231 Wheelock — Nature of Ionization Prodivced hy a Rays. 



ionized by /3 and 7 rays. Those ioiiiziiio; agents seem to effect a 

 complete separation of the ne»;ative ions from their parent mole- 

 cules, lie has shown that lack of satnration for weak ionization 

 with a rays is not due to diffusion of ions, nor does it depend 

 upon a, the coefficient of recombination. " Initial Recombi- 

 nation" is greater the slower the velocity of the ionizing a 

 particle, which prohably means, the slower the velocity of the 

 a particle the less the velocit}' of the ejected electron. 



Moulin* has interpreted the lack of saturation on the hypoth- 

 esis suggested, by Langevin, that in the case of ionization by 

 a rays, where the number of ions pi-oduced by each a particle 

 is very great, tlie ions produced are not at all uniformly disti-ib- 

 uted in the volume of the gas, but are distributed, at least 

 initially, in columns along the trajectory of the a particle. " 



The ordinary recombination obtained between ions of the 

 same column ought to exceed many times that which would be 

 obtained for the same number of ions distributed through- 

 out the volume. According to this hypothesis, recombination 

 should be greater when the field is applied along the path of 

 the a particle than when it is at right angles to the path. The 

 parallel field would not break up the columns, while tlie per- 

 pendicular field would produce a separation of the positive and 

 negative ions, by breaking each column up into two. 



Moulin obtained saturation at about 200 volts per cm. when 

 the field was applied at right angles to the path, while from 

 1200 to 1500 -volts per cm. were necessary for the parallel field. 

 He also found that saturation was more easily obtained when 

 the pressure was decreased. 



In a second paperf Moulin describes some experiments using 

 COj and Hj instead of air, and ol)tained results which confirm 

 those of his previous paper. He also applied a field inclined 

 at an angle of 45° to the path of the a particle and obtained 

 curves which were in good agreement with those obtained by 

 calculation. 



Moulin therefore concludes that the "Initial Recombination" 

 effect is negligible in comparison with that of general recombi- 

 nation (a n') within the columns. 



For volume ionization, general recombination depends upon 

 N, the number of ions per c.c. If the ions are formed in 

 columns, the density of ionization within the columns would 

 be very great, since the ions would occupy only a very small 

 fraction of the total volume of the chamber. 



It would be expected, therefore, on this hypothesis, that re- 

 combination would be different at different parts of the range 

 of the a particle, being greatest where the maximum number 

 of ions is produced by the a particle. 



* Ionization of Gas by a rays and Hypothesis of Initial Eecombination, Le 

 Eadium, May, 1908. f Comptes Eendus, June, 1909. 



