the Cause and Nature of Radioactivity. 



389 



■exciting activity on the latter, and that already excited will 

 decay spontaneously. The experiment was therefore per- 

 formed. A quantity of nitrate was precipitated as hydroxide 

 in the usual way to remove ThX, the precipitate redissolved 

 in nitric acid, and again precipitated after a certain interval. 

 From time to time a portion of the hydroxide was removed 

 and its radioactivity tested. In this way the thorium was 

 precipitated in all 23 times in a period of 9 days, and the 

 radioactivity reduced to a constant minimum. The following- 

 table shows the results : — 



Activity of Hydroxide, 

 per cent. 



After first precipitation 46 



After precipitations at three intervals of 



24 hours 39 



At three more intervals each of 24 hours, 



and three more each of 8 hours 22 



At three more each of 8 hours 24 



At six more each of 4 hours 25 



The constant minimum thus attained — about 25 per cent, 

 of the original activity — is thus about 21 per cent, below 

 that obtained by two successive precipitations without in- 

 terval, which has been shown to remove all the ThX separable 

 by the process. The rate of recovery of this 23 times pre- 

 cipitated hydroxide was then measured (fig. 4). It will be 



Fig-. 4. 



IOC 

























80 











-1^ 



^ 



















/ 



/ 















60 









/ 



















/ 





















40 





/ 





















/ 























zo 



►> 





















































J"ime 



jnDoj, 



5_ 













( 



3 



1 



■ 



2 





/ 



? 



1 



& 



Z 







teen that it is now quite normal, and the initial drop cha- 

 racteristic uof the ordinary curve is quite absent. It is in 



