﻿emitted by Thorium and Actinium. 181 



a maximum earlier than the ft ray activity. The reason of 

 this can be simply explained when we take into consideration 

 the fact that thorium X does not emit ft rays*. If the ex- 

 ponential decay curve of thorium X is drawn through the 

 point of initial a, ray activity, this decay curve represents the 

 loss of activity of thorium X and the emanation together. 

 If the ordinates of the ft ray curve are reduced in the ratio 

 of 1 : 2, such a curve represents the variation of the a ray 

 activity due to the active deposit alone. If these two curves 

 are superimposed, we obtain the observed curve of the varia- 

 tion of the a ray activity. It is easy to see without calculation 

 that the maximum of this curve is reached earlier than that 

 of the ft ray carve. 



A few remarks may be added regarding the period of decay 

 of thorium X. In the original experiments made by Ruther- 

 ford and Soddy, the time required for the activity of thorium 

 X to decay to half value was found to be about four days. 

 v. Lerch finds this period to be 3'637 days. In the above 

 experiments, the period deduced was ,'V()7 days for the a. ray 

 activity, and 3*66 days for the ft ray activity. In these ex- 

 periments with radiothorium, a residual activity was observed 

 after the thorium X had completely lost its activity. Sub- 

 tracting this residual activity, the logarithms of the activity 

 due to thorium X were found to form a straight line. This 

 residual activity was found to be due to radiothorium and a 

 trace of radium, and in accounting for this the values given 

 above should be slightly diminished. The average of some 

 other experiments, including a recovery curve of thorium, 

 gave 3' 65 days as the period. These numbers are thus in 

 good agreement with the value given by v. Lerch. 



Thorium. 



In order to obtain thorium free from thorium X and its 

 later products, the following procedure was adopted. The 

 thorium was precipitated 8 to 10 times by ammonia in order 

 to completely remove the thorium X. The precipitated 

 thorium was then heated to remove the active deposit, which 

 is not soluble in ammonia. For this, the temperature of the 

 blowpipe was not sufficient, so recourse was had to a blast 

 furnace. The a and ft ray activities of the thorium were 

 then examined. Both the u and ft ray activities initially 

 diminished a few per cent., showing that the active deposit 

 had not been completely volatilized. After passing through 

 a minimum, the activities steadily increased, reaching a 



* For the following, cf. Rutherford, ' Radioactivity,' 1905, chap. ix. 



