MON OF rilANV.KS IN KADIOACTTVE I'.MIHKS. 



191 



given by an equation of the same form (see equation 12, section 13) as for thorium 



and actinium, viz., 



| e -v_ e -v 



I T == e -*.T - e-* ' 



where I T is the maximum activity observed, which is reached after au interval T. 

 Since the activity finally decays according to an exponential law (half value in 

 28 minutes), one of the values of X is equal to 4'13 X 10~*. As in the case of thorium 

 and actinium, the experimental curves do not allow us to settle whether this value 

 of X is to be given to Xj or X 3 . From other data (see section 20) it will be shown 

 later that it must refer to X 3 . Thus Xg = 4'13 X KT 4 (sec)" 1 . 



The experimental curve agrees very closely with theory if \. 2 = 3*10 X 10~ 4 (sec)" 1 . 



The agreement between theory and experiment is shown by the table given l>elow. 

 The maximum value I T (which is taken as 100) is reached at a time T = 36 minutes. 



In order to obtain the /8-ray curve, the following procedure was adopted. A layer 

 of thin aluminium was placed inside a glass tube, which was then exhausted. A large 

 quantity of radium emanation was then suddenly introduced by opening a stopcock 

 communicating with the emanation vessel, which was at atmospheric pressure. The 

 emanation was left in the tube for 1'5 minutes and then was rapidly swept out by a 

 current of air. The aluminium was then removed and was placed under an 

 electroscope, such as is shown in fig. 3. The a rays from the aluminium were cut off 

 by an interposed screen of aluminium '1 mi Him. thick. The time was reckoned from 

 a period of 45 seconds after the introduction of the emanation. 



is thus a t airly good agreement between the calculated and observed values 

 of the activity measured by the /8 rays. 



The results are thus satisfactorily explained if it is supposed : 



(1) That the change B into C (half transformed in 21 minutes) does not give rise 



to /8 rays ; 



(2) That the change (' into I ) (half transformed in 28 minutes) gives rise to ft rays. 



