6±6 Profs. J. Joly and E. Rutherford on the 



appearance o£ a tubular halo *. A photograph o£ these 

 veins is given in 'Bedrock' for January 1913. Such conduits 

 of radioactive substances often diverge from heavily-stained 

 areas located on the outer margins of the crystal. Haloes 

 are sometimes found linearly arranged along the veins, 

 showing that they partook of the radioactive substances 

 contained in them. It is known that towards the close of 

 the period of consolidation of a granitic mass, mother liquors, 

 rich in the rarer elements concentrated from the magma, 

 make their appearance and penetrate the granite, and often 

 the adjacent rocks. The genesis of radioactive ore deposits 

 seems in many cases traceable to this phenomenon. It is 

 not impossible that the venation of the mica is referable to 

 similar developments on a minute scale. This, however, 

 cannot have been long subsequent to the period of consolida- 

 tion. The haloes may most safely be regarded as of early 

 Devonian age. 



The experiments on the number of alpha rays required to 

 produce notable staining of the Carlow mica were carried out 

 in the Physical Laboratory of the University of Manchester. 

 The mica was placed beneath and in contact with a lead 

 plate which was perforated with a circular aperture 0'42 cm. 

 in diameter. At a distance of 1*5 cm. vertically over the 

 centre of the aperture a capillary alpha-ray tube, containing 

 25 millicuries of radium emanation to start with, was fixed. 

 The whole was placed under a bell-glass in which a partial 

 vacuum could be made. The thickness of the walls of the 

 alpha-ray tube was equivalent to 1*4 cm. of air at normal 

 density. The average range of the three sets of alpha rays 

 falling on the mica was deduced as 3*5 cm. of air. This 

 would ensure a penetration of about 0*016 mm. in the mica. 

 The quantity of emanation used and the duration of the 

 exposure enable direct determination of the number of alpha 

 rays required to produce a particular effect. 



Two cleavage flakes of the mica were dealt with. To one 

 of these flakes three exposures on adjacent areas were given. 

 On the second flake two exposures on adjoining areas were 

 given. Calling the first three exposures A, B, C, and the 

 second D, E, the number of alpha rays in each case was : — 



Spot A received 3' 7 10 13 alpha rays per square cm. 



B 



55 



1-6 



C 



55 



1-5 



D 



55 



3-7 



E 



11 



1-5 



* Kutherford, Phil. Mag. Jan. 1910. 



