﻿Resonance Radiation of Mercury Vapour. 1101> 



that the actual pressure of the mercury vapour also influences 

 the percentage of polarization, for it was found that it fell 

 rapidly if any other gas was admitted to the tube, helium 

 at 6 mms. pressure destroying practically all traces of 

 polarization. In cases where the vapour was excited by 

 polarized light (electric vector vertical as seen from the 

 camera) the visibility of the fringes indicated nearly 

 complete polarization. AVith unpolarized excitation the 

 visibility was somewhat less, though the fringes were still 

 very distinct. 



These results show that in no case is the polarization 

 complete, for if it were, the visibility of the fringes would 

 be the same with unpolarized as with polarized excitation. 



The fact that the fringes are more distinct when the density 

 of the mercury vapour is lowered by refrigeration of one end 

 of the tube, indicates that molecular impacts probably have 

 something to do with the depolarization. Admixture of other 

 gases at low pressures reduces the fringe visibility, and 

 finally destroys all trace of polarization, though the 

 intensity of the resonance radiation may be quite unimpaired. 



Air at 1 cm. pressure practically destroys the polarization 

 while it is still pronounced in hydrogen at the same pressure. 

 Curiously enough, however, the intensity of the resonance 

 radiation of mercury in hydrogen is less than half of its 

 value in air. This is a marked exception to the law which 

 was found by Wood and Franck, in the case of iodine 

 fluorescence, namely that gases diminished the intensity in 

 proportion to their electro-negative character, air being 

 much more destructive of the fluorescence than hydrogen. 



The results obtained with air, hydrogen^ argon, and helium 

 are given in the following table. 



Intensity of Polarization 

 Resonance Ead. fringes. 



Air 065 mm 2"5 strong. 



Hydrogen 065 mm 1" strong. 



Air 4 mm , 1' nearly gone. 



Hydrogen 4 mm 0*5 faint. 



Air 1cm 0*5 gone entirely. 



Hydrogen 1cm 02 a trace. 



Air 5 mm 3' a trace. 



Argon 5 mm 10' gone. 



Argon 3 mm 5' faint. 



Helium 2 mm 4 - faint. 



Helium 6mm 10" 



gone. 



