>l'i\ OF CIIAN<;i;s IN RADIOACTIVE BODIKS. 



175 



P. COBIE and DANNK* recently determined the curves of decay of the excited activity 

 of radium for different times of exposure. A discussion of their results will be 

 given later. 



The shaj>e of the decay curves of the excited activity of radium depends not only 

 on the time of exposure, but also on which of the types of radiation is used as a means 

 of measurement. The curves of decay of excited activity are quite different for the 

 a and for the /? rays. 



In the experiments a radium solution was placed in a closed glass vessel (see fig. 2, A). 

 The emanation then collected in the air space above the solution. The rod to be 

 made active was introduced through an opening in the stopper and exposed in the 



Rod to be 



active. 



CZ". 



Radium 

 3 Solution. 



Flgz A. 



Electrometer. 



Fig. 2. 



presence of he emanation for a definite interval. For experiments on the excited 

 activity of thorium, some highly emanating thorium hydroxide was placed inside a 

 closed cylinder (see tig. 2, B). The central rod, which was to l>e made active, was 

 connected to the negative pole of a battery of about 100 volts. If the decay of 

 activity was to be measured by the a rays, the rod was made the central electrode in 

 a cylindrical vessel (ser tig. '_', C). A saturating voltage was applied, and the current 

 between the cylinders measured by means of an eJectrometer with a suitable capacity 

 in parallel. A current of dust-free air was continuously circulated through the 

 cylinder in order to remove any emanation which might have adhered to the wire. For 

 experiments on the ft or y rays it was found advisable to use an electroscope instead 



' 'Complex Rendus,' 136, p. 364, 1903. 



