Kigli Vacuum-Spectra of Gases. 255 



'higher pressure than this, the effect of the magnetic field is to 

 restore the original spectrum (PI. IV. fig. II., spectra 1, 3). 



The explanation of the increased illumination and resto- 

 ration of the original spectrum seems to be that the magnetic 

 field forces more ions through the gas — from one electrode 

 to the other, — which were previously proceeding direct to the 

 sides of the vessel, and the consequent increase in ionization 

 and in the number of collisions will naturally produce 

 increased illumination as well as restore the conditions for a 

 complex spectrum, as compared with those that obtained 

 before the application of the magnetic field. 



7. Experiments with hydrogen, so far as they have gone, 

 seem to give similar results. It should be noted, however, 

 that, as is well known, spectra of hydrogen generally present 

 features which are difficult to analyse. The results for 

 hydrogen, therefore, have to be tested by further experi- 

 ments before we can be sure of their accuracy. We 

 accordingly withhold the detailed results for hydrogen till 

 these are duly tested. 



8. Various investigators have studied the spectra of electric 

 discharge through gases. These researches relate mostly, 

 however, to discharge under high pressure. Thus, according 

 to W. J. Humphreys, an increase of pressure causes all iso- 

 lated lines to shift towards the red end of the spectrum, but 

 the lines of bands are not apparently shifted. Also, different 

 series of lines of a given element are displaced to dif- 

 ferent extents, but similar lines of an element, though not 

 belonging to any recognized series, are displaced equally, 

 the pressure-shifts of similar elements being periodic func- 

 tions of the atomic weights. In a later paper he suggested 

 that there might be a direct connexion between the pressure- 

 shift and the Zeeman effect ; but R. Rossi, who employed 

 pressures of 15, 30, 50, and even 100 atmospheres, failed to 

 detect any relation between them. 



9. Influence. of pressure upon absorption spectra has been 

 -studied by A. Dufour and others, while in the case of arc- 

 spectra W. Gr. Dufneld has shown that increase of pressure 

 from 1 to 201 atmospheres broadens all silver lines, and 

 J. Barnes has noticed an increase in the intensity of the 

 spark lines of Al and Mg, and a diminution in the number 

 of arc lines of On under reduced pressure. Reference should 

 also be made in this connexion to the single-line spectra of 

 metals Hg, Zn, Cd, studied by Frank and Hertz and 

 McLennan and Henderson. They have shown that when 

 heated vapour of these elements is traversed by electrons 



