140 



Prol. W. H. Bragg on the Ionization of 



difficulty due to tlie existence of a peculiar phenomenon, 

 which must now be considered. 



It is to be observed that the ionization curves in different 

 gases will not correspond unless the potential employed is 

 enough to saturate at all points of the path of the a particle. 

 More electric force is required as the particle slows down. 

 This may be deduced from figures given in the paper u On the 

 Eecombination of Ions, &c." p. 196. It is there stated that 

 the ratio of the saturated ionization current to that at 25 volts 

 per cm. in the case of the ions made by the a particle of RaC 

 at a distance of 6*25 cm. from its origin was found to be 1*29; 

 whereas, when the distance was reduced to 5*05 cm., it 

 was found to be 1*19. Each of these ratios is the mean of 

 four determinations. (By an arithmetical error, one of the 

 latter is incorrectly given in the paper quoted : 1*23 should 

 be 1'20.) 



Fier. 3. 



C 2 H s Cl.75% + CZiT 25%. 



I A. 50. voUs to 3 



B lOO •■ 



C 200 •• 



D50O •• 



3| £ Too 



Again, the effect is clearly shown by the curves of fig. 3, 

 which represent the results of experiments on a mixture of 

 ethyl chloride and air. It will be seen that the curve does not 

 show the characteristic increase of ionization with distance 

 when the electric force is small, the reason being that it is so 

 much more difficult to collect the ions made by the a particle 

 at the end of its path. 



It is necessary to refer to one more assumption which is 

 made in calculating the results, viz., that the RI of a mixture 

 can be determined from a knowledge of the HI of each 

 component. For example, it is supposed that the EI of air 

 being 100 and of ethyl chloride 132, then the HI of a mixture 

 in such proportions that the a. particle spends half its energy 

 in each is 116. 



