﻿252 Dr. 0. Halm on the Ionization Ranges 



of the a particles o£ actinium X, viz. 6'6 cms., but the ioniza- 

 tion due to the a rays, shown in the curves a and b, begins 

 somewhat higher than this — a difference due to the presence 

 of the emanation for the reasons discussed above. 



The three curves are very similar to the curves obtained 

 for actinium X, shown in fig. 2, the only difference being 

 that the rays of radioactiuium begin to add their effect at a 

 distance of 4' 8 cms. This, however, is not easy to observe in 

 the figure. 



A striking difference is shown, however, in the rates at 

 which the ionization falls off with time. The ionization of 

 actinium X decreases exponentially with a period of ten days, 

 and is hardly distinguishable after about 50 days. The 

 ionization of radioactiuium, however, decreases at a much 

 slower rate, and the ionization after two months' interval is 

 still very marked. 



The decay of the radioactinium preparation could, of 

 course, have been measured in an ordinary electroscope, but 

 this of itself would not have shown whether actinium X 

 and its products were present when the activity had reached 

 a small value. In fact, the ionization curves serve as a 

 means of radioactivity analysis to shoiv what ol ray products 

 are present. 



The Range of the a Particles of the Actinium Emanation. 



The method used for determining the range of the a. particles 

 of the actinium emanation was exactly the same as that used 

 by the writer for the thorium emanation, which was described 

 in the previous paper *. The emanation was passed with a 

 slow air-current through a flat vessel, the top of which con- 

 sisted of a thin mica plate, the stopping power of which had 

 been determined by a special experiment. A small sensitive 

 zinc-sulphide screen was placed vertically above the mica 

 and could be brought to any desired distance. The point, 

 where scintillations just began to be visible, was taken as 

 the maximum range of the a particles when using the scin- 

 tillation method. 



I previously pointed out that the ranges of a particles 

 determined by the electrical method seemed to be nearly 

 3 mms. greater than those determined by the scintillation 

 method as above described. This difference may, possibly, 

 vary somewhat with different observers. 



Taking the mean of several measurements, which were in 



* Loc. cit. 



