151 P;*of. Rutherford and Mr. Soddy on the 



§ 4. The Influence of the Emanation on the Radioactivity 

 of Radium. 



The converse of the changes that occur when a solid radium 

 preparation is dissolved in water has now to be considered. 

 One of the earliest facts observed in connexion with radium 

 was the steady increase of its activity after preparation 

 (Giesel, Wied. Ann, vi. a p. 91, 1899). Consider the case 

 of a preparation of radium that has been kept for some 

 time in solution in the open air and then evaporated to 

 dryness. The emanation that before escaped is now occluded, 

 and the gradual accumulation of the emanation and of the 

 excited activity it produces causes a gradual increase of the 

 activity of the preparation until a maximum is reached some 

 weeks after. A solid compound of radium, on the other 

 hand, that is dissolved and then immediately evaporated to 

 dryness, loses its occluded emanation, but retains the excited 

 activity that the latter has produced. Hence in this case 

 there will occur a fairly rapid decrease at first as the excited 

 activity decays, followed after a few hours by a slow increase 

 as before, due to the production and occlusion of fresh 

 emanation. 



Again, apart from the consideration of the possible exist- 

 ence of RaX analogous to ThX, the general analogy to 

 uranium and thorium would lead us to expect that the 

 removal of the emanation and excited activity will not 

 entirely remove the radioactivity. A certain proportion of 

 the total, constituting a non-separable activity, will remain. 



These considerations are borne out by experiment. Radium 

 chloride was dissolved in water and a current of air aspirated 

 through the solution. After a few hours the radioactivity of 

 the salt obtained from the solution was found to have been 

 reduced to a minimum : and longer aspiration over three 

 weeks did not affect it. This is the non-separable activity. 

 The solutions were evaporated to dryness, and the course of 

 the recovery of the activity observed over a period o£ three 

 weeks, after which the activity remained constant, at about 

 four times the original value. 



The following table expresses the results : — 



Time in Days. 



Activity. 



-rercenrage 

 Activity Recovered. 







25-0 







0-70 



33-7 



11-7 



T77 



12-7 



23-7 



1-75 



68-5 



58-0 



7-83 



83-5 



78-0 



16-0 



96-0 



95-0 



21-0 



1000 



100-0 



