278 ME, J. S. TOWNSEND ON THE DIFFUSION OF IONS PEODUCED IN AIK. 
has shown that if the light acts on moist air for some minutes, a cloud begins to 
appear. This effect must be very small in the present experiments, but it may 
account for the difference between the rates of diffusion of ions produced by Rontgen 
rays and ultra-violet light. 
In conclusion, I must express my thanks to Professor Thomson for many valuable 
suggestions in connection with this research. 
