﻿of tlte a. Rays of Radiothorium. 83 



into the gas without appreciable absorption in the active 

 material itself. 



(2) The thin layer of the material has to be strongly radio- 

 active, so as to obtain well-marked effects in the electrometer 

 using a narrow ionization-chamber. 



In the case of pure radium salts, on account of their 

 extremely high activity, the condition (2) does not present 

 any difficulty. In the present case, the strength of the 

 radiothorium preparation available for this investigation was 

 of the order of magnitude of about 10 * 00 of that of pure 

 radium salts, with the exception of fig. 1, where a stronger 

 preparation was used. One. therefore, could not expect to 

 obtain as accurate results as in the case of radium. 



Another difficulty arises from the presence of the emanation, 

 which so readily escapes from solid radiothorium prepara- 

 tions. The emanation diffuses into the air some distance 

 from the radioactive source, and, unless this emanation is 

 removed, the ionization curves are not at all clearly defined, 

 since the a particles from the emanation, which enter the 

 ionization-chamber, come from varying distances above the 

 radioactive source. 



In order to avoid the disturbance caused by the emanation, 

 the following arrangement was found to be fairly satisfactory. 

 Instead of the little platform, on which the active material 

 was placed, as described in the first communication, a funnel 

 was used. A little dish, generally of porcelain, sometimes 

 of platinum, contained the thin layer of the active material, 

 and this dish was placed inside the funnel. Some strips of 

 metal prevented the dish from fitting too closely to the walls 

 of the funnel. The lower, narrow end of the funnel was 

 connected with rubber-tubing, which passed through the 

 bottom of the metal box to a water-pump. By means of the 

 pump, a constant air-current was pulled through the funnel, 

 and this air-current removed most of the emanation given oft" 

 by the active source as fast as it was formed, and prevented 

 the diffusion upwards of the emanation. 



This arrangement proved satisfactory for large distances 

 between the active source and the ionization-chamber. For 

 short distances, disturbances due to the emanation could never 

 be completely got rid of. The disturbance due to the emana- 

 tion could be entirely avoided by hermetically covering the 

 active dish with a thin uniform sheet of mica. This method 

 was sometimes successfully employed. The sheet of mica 

 reduces the range of ionization of the cc particles in the air. 

 and the stopping power of the a particles for the mica has to 

 be determined in terms of air by a separate experiment. For 



G2 



