﻿of the ol Hays of Radiothorium. 1J1 



strong radiofcborium preparation, carried the emanation slowly 

 through the flat vessel and then escaped through a long rubber 

 tube so as to completely remove the emanation from the 

 neighbourhood of the screen. 



As the air carried the emanation into the vessel the scin- 

 tillations of the a. particles, which had passed through the 

 mica screen, could be easily observed on the little zinc- 

 sulphide screen placed close above the vessel. By altering 

 the distance between the vessel and the screen the distance at 

 which the scintillations disappeared could be determined. It 

 was. however, not very easy to fix tins point, as the scintillations 

 became A'ery weak near the end of the ionization range of the 

 u. particles. The scintillations, finally observed, were due to 

 the a. particles coming from the top of the vessel, while the 

 scintillations of the a rays from the emanation in the lower 

 parts of the vessel had already disappeared. 



This gradual falling off in the number and intensity of the 

 scintillations makes an accurate determination of the range 

 very difficult. In addition, there is another point of un- 

 certainty. After a large amount of emanation has been blown 

 through the vessel, the active deposit begins to appear to a 

 small, but always increasing extent, and, as the rays of the 

 active deposit of thorium contain the most penetrating a, rays 

 yet known, one soon notices a few scintillations at distances 

 much greater than that which corresponds to the maximum 

 range of the a particles of the emanation itself. 



Consequently, the numbers obtained by the scintillation 

 method for this maximum range are uucertain within small 

 limits, but, as the results of several series of measurements 

 were in good agreement, the value of the range observed is 

 probably not much in error. For the arrangement used, the 

 maximum distance, at which scintillations due to the a. particles 

 of the thorium emanation were still visible, was found to be 

 4'0 centims. But since the a rajs had already passed through 

 a thin mica screen, which corresponded in stopping power to 

 1*2 centim. of air, the maximum range in air of the a particles 

 from the emanation by the scintillation method isi'O-fl^ 

 = 5*2 centims. 



I pointed out in my previous paper that the electrical 

 method is somewhat more sensitive than the scintillation 

 method for determination of the range of the a. particles. 

 For example, the ranges in air for the a particles from radium 

 C and thorium C are 7 # 06 and 8'6 centims., respectively, 

 using the electrical method, while for the scintillation method 

 the values found are 6'S and 8*3 centims. The real range in 



