Maximum 



Limit 



mm. 



mm. 



0-0301 



0-0334 



0-0201 



0-0229 



0-0170 



0-0200 



0-0150 



0-0183 



0-0135 



0-0168 



o-oioo 



0-0133 



PLocliroic Ilalos. 639 



of maximum ionization of the various a rays o£ the uranium 

 series in this biotite. 



RaC 



KaA 



Emanation 



RaF 



Ra and Ur 



Ionium 



These ranges appear to be concerned in the genesis of: the 

 halo, but in a manner which can only be completely under- 

 stood by reference to the Bragg curves showing the ionization 

 at every point of the path of an a particle in air. For better 

 convenience we reproduce here the curves (p. 640 J as given by 

 Bragg and Kleeman in the paper which has been the starting- 

 point of all subsequent work of the kind * (loc. cit.). Ioniza- 

 tion being plotted as abscissa^ and distance from the origin 

 as ordinate?, the complete curve for the a rays expelled by 

 RaC (curve chad) shows that the ionization due to the a particle 

 increases slowly at first, but augments very rapidly towards the 

 end of the ionization range. At a disranca of about 6'3 cm. 

 the intensity of ionization is a maximum, and at 7*06 cm. the 

 ray no longer ionizes. Its presence beyond this distance — 

 if it penetrates further — could only be detected by a cumula- 

 tive experiment and spectroscopic analysis; for it will neither 

 affect a photographic plate nor a sensitive zinc sulphide 

 screen, nor will it ionize a gas. The recent, experiments of 

 Geiger ('Nature,' Feb. 24, lit 10, p. 508) support the view 

 that the a ray then comes to rest, and, probably, losing its 

 charge, takes on the character of an atom of helium (Ruther- 

 ford and Geiger, Proc. R. S. Ixxxi. A. p. 162). 



The form of the ionization curve of RaC- is found to apply 

 to the other rays, so that the effects attending the motion of 

 ( 4 ach particle in the medium are defined by transporting the 

 curve for the rays of RaC parallel to itself, and so as to suit 

 the ranges of other rays. Each ray has therefore the same 

 intensification of its effects just before it ceases to ionize: 

 the initial velocities and ranges of the rays alone vary. A 

 radioactive nucleus giving out rays in ail directions from 

 the several a-ray-producing constituents is, therefore, sur- 

 rounded by successive shells of maximum ionization. That 



* We desire to thank Professor Bragg for his permission to reproduce 

 these curves. 



