186 



PROFESSOR E. RUTHERFORD <>N 'nil- 



It is found experimentally that the activity 5 or 6 hours after removal decays very 

 approximately according to an exponential law with the time, falling to half value in 

 11 hours. This is the normal rate of decay of thorium for all time of exposure, 

 provided measurements are not begun until several hours after the removal of the 

 active body from the emanation. 



This fixes the value of the constants of one of the changes. Let us assume for the 

 moment that this gives the value of Xj. 



Then 



AJ = 175 X 10-* (sec)- 1 . 



Since the maximum activity is reached after an interval T=220 minutes (see fig. 1, 

 curve C), substituting the values of Xj and T in equation (12), the value of A., comes 



out to be 



\2 = 2-08 X 10-* (sec)" 1 . 



This value of X 2 corresponds to a change in which half the matter is transformed in 

 55 minutes. 



Substituting now the values of X^ X, 2 , T, the equation (12) reduces to 



If /I T = 1'37 (e~* - e-* 1 '). 



The agreement between the results of the theoretical equation and the observed 

 values is shown in the following table : 



It is thus seen that the curve of rise of activity for a short exposure is explained 

 very satisfactorily on the supposition that two changes occur in the deposited matter, 

 of which the first is a rayless change. 



Further data are required in order to fix which of the time constants of the changes 

 refers to the first change. In order to settle this point, it is necessary to isolate one 

 of the products of the changes and to examine the variation of its activity with time. 

 If, for example, a product can be separated whose activity decays to half value in 

 55 minutes, it would show that the second change is the more rapid of the two. 

 Now PEGRAM* has examined the radioactive products obtained by electrolysis of 



'Phys. Rev.,' p. 424, December, 1903. 



