176 Dr. 0. Halm on Radioactinium. 



very small quantities of working material. But the writer 

 finally succeeded in reducing the a ray activity to 2*5 per 

 cent, of the initial value. The method employed to separate 

 the radioactininm and actinium X from actinium was as 

 follows : — 



Actinium solution in equilibrium was precipitated several 

 times with ammonia. Care has to be taken not to filter too 

 early, as it takes a considerable time for the actinium to come 

 out *. By this method, actinium X remains in the filtrates, 

 actinium and radioactinium remain on the filter. The filter 

 was again dissolved in dilute hydrochloric acid,, and to the 

 solution a little ammonia was added in order to weaken the 

 free acid. To the weak acid solution a small quantity of 

 dilute sodium-thiosulphate solution was added at ordinary 

 temperature. The small sulphur precipitate carries the 

 radioactinium down ; actinium remains in solution. If the 

 filtrate was still decidedly active, the process of precipi- 

 tating traces of amorphous sulphur was repeated. This 

 method of separation works very satisfactorily. In order to 

 avoid the presence of sodium sulphite in the actinium solution, 

 the separation may be carried out in a different way by 

 fractional precipitation with ammonia. The first part always 

 contains an excess of radioactinium, while the actinium is 

 concentrated with the later parts. But this method does not 

 effect a very rapid separation, and one sulphur precipitate 

 is in many cases more effective than many careful ammonia 

 precipitations. 



The activity of the actinium obtained in this way is very 

 small. The activity only increases slightly in the course of 

 the first few days, but then rises more rapidly for over a 

 month and reaches nearly a constant value after four months 

 (compare Curve II. fig. 3, p. 177). It is of interest to 

 compare the experimental curve of recovery of activity with 

 that to be expected from theory. As before, the changes 

 following actinium X may be supposed without any sensible 

 error to be simultaneous. Actinium changes so slowly com- 

 pared with its products that we may suppose that it breaks 

 up at a constant rate, giving rise to radioactinium which in 

 turn is transformed into actinium X. Since actinium itself is 

 rayless, the curve of recovery of activity is complementary 

 to the curve showing the decay of activity of radioactinium 

 and actinium X initially in radioactive equilibrium (see 

 Rutherford, 4 Radioactivity/ vol. ii. p. 336). 



Let P , Q represent the number of atoms of radioactinium 

 and actinium X respectively present in a stage of equilibrium. 



* See also Levin, loc. cit. 



