Radioactive Properties of Uranium, 55 



uranium solution starts to crystallize from the bottom of the 

 dish, first of all uranium itself crystallizes and UrX is 

 pushed in the direction of the surface. When the whole 

 mass is solidified, we get a plate which contains on the 

 surface an excess of UrX, and in the lower layers a deficit 

 of this substance. The ft rays, which come from the UrX 

 present near the surface, emerge with little absorption in 

 the mass of uranium itself, and thus the ft activity must be 

 larger than when UrX is uniformly distributed throughout 

 the plate. In the same way we can explain the steady 

 growth of activity during the actual process of crystallization 

 when UrX is continually passing to the upper layers. 



Many observed experimental facts prove with certainty 

 the correctness of this explanation of the increase of ft 

 activity produced by crystallization. 



For instance, we do not get the increase of ft activity 

 when the solution is continuously stirred daring the 

 crystallization, so that instead of a compact plate there is 

 a powder composed of very small crystals. Moreover, 

 under suitable thermal conditions the crystallization may be 

 started at the surface instead of at the bottom, and then the 

 increase of activity is not observed after crystallization 

 but, on the contrary, there is often a decay. 



This last fact suggested to me a decisive test. If the 

 increase of activity during the crystallization is due to the 

 fact that UrX is pushed to the upper layers when the 

 crystallization starts from the bottom of the vessel, then the 

 lower layers of the plate of crystal should contain less of UrX. 

 In order to see whether this was really the case, I took a 

 plate of crystal of which the ft activity was 1840. The 

 plate was cut across so that it could be removed from the 

 dish, and it was then taken out and inverted so that the under 

 surface faced the electroscope. The ft activity was found to 

 be 528. 



This experiment shows quite clearly the truth of the 

 explanation of the rise of activity during the process of 

 crystallization. By the crystallization, UrX was pushed to 

 the upper layers ; when we turn the plate, the upper layers 

 containing the excess of UrX are now underneath and, 

 before reaching the electroscope, th.Q ft rays, which start 

 from UrX, must pass through the whole thickness of the 

 plate whereby they are to a great extent absorbed. And 

 for this reason the activity of the plate, when it was turned 

 over, was only one-third of the activity measured from the 

 upper side. 



