Pleochroic Halos. 641 



imagine additional rays at work which would prolong the 

 Bragg-Kleeman curve further to the right, and also bring it 

 downwards nearer to the origin. 



Here we may recall Rutherford's suggestion, based on 

 Boltwood's work, that actinium is probably a collateral member 

 of the uranium-radium series; certain of the unstable atoms 

 disintegrating according to a different plan and determining 

 a separate line of descent. As such atoms are known to be 

 subordinate in number it is probable that halos generated by 

 actinium, or even definitely revealing its presence, will never 

 be observed ; its effects being always associated with and, as 

 it were, submerged in, those of the preponderating radium 

 series. There is soms reason to believe that the fastest 

 moving a ray products of actinium, ranging from 5" 5 to 

 6'5 cm. in air, are concerned in finally blackening up the 

 uranium-radium halo ; but so far as our observations have 

 gone no halos referable solely to this radioactive series occur. 

 The extension of the work of Bragg and Kleeman has re- 

 sulted in showing that the laws discovered by these investi- 

 gators apply to a rays however originating, whether from 

 actinium or thorium. 



In the Leinster haughtonite we find that a considerable 

 number of small halos exist, possessing a radius of about 

 013 mm. These are simple, generally structureless, halos, 

 but occasionally showing a darkened peripheral border. They 

 may vary much in intensity, the fainter being associated usually 

 with the smallest nuclei. Jn most cases the nucleus is clearlv 

 and sharply defined as a minute black or nearly opaque speck, 

 which may look square or angular. The colour of these halos 

 varies from that of a faint smoky grey to a deep brown or even 

 black. Their form is perfectly circular. The following are 

 some radial dimensions in millimetres read in cleavage flakes 

 of the mica :— 0-0129, 0*0147, 0'0125, 0-0132, 0*0128, 0*0128, 

 0-0135, 0-0137, 0-0137, 0*0111, 00141, 0-0135, 0*0132, 

 0*0138, 0-0122. We might add many scores of such deter- 

 minations. 



These halos only very rarely reveal any trace of an outer 

 corona or ring. Thus two of the above, having radii in each 

 case of 0*0137 mm., showed a first very faint trace of the 

 corona. Such coronas lie at the correct radial distance for 

 RaC as given in measurements cited further on. It is re- 

 markable that halos at this stage may be much darkened and 

 yet fail to show any trace of the action either of RaC or 

 other of the more penetrating rays, whiie those showing 

 traces of a corona may be only fairly well defined. As we 

 may assume that the time of accumulation of the radioactive 



Phil. Mag. S. 6. Vol. 19. No. 112. April 1910. 2 T 



