﻿emitted hy Thorium and Actinium. 



179 



platinum dish and heated with a blowpipe. As soon as the 

 platinum plate was cold the /3 ray activity was measured. 

 The thorium X was placed beneath a Wilson electroscope, 

 the bottom of which was covered with an aluminium plate of 

 O06 mm. thickness. This is sufficient to absorb all a rays. 

 The a activity was measured with a Dolezalek electrometer. 

 The results of one experiment are shown in fig. 1. In 



the figure the abscissa? denote the time in days reckoned 

 from the end of the heating, the ordinates the activities, the 

 maximum activities being taken as 100. The shape of the 

 ^ and ft curves is very similar. The /3 ray activity is 

 initially only 3*5 per cent, of the maximum value reached 

 one day later. The a ray activity, on the other hand, has an 

 initial value of 60 per cent. The rapid increase of the a. 

 and j3 ray activity is to be expected since the thorium X 

 produces the emanation, which in turn is transformed into 

 the active deposit. A maximum is reached when the active 

 deposit is in approximate equilibrium with the thorium X. 

 The ci and & ray activity then decay exponentially according 

 to the period of thorium X, i. e. the activity falls to half 

 value in about 4 days. The character of these curves is in 

 close agreement with that to be expected from the theory 

 of successive changes. 



The above experiment was made with a preparation of 

 thorium X from radiothorium, kindly provided by Dr. Hahn. 

 Other experiments made with thorium X separated from 

 commercial thorium behaved in a similar manner. The ex- 

 periments are, however, more simply made with the thorium 

 X separated from radiothorium. The thorium X separated 



X 2 



