﻿214 



Mr. L. Vegard on the Properties of 



The theoretical and experimental curve is given in fig. 1, 

 which is taken from Greiger's paper. 



Fig:. 1. 



2.4 6 



Range tucms of air 



From observations of the absorption in various substances 

 Bragg and Kleeman found that the masses of strata of equal 

 absorption are proportional to the square root of the atomic 

 weights. 



Suppose we have a homogeneous layer of matter consisting 

 of elements of atomic weight A x A 2 ... A„, then a stratum of 

 total mass M per unit area has an absorbing power equivalent 

 to a mass W of air given by the expression 





M'=3 



+ 



Va 2 + 



+ 



k*)*. 



VA 



(o) 



where I : is the mass of the element A x for each unit of the 

 total mass of the mixture. 



H. Greiger * measured what he called the most probable 

 scattering angle (A.) for the a-rays, and found 



X = K 0V /AX, 



where X is the air equivalent traversed, and A the atomic 

 weight This result, when interpreted, shows that on an 

 average the at- particle will follow very nearly a straight Jine, 

 the orbits will most of them only be slightly curved near the 

 point where the particle stops, and consequently the track 

 followed by a pencil of initially parallel rays will only very 

 slightly broaden out from the track followed provided no 

 scattering took place. 



* H. Geiger, Proc. Roy. Sue. A. lxxxiii. p. 492 (1910). 



