476 Prof. R. W. Wood on the 



Transformation of Resonance Spectra by Gases of the 

 Helium Group. 



The introduction of helium into the iodine vacuum-bulb 

 results, as has been pointed out, in the development of the 

 band spectrum which we have when we excite the vapour 

 with white light. 



A photograph of the band spectrum, with the resonance 

 lines superposed, excited by the light of the mercury arc 

 when the iodine vapour is mixed with helium at 5 mm. 

 pressure, is reproduced on PL IV. fig. 5. The band spectrum 

 appears faintly when the helium is at a pressure of a milli- 

 metre or less, and as the helium pressure increases, the 

 intensity of the band spectrum increases, while that of the 

 resonance spectrum decreases, the total amount of emitted 

 light remaining about the same, however. It is still an open 

 question whether both spectra are emitted simultaneously by 

 the same molecule. I am rather doubtful about this, for it 

 seems more probable that when the helium is at a very low 

 pressure, the band spectrum is emitted only by those 

 molecules which at the moment happen to be within the 

 sphere of influence of a helium molecule. It is possible that 

 a momentary combination between a helium atom and one 

 of iodine may exist either normally or under the action of 

 light. Sir J. J. Thomson has found that such momentary 

 combinations exist between atoms in vacuum-tubes developing 

 canal rays, which would be considered impossible by the 

 chemists. It would be extremely interesting to investigate 

 the positive rays excited in a mixture of helium and iodine 

 by Thomson's very beautiful method. 



It appeared to be of the greatest importance to investigate 

 the behaviour of the other gases of the helium group in this 

 respeet, for, as has been shown in a previous investigation 

 by Wood and Franck, the commoner gases, hydrogen, 

 nitrogen, oxygen, etc, are wholly without influence in 

 developing the band spectrum. Thanks to the kindness and 

 interest of Sir William Ramsay and Professor Collie, the 

 gases argon, neon, krypton, and xenon were placed at my 

 disposal. 



We will consider the behaviour of the gases in the order 

 of their molecular weight. 



In helium, with a molecular weight of 4, at a pressure of 

 10 mm., the band spectrum is very strong and there is 

 scarcely a trace of the resonance spectrum. In neon at the 

 same pressure (weight 20) the resonance spectrum is rela- 

 tively much stronger. This is partially in accord with the 

 views expressed in a previous paper by Franck and Wood, 



