58 Dr. T. Godlewski on some 



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 

 influence 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 completely dry "*, and the diffusion 

 took place even when a part of water of crystallization had 

 escaped from the crystals on the surface. 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 

 crystals and the total mass of uranium are the solvent and 

 UrX is the dissolved body. And then the UrX diffuses 

 through the crystals from places 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 penetration of both substances. 



The process of formation of UrX points to the fact that 

 we really have here a mixture of this kind. An atom of 

 uranium, breaking up by expulsion of one a 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 the deeper molecular penetration as existing for 

 the atoms which previously were the atoms of the parent 

 body. 



Throughout a given mass of uranium, single atoms of 

 UrX are dispersed. Thus if we consider the total amount 

 of UrX present at a given moment in a given quantity of 

 uranium, we may assume that all this UrX is " dissolved " 

 in the uranium. The observed fact of the diffusion of UrX 

 confirms this supposition. The diffusion of UrX goes in the 

 direction from higher to lower concentration ; we may 



* Meyer and Schweidler, who first observed this decay of activity 

 after crystallization, due, as we saw, to diffusion of UrX, pointed out 

 that the plates investigated were completely dry (vollkommen trocken). 

 Loc. cit. p. 1068. 



