636 Mr. H. G. J. Moseley and Dr. K. Fajans on 



The curve drawn has been calculated on the assumption that 

 the recoil product consists entirely of actinium D, and 

 the agreement between theory and experiment is seen to be 

 satisfactory. 



It is to be remarked that the recoil experiments of Mile. 

 Blanquies were complicated by the presence in the recoil 

 product of some actinium B. We, on the contrary, always 

 obtained by recoil, either from an active plate or from a fine 

 wire, actinium D alone. 



The presence of a product of short life would also be 

 shown by a rapid initial fall of activity on removing the 

 active source. The rise of activity on the disk was measured 

 as before, until, after 15" 7 minutes, the maximum was 

 reached. The active plate was then earthed, and the activity 

 on the disk was found to decay exponentially with the period 

 of actinium D, the activity, immediately after the plate 

 was earthed, being found by extrapolation to be the same as 

 immediately before. 



In the light of these experiments, the observation of 

 Geiger and Marsden * that 10 per cent, of the a particles 

 from actinium active deposit are emitted in pairs, seems only 

 to be explained by the existence after actinium C of a side- 

 branch product of very short life. Observations have been 



made with a small ionization vessel, and the disk rotating at 

 high speed, in which such a product should have been detected 



had its period been greater than 10 ~ 4 second. 



A short description will now be given of a simple method 



by which products obtained by recoil may be analysed, and 



products of short life detected. The method is also suited to 



the examination of the radiation from such products. 



The bottom of an electroscope is closed by a sheet of lead, 



in which is a large hole covered over with aluminium leaf. 



The active plate, the source of the recoil, is fixed, active side 



downwards, to the lead sheet. 



Any ionization produced in the electroscope is due to 



7 radiation, and to /3 radiation reflected into the opening 



from surrounding objects. 



Directly underneath the opening and in full view of the 



active plate is placed a small insulated metal plate, or reflector. 



This reflector being charged to a high negative potential by 



an influence machine, attracts to itself the product recoiling 



from the active plate, and the radiation from this product 



enters the electroscope. 



Using actinium active deposit as the source of activity the 



results shown in fig. 3 were obtained. 



* Loc. tit. 



