[MCLENNAN] DIFFUSION OF ACTINIUM EMANATION 13 
In seeking an interpretation of these results it seems clear from 
the first set of observations that the maximum deposits found in the 
second series of observations were not due to the presence in the space 
between the plates of a maximum concentration of either the emana- 
tion from the salt or of the active deposit particles produced by the 
emanation, but that they arose from the fact that the emanation from 
actinium was an exceedingly short-lived radioactive product. On 
account of this characteristic the emanation at higher pressures, such 
as atmospheric, was wholly transformed before it diffused very far 
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from the salt, and but little of it, therefore, reached the point on the 
plates under observation in the experiments illustrated by the curve 
in Fig. Il. With decreasing pressures, however, the emanation dif- 
fused more and more rapidly, and the effect of this produced at first an 
increase in the concentration of the emanation at the point of observa- 
tion. With the pressures decreased still further, the emanation diffused 
more and more rapidly, and finally at the lowest pressure obtainable, 
the diffusion became so rapid that the concentration at the point of 
observation became exceedingly small. With this explanation, it 
