2 RUSS, Note on Radio-active Recoil. 



has been to find the relative numbers of Radium A, 

 Radium B, and Radium C particles radiated away from 

 the condensed Emanation, by an analysis of the decay 

 curves of the active deposit collected on the aforesaid 

 disc for different times of exposure. 



The method of experiment consisted in condensing 

 the Emanation from a few milligrammes of Radium at 

 the bottom of a glass tube by means of liquid air. After 

 waiting three hours, by which time the Emanation is in 

 equilibrium with its products Radium A, Radium B, and 

 Radium C, a disc was suspended about 7 cms. above the 

 condensed Emanation, and the system evacuated to a 

 pressure of about i/ioo mm. After a suitable exposure 

 the disc was removed and the decay of its activity tested 

 by means of an electrometer. During the course of these 

 experiments, a short account of which is given in the 

 paper referred to, the times of exposure of the disc were 

 10 minutes, 40 minutes, and 3 hours respectively, but for 

 the purposes of this analysis attention will be confined to 

 the 10 minutes and the 3 hours exposures. This selection 

 has been made owing to the comparative simplicity of 

 the analytical expressions under these conditions, at the 

 expense of no essential features of the phenomena. 



The decay curve obtained for an exposure of 10 

 minutes is marked I in Fig. i, and it may at once be 

 seen, by inspection of its marked variation from curve 

 Ia, which is the well-known curve due to 10 minutes' 

 exposure to the Emanation, that a considerable amount 

 of Radium B was projected on to the disc as well as 

 Radium A, a result which might have been anticipated. 



Curve II shows the decay curve obtained when the 

 exposure was 3 hours, and its deviation from the ordinary 



3 hours exposure curve IIa is still more marked than for 

 the shorter exposure. 



