﻿194 



Mr. A. S. Eve on the Radioactive Matter 



the radiation from the limited quantity o£ emanation which 

 escapes from it. It is not unreasonable to expect a much 

 larger effect from the 7 rays of the radium (and its successive 

 products) in the earth than from the 7 rays due to the emana- 

 tion (or its quick transformation products) found in the 

 atmosphere. The question may be investigated quantitatively. 

 Before doing so it is well to note the rate of absorption of 

 the 7 rays under different conditions. 



Table II. 



Substance. 



Density. 

 11-6 



A. 



Thickness in cms. 

 to half value. 



Lead 



•5 



1-4 



Earth 



27 



•092 



7'5 



Water 



10 



034 



20-4 



Air 



•0013 



•000044 



15700 



In this table the value of X for water is taken from 

 McClelland' s results*, and the values of \ for air and for the 

 constituents of the upper crusts of the earth are deduced 

 from the density law which he established. It is assumed 

 that 2' 7 is a fair average value to take for the density of the 

 constituents of the upper crust of the earth. It follows that 

 the 7 rays are cut down to one per cent, after traversing 

 half a metre of the earth's crust, or 1000 metres of the 

 atmosphere. 



The Amount of Radium in the Earth estimated by the 

 Penetrating Radiation. 



We have seen that the number of ions produced by the 

 penetrating radiation near the earth's surface is found from 

 H. L. Cooke's experiments with a brass vessel to be about 

 4*5 per c.c. per second. If a small hollow cavity were taken 

 many metres below the earth's surface, we should expect the 

 ionization due to the penetrating radiation to be twice as 

 great, because rays would come from above and below, not 

 from below only as on the earth's surface. If ^( = 9) be 

 the number of ions which would be produced per c.c. per 

 second in a brass vessel, supposed to be within such a cavity, 

 by the 7 rays from the active matter in the earth surrounding- 

 it ; if Q / be the equivalent number of grams of radium 

 bromide which is a measure of the active matter per c.c. in 

 the earth's crust ; then, by a calculation precisely similar to 



* Phil. Mag. July 1904. 



