152 Dr. G. A. Blanc on 



It seemed to me therefore evident that the greatest part of 

 the activity shown hy a certain number of sediments must 

 be due to the presence of a radioactive substance without 

 any emanating power, or maybe whose emanation lost its 

 activity very rapidly with time. This hypothesis could also 

 explain how the waters of Salins-Moutiers, when examined 

 by Curie and Laborde, had proved to contain very little 

 emanation *, while the sediments were among the most active 

 which I had found. 



To settle this I proceeded in the following way : — Having 

 left for a few minutes some 50 grams of sediment from the 

 Grande Source of Salins-Moutiers in the dish of the apparatus 

 shown in fig. 1, I managed to slide rapidly a metal screen 

 over the dish, so as to close the opening under the vessel and 

 remove the air inside it from the direct action of the sediment. 

 (I had previously ascertained that the metal plate used as a 

 screen was sufficient to cut away any direct radiation from 

 the active substance.) In this way I was able to see that the 

 conductivity of the air in the testing-vessel did not drop 

 instantly when the screen was introduced, but that it gradually 

 decayed with time. 



By noting the rate at which the aluminium leaf fell I was 

 able to determine with sufficient approximation the law 

 according to which the emanation lost its activity. Repeated 

 measures showed that the conductivity is reduced to half 

 its value in about one minute. 



Similar results were obtained by examining in the same 

 manner the various sediments of the " Petite Source >} of 

 Salins-Moutiers and of the Echaillon spring. 



The results obtained with the sediments of Echaillon are 

 shown graphically in fig. 3. These results seemed to point 

 to the conclusion that a great part of the activity shown by 

 the materials now mentioned was due to the presence of 

 a radioactive constituent similar to, if not identical with, 

 thorium. 



I thought therefore that it would be interesting to see 

 whether an inactive body could be made temporarily radio- 

 active by exposing it to the emanation, and in this case verify 

 whether the rate of decay of the excited activity was the 

 same as in the case of thorium. After keeping for a couple of 

 hours a disk of tinfoil in a glass vessel containing a certain 

 amount of sediment from airy one of the three springs mentioned 

 above, it showed a notable activity. Considerably stronger 



* Comptes Rendus, cxxxviii. p. 1150. 



