oj the u Particles from- Radium C and Thorium C. 549 



variations in the number of electrons encountered by the 

 a particle along its path. 



Thus it does not appear probable that these electronic 

 variations would give rise to a " hump " on the ionization 

 curve. On the contrary, the flattening at the extreme end of 

 the range seems to be the necessary and sufficient result of 

 such variations. 



It should be clearly understood that only the small portion 

 of the ionization curve near the extreme end is accounted for 

 on this view. For the sake of clearness, the extremities of 

 this portion are indicated on fig. 3 by the letters A and B. 

 That larger portion of the ionization curve lying between B 

 and C has been shown by these experiments to be a straight 

 line. It should be noticed that the slopes of these lines for 

 RaC, ThOx, and ThC 2 agree within the experimental error, 

 their mean being 3°48. This information regarding the BC 

 portion of the curve is purely empirical, and a theoretical 

 interpretation on the basis of our present views on a-ray 

 scattering seems difficult. Further discussion about these 

 points is reserved for the present, but it is hoped to return to 

 them in a future paper. 



§ 7. The Ranges of the x Particles. 



We are now in a position to consider the ranges of the 

 three types of u particles studied. The range is generally 

 understood to be the distance from the source at which the 

 ionization due to a parallel bundle of a. rays becomes zero. 

 From both the present observed and calculated values of the 

 ionization current, it will be seen that this current seems to 

 fall to zero in an asymptotic manner. It is thus a matter of 

 uncertainty to fix accurately on a point which may be called 

 the end of the range. This point, moreover, would be fixed 

 differently by different observers, depending on the sensi- 

 tivity of their apparatus. 



Furthermore, on the theory outlined above, this " asym- 

 ptotic " end of the curve is not due to all the a particles in the 

 bundle, but to the exceptional u particles which by chance 

 penetrate farther than their fellows. The point at which the 

 straight portion of the curve cuts the x axis when extra- 

 polated would appear on this view to represent the " range " 

 of the a particle after eliminating this extraneous probability 

 effect. In any comparison of theory with experimental 

 evidence, it would seem more justifiable to use the experi- 

 mental values of the " extrapolated range" rather than those 

 of the range as ordinarily understood. Moreover, this 



