544 Mr. G. H. Henderson on the Range and Ionization 



For thorium C 2 the equation of the line is 



1=3-43(8 167-^), ..... (2) 



while for thorium Q x the equation is 



1 = 1-96 (4-529-a?) 

 or 



1 = 3-47 (4-529 -a?), (3) 



if the maximum of this curve be taken as unity. 



The observations do not fit the straight lines at the very 

 end of the range, but spread out so as to approach the x axis 

 in an asymptotic manner. Before discussing this character- 

 istic spreading out of the curve at more length, one or two 

 minor points about the general form of the ionization curve 

 may be noticed. 



The general form of the ionization curve found for 

 radium C may be compared with the dotted curve in fig. 2, 

 which is obtained from one given by Geiger * with two 

 modifications. Geiger's scale of abscissae has been reduced 

 to 0° C, and his scale of ordinates altered in such a way 

 as to make the total areas under the two curves equal, 

 which simply refers the two curves to the same number 

 of a particles. Remembering that the ionization chamber 

 in Geiger's experiments was filled with hydrogen, it could 

 not be expected that the two curves would agree. A 

 figure given by Taylor f shows the ionization curves for 

 polonium a. rays in atmospheres of air and hydrogen. 

 Although a strict comparison is not justified for several 

 reasons, it may be seen that the relative shape of the 

 two curves given bv Taylor is very similar to that shown in 



In the light of Taylor's experiments it was pointed out 

 by Geiger \ that his value for the total number of ions 

 produced by one a particle might be in error. The doubt 

 fell on the ratio between the number of ions produced in 

 the first *39 cm. of the range in air, and the total number 

 of ions produced, which Geiger measured in hydrogen. 

 In the present experiments measurements have been con- 

 tinued down to *9 cm. from the source and the initial 

 portion of the curve extrapolated. If this portion be taken 

 as correct, then since the total area under each curve in 

 fig. 2 is the same and since the initial *39 cm. of the two 



* Geiger, Proc. Roy. Soc. A, lxxxii. p. 486 (1909). 

 t Taylor, Amer. Journ. Sci. xxviii. p. 357 (1909) ; Phil. Mag. xxi. 

 p. 571 (1911). 



X Geiger, Proc. Roy. Soc. A. lxxxiii. p. 505 (1910). 



