[ 644 ] 



LXIII. The Age of Pleochroic Haloes. 

 By J. Joly 7 F.R.S., and E. Rutherford, FM.S. 



[Plate IX.] 



T is now well established that the minute circular marks 

 seen in sections of certain coloured rock minerals — 

 notably the coloured micas — are due to the effects produced 

 by the alpha radiation o£ a central radioactive particle. The 

 circular mark, in fact, represents the section or projection of 

 a sphere defined by the range in the particular containing 

 mineral of the most penetrating alpha ray emitted. If the 

 parent radioactive substance is uranium, the fully developed 

 halo is defined by a sphere having the radius of the furthest 

 reaching ray, RaC. If the parent substance is thorium, the 

 extreme radius of the sphere will be defined by the range of 

 ThC. 



Haloes are found in which the effects of other and less 

 penetrating alpha rays of the uranium and thorium families 

 may be clearly shown, and in some the quantity of radio- 

 active material is so small that the halo may be described as 

 " under-exposed " ; so that the maximum effects of RaC or 

 ThC being distributed over a relatively large spherical 

 surface or shell, may be faint or absent, and the containing 

 mineral only notably darkened by those inner rays whose 

 actions are more concentrated owing to the lesser spherical 

 shells to which their maximum ionization effects are confined. 

 Thus, in the case of the uranium derivatives, the halo may be 

 limited by the range of RaA, by Ra emanation, by Ra, or 

 even by ionium or uranium. 



The halo is in every case the result of the integral actions 

 of rays emitted since a very remote period. Haloes in the 

 vounger rocks are unknown. The quantity of radioactive 

 material involved is very small. More especially haloes of 

 true spherical form are necessarily formed around very 

 minute nuclei, so that even if these were entirely composed of 

 a parent radioactive element, it is only by the integration of 

 effects over a very prolonged period that any results are 

 brought about. 



It is of interest to seek some estimate of the time which 

 may have been required to generate these haloes. This can 

 be done if the following data are available : — 



(a) The number of alpha rays which will produce a certain 

 intensity of staining in a particular mineral. 



(b) The mass of the nucleus of a halo of similar or com- 

 parable intensity of staining in this same mineral, and from 



* Coinnmnicatsd by the Authors. 



