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of uranium nitrate in which the substance UrX is unequally di- 
stributed UrX diffuses from the places where it is in higher con- 
centration to places where its concentration is lower. 
This diffusion of UrX therefore explains the first rapid decay 
after crystallization. We see also that the period during which the 
minimum activity is reached should depend on the thickness of 
the plate, and such is the case. 
6. The possible causes of the diffusion. 
The question now arises in what manner and under the in- 
fluence of what forces does this diffusion take place? Only two 
explanations appear possible. 
It may be supposed that some part of the UrX is dissolved 
in a small amount of water and diffuses in a state of solution 
between the crystals under the influence of capillary forces. 
The crystal plates under investigation, however. seemed to be com- 
pletely dry!) and the diffusion took place even when a part of 
water of crystallization had escaped from the crystals on the sur- 
face. Therefore the supposition that the UrX diffuses in the state 
of solution does not seem to be probable. 
And if the diffusion does not take place under capillary forces 
we are here dealing with a „solid solution“. The erystals and the 
total mass of uranium are the solvent and UrX is the dissolved 
body. And then the UrX diffuses through the crystals from pla- 
ces of the higher to lower concentration. 
We define the solution as a mixture of two substances, which 
is not a mechanical one, but is accompanied by the molecular pe- 
netration of both substances. 
The process of formation of UrX points io the fact that we 
really have here a mixture of this kind. An atom of uranium bre- 
aking up by expulsion of one particle, changes into an atom of 
UrX. But it always remains surrounded by the other atoms and 
molecules of uranium. It is not possible to imagine deeper mole- 
cular penetration as existing for the atoms which previously were 
the atoms of the parent body. 
1) Meyer and Schweidler who first observed this decay of activity after ery- 
stallization, due as we saw to diffusion of UrX, pointed out that the plates in- 
vestigated were completely dry (vollkommen trocken). Loe. eit. 
