574 Mr. T. S. Tavlor on the Ionization of Different 



respectively. The dotted portion of each curve in figs. 1 

 and 2 is assumed to be the form it would take were it 

 possible to move the polonium entirely up to the ionization 

 chamber. At any rate, such assumed portions of the curves 

 can differ but little from the actual curves. It is to be noted, 

 that the ionization curves shown in figs. 1 and 2 are plotted 

 differently from the regular Bragg ionization curve in that 

 the values of ionization are taken as ordinates and distances 

 of the source of rays from the chamber as abscissse, instead 

 of vice versa as is usually done. 



Although the curves in figs. 1, 2, and 3 represent some 

 differences from one another in regard to the relative amounts 

 of ionization for corresponding distances of the source of 

 rays from the ionization chamber, all of them are of the 

 same general form. From a re-determination of the velocity 

 of the alpha particle at different points in its path, and the 

 assumption that the ionization produced at any point in the 

 path of the particle is proportional to the energy consumed. 

 Geiger* has shown that the ionization I at any point in the 

 path is given by the relation 



1= c __ 



(>_ i?; .)l/3> 



where c and r are constants and x is the distance from the 

 source of rays. By comparing this theoretical ionization 

 curve with the experimental curve obtained in hydrogen for 

 a pencil of rays, Geiger found the two to agree very 

 closely. 



This theoretical curve has been compared with the experi- 

 mental curves obtained in each of the gases and vapours 

 given in Table I., and a very close agreement between 

 theoretical and experimental curves was found for each gas. 

 To make this comparison, it was necessary to determine the 

 constants r and c for each gas. For the value of r, Geiger 

 used the average range of the alpha particles in the pencil 

 of rays. Since the maximum range of the alpha particles 

 in the cone of rays used in the present experiments was 

 always 11*1 centimetres, the average range of the alpha 

 particles in this cone of rays emerging from the cylinder 

 containing the polonium was slightly less than 11* L centi- 

 metres. Consequently 10*8 centimetres was taken as the 

 value of the average range of the alpha particle, that is 

 10*8 centimetres is supposed to represent the average dis- 

 tance the alpha particles travelled in each gas before losing 



* True. Royal Society, Series A, vol. Ixxxiii. no. A 365, p. 505. 



