum illumination ascertained, and their 

 outlines on the wall marked with a pencil. By frequent 

 hand regulation they were kept always in the same relative 

 position. 



The helium gas, carefully purified, was furnished by the 

 firm of Siemens and Halske, through the courtesy of 

 Dr. Holm. The argon was prepared in the Institute. The 

 nitrogen was obtained from commercial bombs, and was not 

 very pure, and the chlorine and oxygen prepared by heating 

 gold chloride and potassium permanganate respectively. 



The results are given in the curves (Plate III.), in which 

 the ordinates are the intensities of the iodine fluorescence and 

 abscissae the pressures in mms. of the admixed gases. The 

 values obtained in the previous work with air, hydrogen, C0 2 , 

 and ether are given on the same plate. Check observations 

 were also made in the present work, with air, io make 

 sure that the results were comparable with the earlier ones. 



Y2 



