PLEOCHROIC HALOES 135 



in any substance of known chemical composition 

 and density will be, compared with its range in 

 air. 



This is of special importance in connection 

 with phenomena we have presently to consider. 

 It means that, knowing- the chemical composition 

 and density of any medium whatsoever, solid, 

 liquid or gaseous, we can calculate accurately the 

 distance to which any particular alpha ray will 

 penetrate. Nor have the temperature and pres- 

 sure to which the medium is subjected any influ- 

 ence save in so far as they may affect the prox- 

 imity of one atom to another. The retardation 

 of the alpha ray in the atom is not affected. 



This valuable additive law cannot, however, in 

 strictness be applied to the amount of ionisation 

 attending the ray. The form of the molecule, or 

 more generally its volume, may have an influence 

 upon this. Bragg draws the conclusion, from this 

 fact as well as from the notable increase of ionisa- 

 tion with loss of speed, that the ionisation is 

 dependent upon the time the ray spends in the 

 molecule. The energy of the ray is, indeed, 

 found to be less efficient in producing ionisation 

 in the smaller atoms. 



Before leaving our review of the general laws 

 governing the passage of alpha rays through 

 matter, a point of interest must be referred to. 

 We have hitherto spoken in general terms of the 

 fact that ionisation attends the passage of the 

 ray. We have said nothing as to the nature of 



