122 Mr. J. S. Townsend. TJie Diffusion of Ions produced 



a manner as to make the density of each part of the negative the same 

 on development, then all objects photographed with such a screen 

 interposed on similar plates will be rendered in proper gradations of 

 light and shade regardless of their colour or colours. 



" The Diffusion of Ions produced in Air by the Action, of a Radio- 

 active Substance, Ultra-violet Light, and Point Discharges." 

 By JOHN S. TOWNSEND, M.A., Clerk Maxwell Student, 

 Cavendish Laboratory, Fellow of Trinity College, Cambridge. 

 Communicated by Professor J. J. THOMSON, F.R.S. Received 

 May 17, Read June 14, 1900. 



(Abstract ) 



The researches described in this paper form a continuation of those 

 published on the Diffusion of Ions into Gases.* The latter paper 

 gives the results of experiments made with ions produced in air, 

 oxygen, hydrogen, and carbonic acid by the action of Rontgen rays. 

 The gases in these experiments were at atmospheric pressure. 



The present paper contains similar investigations for ions produced 

 in air at various pressures by the action of a radio-active substance, 

 and also determinations of the rate of diffusion of ions produced in 

 air at atmospheric pressure by the action of ultra-violet light and 

 point discharges. 



The principle of the method consists in calculating the coefficient of 

 diffusion from observations on the loss of conductivity of a gas as it 

 passes along metal tubing. The experiments were arranged so that in 

 all cases the loss of conductivity due to diffusion should be much 

 greater than the loss due to other causes, so that it was not necessary 

 to apply any corrections for losses arising from recombination or from 

 the mutual repulsion of the ions. 



The results of the experiments are given in the following tables. 

 Tables I, II, III, and IV give the coefficients of diffusion, K, of positive 

 and negative ions in dry and moist air at various pressures, P, the 

 ionization being produced by the action of a radio-active substance. 

 The temperature of the air during each experiment is given in the 

 column 6. 



These tables show that in each case the rate of diffusion of ions into 

 a gas is inversely proportional to the pressure of the gas. 



The coefficients of diffusion at 772 mm. show a discrepancy from 

 this law, which is somewhat greater than the probable error of the 

 experiments, but we should not expect a closer agreement between the 

 products P x K unless the temperature of the air was the same in 



' Phil. Trans.,' A, ?oL 193, pp. 129158. 



