PLEOCHROIC HALOES 131 



cms. 



Thorium 2.72 



Radiothorium 3.87 



Thorium X 4.30 



Th Emanation 5.00 



Thorium A ... 5.70 



Thorium C a 4.80 



Thorium C s 8.60 



cms. 

 Radioactinium ... ... ... ... 4.60 



Actinium X 4.40 



Act Emanation 5.70 



Actinium A 6.50 



Actinium C ... ... ... ... 5.40 



It will be seen that the ray of greatest range is 

 that proceeding from thorium C 2 > which reaches 

 a distance of 8*6 cms. In the uranium family the 

 fastest ray is that of radium C. It attains 6*94. 

 cms. There is thus an appreciable difference 

 between the ultimate distances traversed by the 

 most energetic rays of the two families. The 

 shortest ranges are those of uranium i and 2. 



The ionisation effected by these rays is by no 

 means uniform along the path of the ray. By 

 examining the conductivity of the gas at different 

 points along the path of the ray, the ionisation 

 at these points may be determined. At the limits 

 of the range the ionisation ceases. In this man- 

 ner the range is, in fact, determined. The dotted 

 curve (Fig. i) depicts the recent investigation of 

 the ionisation effected by a sheaf of parallel rays 

 of radium C in air, as determined by Geiger. 



