7 
296 RADIO-ACTIVE PROCESSES [CH. 
end of 22 hours. As soon, however, as the Th X has been separated, 
new Th X is produced in the thorium compound. The activity of 
this new Th X is not, however, sufficient to compensate at first for 
the loss of activity due to the change in emanation X, so that, as 
a whole, the activity will at first decrease, then pass through a 
minimum, then increase again. 
The correctness of this point of view has been tested by Ruther- 
ford and Soddy? as follows: If the precipitated thorium hydroxide 
after the removal of Th X is put through a series of precipitations 
with ammonia at short intervals, the Th X is removed almost as 
fast as it 1s formed, and, at the same time, the activity of the 
emanation X decays, 
The following table indicates the results obtained. A portion 
of the precipitated hydroxide was removed after each series of 
precipitations and its activity tested in the usual way. 
Activity of 
hydroxide per cent. 
After 1 precipitation sat : ie ee 46 
After 3 precipitations at intervals oe a4 Heist 600 39 
After 3 more precipitations at intervals of 24 hours and 
3 at intervals of 8 hours ads Age 400 Rc 22 
After 3 more each of 8 hours _... det oe Sel 24 
After 6 more each of 4 hours... 43 Se 506 25 
The differences in the last three numbers are not significant, 
for it is difficult to make accurate comparisons of the activity of 
thorium compounds, which have been precipitated under slightly 
different conditions. It is thus seen that as a result of successive 
precipitations, the activity is reduced to a minimum of about 25 per 
cent. The recovery curve of the activity of this 23 times precipitated 
hydroxide is shown in Fig. 56. The initial drop in the curve is 
quite absent, and the curve, starting from the minimum, is practi- 
cally identical with the curve shown in Fig. 35, which gives the 
recovery curve of thorium hydroxide after the first twodays. This 
residual activity—about 25 per cent. of the maximum—is non- 
separable from the thorium by any chemical process that has been 
tried. 
The initial rise of activity of Th X, after it has been separated, 
will now be considered. In all cases, it was found that the activity 
1 Trans, Chem. Soc. 81, p. 837, 1902. Phil, Mag. Nov. 1902. 
q 
