380 Prof. E. Rutherford and Mr. F. Soddy on 



will be seen that neither of the curves is regular for the first 

 two days. The activity of the hydroxide at first actually 



diminished and was at the same value after two days as when 

 first prepared. The activity of the ThX, on the other hand, 

 at first increases and does not begin to fall below the original 

 value till after the lapse of two days (compare section IX.). 

 These results cannot be ascribed to errors of measurement, 

 for they have been regularly observed whenever similar 

 preparations have been tested. The activity of the residue 

 obtained from thorium oxide by the second method of wash- 

 ing decayed very similarly to that of ThX, as shown by the 

 above curve. 



If for present purposes the initial periods of the curve are 

 disregarded and the later portions only considered, it will be 

 seen at once that the time taken for the hydroxide to recover 

 one half of its lost activity is about equal to the time taken 

 by the ThX to lose half its activity, viz., in each case about 

 4 days, and speaking generally the percentage proportion of 

 the lost activity regained by the hydroxide over any given 

 interval is approximately equal to the percentage proportion 

 of the activity lost by the ThX during the same interval. If 

 the recovery curve is produced backwards in the normal 

 direction to cut the vertical axis, it will be seen to do so at a 



