66 THE ROYAL SOCIETY OF CANADA 
assumed that the same proportion holds for 8 and for y rays in passing 
through air, it follows that 
L'/-004 = - 103/13 
5040000008? 
This is less than the result obtained from the value of #//D found 
by McClelland,* namely .034, which leads to a value 
pv’ = -000044 
Soddy and Russell’s** value for #"/D gives a still larger result, so 
that until further light is obtained, the intermediate value -000044 
may be adopted. 
Just as when the radioactive constant is À, the average life is 1/2, 
so we may say that when the coefficient of absorption is 1, the average 
distance reached is 1/#. But it will be remembered that the distance 
is measured ‘as the crow flies,” whilst the actual path is zig-zag. 
If in a parallel pencil of § rays, N, electrons start their flight then 
at a distance x, only N remain effective for ionization, so that —dN 
have arange between x and x + dx. 
Hence 2 
—xzdN/N, 
N, 
is the average distance attained from the 
source. 
It follows that the average range is the area below the curve, di- 
vided by No, or 
Ne on on cite 
Thus, the f particles reach an average distance from the source 
in ar 7 -004, or 2-5 metres. 
If it is correct to regard a y ray as an entity, with a linear path, 
then its average range is the inverse of -00004, or 250 metres. 
If the 8 rays were ejected all with the same velocity, the above 
statement would have a conciseness which is lacking in the case of the 
complex rays from radium C, 
The writer proposes to employ the method above described to 
determine the value of y for the ? rays from radium E, and for Réntgen 
rays. 
*° Phil. Mag. Aug. 1904. 
*3 Phil. Mag., Oct., 1909. 
