the Ions in Gases at Low Pressures. 



559 



in the gas through which the light passes; these are distinct 

 from those produced near a metal plate on which the light falls, 

 and they can produce a cloud with very little supersaturation; 

 these nuclei are not ions, for they do not move in an electric 

 field, and the drops formed round these nuclei ought there- 

 fore not to be counted in estimating the number of negative 

 ions. For this reason it is necessary to use ultra-violet light 



Fig. 2. 



i! 



of small intensity, and there are in addition other reasons 

 which make it impossible to work with strong light. I found 

 when working with the ions produced by Rontgen rays, that 

 it was impossible to get good results unless the rays were 

 weak and the clouds therefore thin. If the rays were strong, 

 one expansion was not sufficient to bring down all the ions 

 by the cloud; sometimes as many as five or six expansions 

 were required to remove the ions from the vessel. Another 



