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BELL SYSTEM TECHNICAL JOURNAL 



ceasing from ionization altogether. The radius of the outer boundary 

 of every such sphere is the range, in mica, of the kind of alpha-rays 

 which caused it. All of the alpha-ray-emitting descendants of the 

 initially-imprisoned substance form their individual spheres; in the 

 cross-sections of the best haloes one can discern nearly all of the rings 

 due to uranium I and its seven alpha-ray-emitting descendants on the 

 direct line to radium G, or those of the seven members of the thorium 



Fig. 3. Pleochroic haloes (B. Gudden, ZS.f. Physik) 



a. Rings of UI and UII (innermost, merged into a single broad ring), lo, and Ra. 



b. Rings of RaF (innermost), Rn, RaA and RaC 



c. Rings of various substances of the uranium-radium series. Magnifications 

 665, 500, 480 respectively. 



series which disintegrate in this way. There are no extra rings in these 

 haloes, which strengthens the presumption that no radioactive sub- 

 stances in either series lie undetected. But there are also haloes of 

 which the rings have not the proper radii to be identified with any 

 known radiating substance. Are these possibly evidence for the pre- 

 historic existence of others belonging to other series, all of which were 

 too short-lived to survive into the days of scientific research, but dis- 

 appeared with the dinosaur and the pterodactyl? 



