268 
t in days Activity t in days Activity 
99 545 97 56:5 
109 50:8 10:7 61-4 
12:9 43:6 127 625 
13-9 412 13:7 68:6 
149 405 15:8 76:2 
159 36.2 170 78:0 
171 354 20:7 823 
179 340 227 857 
210 265 250 83:6 
22.9 244 270 900 
249 218 30:8 923 
212 185 36: 971 
292 17:6 45:0 995 
309 16:1 520 940 
340 135 640 100 
36:0 12-1 670 100 
42:0 882 
450 al 
520 5:89 
590 465 
670 369 
In Fig. 1 curve A represents the activity of AcX as a fune- 
tion of the time, the activity of AcX being expressed in the same 
units as in table I. In the recovery curve B the difference between 
the maximum and the first value is taken — 100. 
The activity of AcX immediately after removal was weight for 
weight more than a hundred times as great as that of the original 
actinium. In the various experiments the value of initial activity 
of AcX was proportional to the amount of actinium used, but was 
by no means proportional to the total weight of matter obtained 
from the filtrate. In some cases, for example, only a few milli- 
grams of the substance was obtained, which exhibited as great 
activity as a few centigrams obtained in other cases. This shows 
clearly that in case of actinium, the substance obtained from the 
filtrate, which we see and weigh, does not all consist of AcX. 
The substance contains some impurities; in the present case, pro- 
bably some of the rare earths. The amount of actinium X actually 
