233

For what are the conditions attending the passage of the ray in a medium such as mica?  According to crystallographic conceptions we have here an orderly arrangement of molecules, the units composing the crystal being alike in mass, geometrically spaced, and polarised as regards the attractions they exert one upon another.  Mica, more especially, has the cleavage phenomenon developed to a degree which transcends its development in any other known substance.  We can cleave it and again cleave it till its flakes float in the air, and we may yet go on cleaving it by special means till the flakes no longer reflect visible light.  And not less remarkable is the 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 molecules 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

234

direction it is approached.  Or, again, if the encounters indeed 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 properties are unaltered in nature but are increased in intensity.  This applies till the halo has become so darkened that light is no longer transmitted under the conditions of thickness obtaining in rock sections.  It is well known that there is in biotite a maximum absorption of a plane-polarised light ray, when the plane of vibration coincides with the plane of cleavage.  A section across the cleavage then shows a maximum amount of absorption.  A halo seen on this section simply produces this effect in a more intense degree.  This is well shown in Plate XXIII (lower figure), on a portion of the halo-sphere.  The descriptive name “Pleochroic Halo” has originated from this fact.  We must conclude that the effect of the ionisation due to the alpha ray has not been to alter fundamentally the conditions which give rise to the optical properties of the medium.  The increased absorption is probably associated with some change in the chemical state of the iron present.  Haloes are, I believe, not found in minerals from which this