VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 181 
filtering the water, and evaporating down, a very active residue 
was obtaied which was analogous in all respects to Th X. 
On examining the products a month later, it was found that 
the Th X was no longer active, while the thorium had completely 
regained its activity. A long series of measurements was then 
undertaken to examine the time-rate of these processes of decay 
and recovery of activity. 
The results are shown graphically in Fig. 34, where the final 
activity of the thorium and the initial activity of the Th X are in 
each case taken as 100. The ordinates represent the activities 












Time in Days 
Fig. 35. 
determined by means of the ionization current, and the abscissae 
represent the time in days. It will be observed that both curves 
are irregular for the first two days. The activity of the Th X 
increased at first, while the activity of the thorium diminished. 
Disregarding these initial irregularities of the curves, which will be 
explained in detail in section 190, it will be seen that, after the 
first two days, the time taken for the thorium to recover half its 
lost activity is about equal to the time taken by the Th X to lose 
