the Radioactive Emanations by Charcoal. 377 



diffuse to the open air, the decay of activity did follow an 

 exponential law, the time for diminution to half-value being 

 1*0 day. This result may be expected, for it means that the 

 amount of emanation diffusing to the open air per second is 

 proportional to the amount of emanation in the reservoir. 

 P. Curie and Danne * performed experiments of this kind 

 with capillary tubes as the communication from the emanation 

 reservoir to the outside air, and found that the loss of emana- 

 tion from the reservoir was exponential. 



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' 7 5 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 day ; 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 

 absorbent until it becomes saturated, very littles emanation 

 passing to the under layers until the top layer reaches this con- 

 dition. 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 lessening 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 explanation. In this case the activity diminished 

 at first at the rate of half- value in 1*3 day, but kept gradually 

 changing until, after twenty-four hours, the rate was half- 

 value in S'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 

 * Curie & Danne, Comptes Rendus, exxxvi. p. 1314 (1903). 



