﻿the Active Deposit of Radium in an Electric Field. 727 



Xo indication of any decided effect upon the distribution 

 of the activity was obtained by subjecting the gas throughout 

 the exposure to the influence of a strong source of Rontgen 

 rays. In this experiment the vessel contained emanation in 

 air at a pressure of 3J atmospheres ; the applied positive 

 potential was 1100 volts ; the equilibrium rate with the 

 added cnpacitv was 5 mm. per sec, and with the Rontgen 

 rays acting was 25 mm. per sec. The percentage cathode 

 activity was 56'2, which is of the order that might have been 

 expected from the previous results. 



7. Discussion of Experimental Results. 



In the interpretation of the experimental results which 

 have been obtained in connexion with the distribution of 

 activity the question arises immediately: what is the reason 

 for the difficulty in saturating the cathode activity? Even 

 under the most favourable conditions which have been em- 

 ployed in our work there is still about 10 per cent, of the 

 activity deposited on the walls of the testing vessel. In an 

 attempt to answer this question we are at once led to the 

 corresponding problem in connexion with the ionization 

 current which passes through the gas during the activation 

 of the electrodes. Mention has already been made of the 

 researches of Bragg, Kleeman, Moulin, and Wheelock in 

 connexion with this aspect of the problem. Bragg explained 

 the difficulty of saturating the ionization due to a particles 

 by introducing the conception of initial recombination ; viz. 

 that the electron which is expelled in the process of ionization 

 returns in a number of cases to its parent atom, and excep- 

 tionally strong electric fields are needed to exercise any 

 appreciable preventive effect upon this tendency to recombine. 

 Kleeman extended the experimental work, using Bragg's 

 theory as a basis. Moulin ascribed the difficulty of obtaining 

 saturation to the fact that the a particle ionized in columns, 

 so that the density of ionization is not uniform throughout 

 the gas when sufficiently small volumes of gas are considered. 

 This localization of the ions would naturally result in a re- 

 combination more intense than that which would correspond 

 to a uniform distribution in the usual acceptation of the term. 

 Moulin showed that saturation appeared to be most difficult 

 when the columns were parallel to the lines of force of the 

 electric field ; when, however, the a particles moved across 

 the lines the ionization tended more readily to saturation. 

 Wheelock continued the work, adopting the idea of columnar 

 ionization ; in particular, he showed that when the pressure 



