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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 diffu- 
sion of UrX goes in the direction from higher to lower concentra- 
tion; we may conjecture from higher to lower osmotie pressure. 
But this osmotie pressure whilst it might control the diffusion cannot 
be imagined as completely analogous to the osmotie pressure as 
known in fluid solutions. In the case of extremely weak concen- 
tration of UrX the ordinary osmotie pressure would be a vanishing 
quantity. But in the present case the forces which guide the diffu- 
sion must be extremely great in order to overcome the immense 
resistance due to friction. These forces can only result from tbe 
reciprocal action between the molecules of the parent body and 
the atoms of its product and appear to be of a special radioactive 
type. 
Just as UrX is dissolved in its parent uranium, so the other 
radioactive produets should be dissolved in their parent. There are 
some experimental facts which confirm this supposition. We know 
that radium and thorium give out a gaseous emanation as one of 
their successive products. The emanation is produced at a constant 
rate, which does not depend on any physical or chemical agencies, 
but the escape of the emanation from the body is variable in cha- 
racter and dependent on different eonditions. For instance, radium 
and most of the compounds of thorium give off little emanation 
when in a solid state. The emanation is stored in the body itself 
in eonsiderable amount. We may suppose that in reality just as 
UrX was dissolved in uranium so the radium and thorium ema- 
nations are dissolved in radium and thorium. 
When the parent body is dissolved the emanation is no longer 
held bound in the solid solution, and it can readily escape from 
the water. And it is a fact that all substances have the maximum 
emanating power when dissolved. The inerease of emanating power 
in presence of moisture can be explained in the same manner. 
We know further that generally the solubilities of gases de- 
crease with the temperature. And indeed the emanating power 
of almost all radioaetive bodies increases when the temperature is 
raised, reaching a maximum at a dull red heat. At this tempera- 
