﻿emitted by Thorium and Actinium. 187 



He has further shown that the radioactinium gives rise to 

 actinium X. The activity cf radioactinium rises to a maxi- 

 mum in about 20 days, and after passing the maximum 

 finally decays, according to an exponential law, with a period 

 of about 20 davs. The rise is due to the formation of acti- 

 nium X and its successive products, as Hahn has clearly 

 shown, by separating actinium X from a radioactinium pre- 

 paration several weeks old. 



In the light of these experiments the anomalous result-. 

 which I had observed, at once receive a simple explanation. 

 The residual a ray activity observed was not due to actinium 

 itself but to radioactinium. The recovery curve, which rose 

 to a maximum in 20 days, is the recovery curve not of acti- 

 nium itself, but of radioactinium. This was confirmed by the 

 fact that the curve decreased again after passing the maximum. 

 The unexpected initial value of the a activity can be explained 

 in this way too. The radioactinium is separated free from 

 actinium by immediately filtering after precipitation by 

 ammonia. When the solution so treated stands for some 

 time, it becomes turbid and, after boiling, the precipitate then 

 consists of the actinium itself. The a. ray activity of this 

 precipitate is very small at first, but gradually rises for three 

 months in consequence of the gradual production of radio- 

 actinium and its successive products. 



It may be of interest to record some experiments which 

 show conclusively that actinium X can be almost completely 

 separated from actinium by successive precipitations with 

 ammonia. After ten rapid precipitations, a current of air 

 was aspirated through the actinium solution and passed into 

 a suitable electroscope. The rate of discharge observed was 

 less than 1/1000 of that observed, when the same current of 

 air was passed through the solution of actinium X separated 

 from it. Such a result shows clearly that hardly a trace of 

 actinium X remained with the actinium. 



Godlewski states that he found some difficulty in com- 

 pletely freeing actinium of a, ray activity by successive pre- 

 cipitations with ammonia. This difficulty does not arise from 

 the presence of actinium X, but of radioactinium. There 

 is no doubt that in the course of the successive precipi- 

 tations of actinium, Godlewski had in some way also sepa- 

 rated radioactinium. The recovery curve of actinium 

 given by him, which starts from a small initial value and 

 rises with a ten-day period, would at once be explained if 

 the /3 rays, and not the a rays, were used as a means of 

 measurement. 



