344 The Rays and Product of Uranium X. 



nearly 18 months ago. Any change has been in the nature 

 of a slight decay rather than an increase. 



Measured in an ordinary cylindrical electroscope (13 cm. 

 high by 10*5 cm. diameter), the a-activity of the more active 

 of the preparations, namely, those first prepared and therefore 

 containing most of the initial impurities in the uranium, are 

 comparable with that of a similar surface (10 sq. cm.) of 

 uranium oxide. The least active preparations (those of the 

 third separation) possess only about a third of this activity. 

 For the latter it can easily be calculated that the permanent 

 a-radiation produces only about -^qqq part of the leak which 

 would have been produced in the same electroscope initially 

 by the ^-radiation. The calculated initial ^-activity of each 

 of the more active preparations corresponds to about half a 

 million scale-divisions per minute. That produced by the 

 a-rays per sq. cm. of uranium oxide surface is about 44. 



Other tests have been done to detect a possible growth of 

 actinium in the preparations. The method employed consists 

 in placing closely over the (positively charged) platinum 

 tray containing the preparation a negatively charged brass 

 plate, removing the latter after a definite period, and mea- 

 suring its a-activity and its rate of decay. Evidence of 

 actinium was first found when the first preparation had 

 decayed far enough to allow of measurements in an ordinary 

 electroscope. The readings steadily increased after inserting 

 the preparation due to an emanation being generated. The 

 active deposit test showed that actinium was undoubtedly 

 present. The activity of the active deposit decayed to half- 

 value in 37*5 minutes, with an initial delay characteristic of 

 actinium A. Alter two hours exposure the activity obtained 

 initially increased the natural leak of the electroscope about 

 10 times, and corresponded to the specific a-activity of 

 0*65 sq. cm. of uranium oxide. The second preparation 

 gave a detectable active deposit, but it was only one-fifteenth 

 as much as the first. In the single preparations of the third 

 and fourth separations, no actinium whatever could be 

 detected. Quite recently, a combined test with all four pre- 

 parations of the third and fourth separations showed a just 

 detectable active deposit. Naturally these observations will 

 be continued. Some of them have been in progress seven 

 months, and have shown that if a growth of actinium occurs 

 at all it must be very slow. Its presence in the first prepa- 

 rations may therefore be ascribed to initial impurities in the 

 uranium, which are rapidly separated by the successive 

 crystallizations. 



The general result of this investigation is to show that 



