412 Mr. Grill on the Intensity of the Ultra-Violet Light 



It is scarcely too much to say that all the phenomena con- 

 nected with the transmission o£ X-rays through matter may 

 be readily explained in terms of a few simple laws expressed 

 with reference to these spectra. 



[[ wish to express my indebtedness to Mr. G. H. Marty n 

 for his assistance in some of these experiments. — 0. G. B.] 



XL. The Intensity of the Ultra-Violet Light emitted by an 

 Electrical Discharge at Loic Pressures. By E. W. B. Gill,*. 

 ALA., B.Sc, Fellow of Merton College, Oxford"". 



THE phenomena accompanying the discharge of electricity 

 through gases at low pressures have been examined in 

 great detail by many observers, and accurate measurements 

 have been made of the distribution of electric force along 

 the discharge, the number of ions at various points, the 

 temperature and other properties. 



The experiments here described are a set of measurements 

 of a different nature relating to the distribution of intensity 

 of the light emitted^ or more strictly of the intensity of a 

 certain portion of the spectrum of the light emitted from 

 various parts of the discbarge, The results obtained, though 

 not throwing much light upon the actual mechanism of the 

 discharge, are of some interest as showing a systematic 

 variation with change of pressure of rather a curious kind. 



When ultra-violet light falls upon a metal plate, zinc being 

 usually used, negative ions are set free at its surface; and 

 Griffiths has shown that, other things being equal, the 

 number of ions set free is nearly proportional to the intensity 

 of the light, 



An estimate of the number of ions set free by the light 

 coming from different portions of the tube will therefore 

 give at once the relative intensities. A previous research f 

 has shown that the rays chiefly instrumental in setting free 

 the ions vary from about 250 /jl/jl to 180 /jl/ul, and it must be 

 understood that the results of these experiments refer to the 

 intensity of this particular range of wave-lengths. 



Description of Apparatus. 



The discharge-tube was constructed of quartz, and was 

 joined to a large glass reservoir containing mercury in its 



* Communicated bv Prof. J. S. ToWnseiicL 

 f Phil. Mag. Feb. 1910, p. 290. 



