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VIII. On Radioacthihnn. By Otto Hahn, Ph.D* 



IN a previous paper f I gave a brief account of the 

 methods of separation and the radioactive properties of 

 a new product of actinium, which was called ; ' radioactinium," 

 as its position in the disintegration series of actinium is the 

 same as that of radiothorium in the thorium series. 



The ohject of the present paper is to consider in more 

 detail the properties of radioactinium, and also the experi- 

 mental and theoretical curves, which throw light on its 

 properties as the intermediate product between actinium and 

 actinium X. 



I. Preparation of RADioACTixior. 



The experiments on radioactinium were made both with 

 actinium of Debierne (kindly lent to me by Sir William 

 Ramsay) and actinium (emanium) of Giesel. Again, as in 

 previous cases, no differences in the radioactive behaviour of 

 the two substances could be observed — a result which proves 

 beyond question the identity of the radioactive constituents 

 of the two preparations. 



On dissolving in hydrochloric acid an actinium preparation 

 in the form of a fine powder which has previously been heated, 

 the resulting solution is generally not clear. A small portion, 

 depending on the strength and nature of the acid and on the 

 active preparation employed, remains undissolved. 



If a small quantity of the jilt rate is evaporated to dryness 

 and tested from time to time in an electroscope, it is soon 

 found that the activity continues to increase for a long time. 

 The activity curves first obtained in this way were not very 

 regular, but they showed a different shape from that to be 

 expected, if the rise in activity were only due to the pro- 

 duction of actinium X and its successive products. 



In addition, the change in activity of the insoluble part was 

 very different from the well-known behaviour of actinium X 

 or of a mixture of actinium and a surplus of actinium X. 



In some cases, the activity of the filter, instead of decreasing 

 remained nearly constant or actually increased. This result 

 was very puzzling, as the activity of the filtrate was also 

 increasing at the same time. 



All these results were quite unexpected, and at the time no 

 reasonable explanation suggested itself. 



In order to find out whether physical conditions, such as 

 diffusion, moisture, &c, were in any way responsible for this 



* Communicated by the Author. 



t 'Nature/ Ma^TOOO ; Ber. xxxix. p. ]60o. 



