PLEOCHROIC HALOES 149 



uniplanar nature of its cleavage. There is little 

 cleavage in any plane but the one, although it is 

 easy to show that the molecules in the plane of 

 the flake are in orderly arrangement and are more 

 easily parted in some directions than in others. 

 In such a medium beyond all others we must look 

 with surprise upon the perfect sphere struck out 

 by the alpha rays, because it seems certain that 

 the cleavage is due to lesser attraction, and, 

 probably, further spacing of the molecules, in a 

 direction perpendicular to the cleavage. 



It may turn out that the spacing of the mole- 

 cules will influence but little the average number 

 per unit distance encountered by rays moving in 

 divergent paths. If this is so we seem left to 

 conclude that in spite of its unequal and polarised 

 attractions there is equal retardation and equal 

 ionisation in the molecule in whatever direction it 

 is approached. Or, again, if the encounters in- 

 deed differ in number, then some compensating 

 effect must exist whereby a direction of lesser 

 linear density involves greater stopping power 

 in the molecule encountered, and vice versa. 



The nature of the change produced by the 

 alpha rays is unknown. But the formation of the 

 halo is not, at least in its earlier stages, attended 

 by destruction of the crystallographic and optical 

 properties of the medium. The optical proper- 

 ties are unaltered in nature but increased in 

 intensity. This applies till the halo has become 

 so darkened that light is no longer transmitted 



