376 Active Deposits of Radium, Thorium., and Actinium. 



thus having a large chance of neutralizing itself. Assump- 

 tion (1), besides appearing necessary in order to explain 

 the present results, is a natural deduction from Werten- 

 stein's * observation that RaD recoiling from RaC in very 

 low vacuo is uncharged at the moment of expulsion, and 

 that the rate of acquisition of a positive charge increases 

 with the pressure. 



The ionization potential of the elements increases generally 

 from left to right in the periodic table. From this it is to 

 be expected that the ionization potential of RaA is greater 

 than that of RaB. If a similar relationship holds for 

 ionization by collision with molecules, then the theory 

 given would require that the percentages for RaA should 

 be less than for RaB, as is found in all the gases examined, 

 thougli it. is quite possible that the effect is due to a difference 

 in dissociating power of the two. 



Summary. 



1. Wellish's conclusion that a definite percentage of the 

 active deposit atoms from radium emanation is initially 

 positivel} r charged at the end of the recoil path, the 

 remainder being neutral, has been verified ; and the error 

 in Henderson's experiments, from which he concluded that 

 all the deposit atoms are initially positive, pointed out. 



2. The importance of the part played by the recoil of 

 RaB from the electrodes has been indicated, and the per- 

 centages of RaB initially positively charged in various gases 

 have been measured ; and from this the percentages of RaA 

 positively charged calculated from experiments with radium 

 emanation 



3. An explanation of the difference in distribution on 

 the anode and cathode of the active deposits of radium, 

 thorium, and actinium has been given on the assumption 

 that the percentages of ThA and Ac A and of ThB 

 and AcB initially positive in a gas are identical with 

 those of RaA and RaB respectively, as is to be expected 

 from their isotopic nature. The distribution of thorium 

 active deposit was experimentally determined for eight 

 gases in a parallel plate vessel, and found to be consistent 

 with the theory, as also were Lucian's results for the dis- 

 tribution of actinium active deposit in air in a cylindrical 

 vessel. 



■4. An apparent connexion between the percentage of 

 recoil atoms initially positive in a gas and the temperature 



* Wertenstein, C. R. clxi. p. 696 (1915). 



