Dr. 0. Halm on Radioactinium. 



169 



For an example the following table gives the experimental 

 (I.) and theoretical (II,) values of the activity ; II. supposing 

 the period is 19*5 days. This only refers to the part of 

 the decay curves obtained about one month and a half after 

 the preparation of the radioactinium, when the decay is 

 exponential. The numbers at I. are corrected by subtracting 

 a small constant residual activity. In both cases, the value 

 after l'.V5 days is taken as 50'0. 



Time in days. 



I. 



Observed. 



II. 



Calculated. 







5 



98-2 

 84-8 

 772 

 50-0 

 212 

 11-4 

 9-30 

 5-85 

 425 

 2-45 



100 

 83-8 

 751 

 500 

 200 

 110 

 9-25 

 5-80 

 4-25 

 21 



8 



195 



45 



62 



67 



80 



89 



109 





Thus over an interval of nearly four months the numbers 

 are in a satisfactory agreement, showing that the decav is 

 exponential with the period stated above. 



c. Position of Radioactinium in the Radioactive Series. 



The following experiments were made to throw light on 

 the position of radioactinium in the actinium series. The 

 radioactinium was prepared after one of the methods pre- 

 viously described and was tested on the filter. It exhibited 

 the characteristic variation of a and ,8 ray activity, viz. a 

 rapid rise measured by the /3 rays and a more gradual rise 

 measured by the a rays. The maximum value was reached 

 after two and a half weeks. Shortly after reaching the 

 maximum, the filter containing the sodium thiosulphate pre- 

 cipitate was burnt in a porcelain crucible and a solid white 

 residue remained, which probably consisted in part of thorium 

 oxide and other rare earths. 



This residue, which weighed only a few milligrams, was 

 then treated with strong nitric acid. A part of the residue 

 remained undissolved. After dilution and filtration, the 

 filtrate was precipitated with ammonia and left standing for 

 a short time. A very small quantity of a colourless jelly-like 

 precipitate appeared and was filtered through the first filter, 

 which contained the trace of undissolved residue. The 



