142 HUXLEY MEMORIAL LECTURES 



the accumulated effects of the influences of all the 

 rays. Each has its own sphere of influence, and 

 the spheres are all concentric. 



The radii in biotite of the several spheres are 

 given in the following table : 



URANIUM FAMILY. 



mm. 



Radium C 0.0330 



Radium A 0.0224 



Ra Emanation 0.0196 



Radium F 0.0177 



Radium 0.0156 



Ionium 0.0141 



Uranium i 0.0137 



Uranium 2 0.0118 



THORIUM FAMILY. 



mm. 



Thorium C 2 0.040 



Thorium A 0.026 



Th Emanation 0.023 



Thorium C t 0.022 



Thorium X 0.020 



Radiothorium 0.019 



Thorium 0.013 



In the photograph (Fig. 5) we see a uranium 

 and a thorium halo in the same crystal of mica. 

 The mica is contained in a rock-section and is cut 

 across the clevage. The effects of thorium C 2 

 are clearly shown as a lighter border surrounding 

 the accumulated inner darkening due to the other 

 thorium rays (upper halo). The uranium halo 

 (to the right) similarly shows the effect of radium 

 C, but less distinctly. 



