Resonance Spectra, of Iodine Vapour. 4z77 



regarding the relation between the affinity of a gas for 

 electrons and its action in destroying fluorescence. According 

 to the investigations of Franck neon has the smallest electron 

 affinity of all the gases, le^s even than that of helium. 

 According to our theory, if a gas has a strong affinity for 

 electrons it destroys completely the emission of all of the 

 fluorescing molecules which at the moment happen to be 

 within the sphere of action of one of the gas molecules, 

 without, however, affecting the nature of the radiation of 

 the iodine molecules which at the moment happen to be out- 

 side of the sphere of action. This means that as the pressure 

 of the gas is increased, more and more of the iodine molecules 

 are within the sphere of action at any given moment and 

 fewer and fewer of them emit light ; the resonance spectrum 

 therefore gradually fades away without any trace of the 

 band spectrum appearing. In the case of a gas having 

 small affinity fur electrons, we find that we can have it 

 present at a relatively high pressure without any great 

 reduction in the total amount of light emitted by the iodine, 

 and that it operates in some manner, the nature of which u e 

 cannot at present surmise, in transferring the energy from 

 the excited system of electrons to all of the other systems 

 present in the molecule. The question now arises as to 

 whether the efficiency of these gases in developing the band 

 spectrum depends upon their molecular weight, or upon 

 some other property. Neon is five times as heavy as helium, 

 yet at the same pressure (10 mm.) the resonance spectrum 

 is relatively much stronger. 



We cannot be quite sure of the exact relations until 

 photometric measurements have been made of the intensity 

 of the resonance spectrum and the band spectrum of iodine 

 in the gases of the helium group at different pressures. 

 In argon, at 6 mm. pressure the total light emitted is much 

 less than in neon and helium at 10 mm., but the resonance 

 spectrum is stronger in proportion to the band spectrum 

 than in the case of neon at 10 mm. In krypton (weight 83), 

 at 1*7 mm. pressure the intensity of the emitted light is 

 reduced to about -J of its original value and the spectroscope 

 shows only the barest trace of the band spectrum, practically 

 all of the light remaining localized in the resonance lines, 

 as is the case with the common gases. In xenon (weight 

 130), at 1*5 mm. pressure the intensity is found reduced to 

 about ^ of its original value with no trace whatever of the 

 band spectrum. Helium, the lightest of these rare gases, 

 thus appears to be the most efficient in developing the band 

 spectrum. I had expected to find neon more efficient than 



