the Cause and Nature of Radioactivity . 379 



was tested at regular intervals to determine the rate of 

 recovery of its activity. For this purpose the original speci- 

 men of *5 gram was left undisturbed throughout the whole 

 series of measurements on the plate over which it had been 

 sifted, and was compared always with *5 gram of ordinary 

 de-emanated thorium oxide spread similarly on a second plate 

 and also left undisturbed. The emanation from the hydroxide 

 was prevented from interfering with the results by a speeial 



arrangement for drawing a current of air over it during the 

 © © © 



measurements. 



The active filtrate from the preparation was concentrated 

 and made up to 100 c.c. volume. One quarter was evaporated 

 to dryness and the ammonium nitrate expelled by ignition in 

 a platinum dish, and the radioactivity of the residue tested at 

 the same intervals as the hydroxide to determine the rate of 

 decay of its activity. The comparison in this case was a 

 standard sample of uranium oxide kept undisturbed on a 

 metal plate, which repeated work has shown to be a perfectly 

 constant source of radiation. The remainder of the nitrate 

 was used for other experiments. 



The following table gives an example of one of a numerous 

 series of observations made with different preparations at 

 different times. The maximum value obtained by the 

 hydroxide and the original value of the ThX are taken 



as 100:— 







Time in davs. 



Activity of Hvdroxide. 



Activity of TkX. 







44 



100 



1 



37 



117 



2 



48 



100 



3 



54 



88 



4 



62 



72 



5 



6S 





6 



71 



53 



8 



78 





9 



. 



29-5 



10 



83 



25-2 



13 



... 



15-2 



15 



... 



11-1 



17 



96-5 





21 



99 





28 



100 





Fig. 2 shows the curves obtained by plotting the radio- 

 activities as ordinates. and the time in days as abscissas. 

 Curve II. illustrate- the rate of recovery of the activity of 

 thorium, curve I. the rate of decav of activity of ThX. It 



2 C 2 * 



