330 



Prof. J. Joly on Pleocliroic Ilalos. 



stages, intensely dark to their extreme radial limits ; and 

 again others, surrounding very minute specks, where the 

 earliest development as an undersized border of brown 

 coloration is apparent. Both the age of the rock and the 

 intensity of the source of radiation affect the development. 

 A greisen is an alteration -product and halos in it may be 

 much younger than the original granitic mass, for not only 

 the profound mineral changes but any excessive heating will 

 destroy original structures of the kind. 



The complex halos described above evidently contain no 

 effective quantity of thorium. Thus Hahn (loc. cit.) gives 

 for the ranges in air of the a. rays of thorium derivatives the 

 values in centimetres: Radio-thorium 3*9, ThX 5*7, Emana- 

 tion 5-5, ThB 5*0, ThC S'6. Such rays if present would 

 obliterate the structure observed ; nor would they agree with 

 the measured maximum radius considered in connexion with 

 Bragg's law. In studying the ordinary pleocliroic halos, 

 petrologists would seem possessed of a means of determining 

 the average atomic weight of the containing mineral : the 

 alternatives, in fully-developed halos, being limited to 

 calculations based on the ranges of the rays of Ra and 

 ThC. 



One other point remains to be noticed. It would be 

 expected, I think, that a spherical structure derived in the 

 manner of a halo and viewed in plan should not be very 



definitely denned 

 of the corona is, 

 be very perfect, 

 what intensified.) 



at its extreme boundaries. The boundary 

 however, when carefully focussed found to 

 (I am not sure that it is not even some- 

 Might not this definition be ascribed to 

 the known fact, appearing so clearly in Bragg's curves, that 

 the ionization of the a ray increases in intensity just before 

 it becomes finally ineffective ? The halo, in short, approxi- 

 mately represents a projection, on the spherical radius, of a 

 Bragg curve for the uranium-radium family of a rays ; 

 intensity of colouring replacing the measured effects in the 

 ionization chamber. 



That the effects visible in the halo are due to ionization 

 and not to mere storage of helium appears to be set beyond 

 doubt by the observed limit of the rays in the mica agreeing 

 with the observations on the ionization ranges as obtained in 

 the laboratory. A mere accumulation of helium should 

 exhibit considerably larger radial dimensions. 



The beautiful artificial halos obtained by Rutherford (Phil. 

 Mag. Jan. 1910) would, probably, reveal structural particulars 

 similar to those described above, if examined at a sufficiently 

 early stage of their development. 



