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cess of erystallization when UrX is continually passing to the 
upper layers. 
Many observed experimental facts prove with certainty the cor- 
rectness of this explanation of the increase of 8 activity produced 
by crystallization. 
For instance we do not get the increase of f activity when the 
solution is continuously stirred during the crystallization so that 
instead of a compact plate there is a powder composed of very 
small crystals. Moreover, under suitable thermal conditions, the 
erystallization may be started at the surface instead of at the 
bottom, and then the increase of activity is not observed after 
erystallization, 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 if this was 
really the case, I took a plate of crystal of which the 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 activity was found to 
be 528. 
This experiment shows quite clearly the truth of the explana- 
tion of the rise of activity during the process of erystallization. 
By the erystallization 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, the ß 
rays. which start from UrX, must pass through the whole thick- 
ness 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. 
5. Diffusion of UrX. 
The results obtained in the preceding section can now be used 
to explain the first rapid decay of ß activity of uranium nitrate 
after erystallization from the water solution. 
We saw that in the case wben uranium nitrate was obtained 
by evaporation from the solution, and not by erystallization, this 
first decay was not exhibited. Mereover, it was pointed out that 
