﻿192 Mr. A. S. Eve on the Radioactive Matter 



spherical shell of thickness dr the total number of ions 

 per second would be 4zirr 2 dr times as great. Hence N, the 

 total number of ions produced per second in all the sur- 

 rounding air, is given by 



N^f^.-W^- 47 ^. . -. .(3) 

 Jo r X 



But McClelland has shown that the coefficients of absorp- 

 tion of the very penetrating rajs vary approximately as the 

 densities of the absorbing substances. The value of A for 

 water is *034, hence A, for air is about '000044. Thus the 

 7 rays are reduced to half value after penetrating 157 metres 

 of air, and to one per cent, after passing 1000 metres of air. 



From (3) we find the total number of ions per second, due 

 to the 7 rays from 1 gram of pure radium bromide, when the 

 rays are wholly absorbed in air, equal to 



4ttx 3-1x107-000044 

 or about 



S-Vx 10 14 ; (4) 



and this is probably an overestimate of the true value. 



A comparison can therefore be made between the total 

 number of ions, due to the a and 7 rays respectively, pro- 

 duced per second by a gram of radium bromide, when all the 

 rays are absorbed in air. Professor Rutherford has found 

 that the a rays from one gram of radium bromide in radio- 

 active equilibrium produce about 1*24 x 10 16 ions per second if 

 wholly absorbed in air. By an independent method the writer 

 found about 1*67 x 10 16 ions per second due to the same 

 cause. Hence the 7 rays appear to produce less than one- 

 sixteenth part of the total number produced by the a rays. 

 Without insisting on the exact value of this ratio, it is clearly 

 seen that all the ions due to the a rays of a given mass of 

 radium are more numerous to a considerable extent than all 

 the ions due to the 7 rays. And this might be expected, 

 inasmuch as the heating effect of the 7 rays is only a small 

 percentage of that of the a rays. 



On the Io7is due to the 7 Rays from the Emanation in the 

 Atmosphere. 



In a recent paper * in this Magazine it has been shown 



that the equivalent amount of radium bromide which would 



be required to provide the emanation actually present in one 



cubic kilometre of the atmosphere near the earth's surface is 



* Eve, Phil. Mag. July 1905. 



