[BoYLE] TEMPERATURE AND VELOCITY OF GAS CURRENT 89 
It should be noticed that, in these experiments, if the emanation 
reservoir were sealed, so that there could be no diffusion to charcoal or 
to open air, the activity would fall to half-value in 3-75 days. 
From the figures above, the rate of diffusion of the emanation from 
the reservoir to the charcoal was at first the same as the rate of diffusion 
to the open air, viz.: half-value in 1-0 days; but as time went on this 
rate gradually became less. A probable explanation of this fact is that 
the layer of charcoal first reached by the emanation acts as a good ab- 
sorbent until it becomes saturated, very little emanation passing 
to the under layers until the top layer reaches this condition. In 
consequence, it will take time for the emanation to diffuse from the 
upper to the lower layers of charcoal, with the result of a gradual lessen- 
ing of the amount of emanation absorbed from the reservoir. 
Another experiment was made with the emanation diffusing to a 
very thin layer of charcoal, and the results obtained support this ex- 
planation. In this case the activity diminished at first at the rate ot 
half-value in 1-3 days, but kept gradually changing until, after twenty- 
four hours, the rate was half-value in 3-2 days. At this stage there 
could be very little absorption, since the natural decay of the emanation 
. itself is half-value in 3-7 days. We may say, then, that it took about 
a day to saturate the thin layer of charcoal. 
In the experiments just described the charcoal has been contained 
in a vessel outside the emanation reservoir; when the charcoal is in 
the reservoir itself, the absorption takes place more rapidly, as the time 
for the emanation to diffuse through the connecting tubes is eliminated. 
In an experiment where a tray containing charcoal was placed in the 
bottom of the emanation reservoir, on admitting some emanation the 
absorption took place so quickly that there was not the usual rise of 
activity due to the active deposit, but, on the contrary, an immediate, 
rapid fall. With only a thin layer of charcoal—a sprinkling covering 
the bottom of the tray—there is a slight rise of activity due to the active 
deposit, but not nearly in the same degree as if the charcoal had been 
absent. 
The above experiments were all performed in a constant tempe- 
rature room, so that all effects on the diffusion by changes of temperature 
and draughts of air were avoided. 
More experiments, along the same lines as the above, are being 
carried on. 
Thorium Emanation. 
In the case of thorium emanation, owing to its short period of 
decay — to half-value in 54 seconds—it is necessary to use a flow 

