144 HUXLEY MEMORIAL LECTURES 



centric spherical shells of darkened biotite. This 

 is a very interesting fact. For it proves that in 

 the mineral the alpha ray gives rise to the same 

 increased ionisation towards the end of its range 

 as Bragg determined in the case of gases. And 

 we must conclude that the halo in every case 

 grows in this manner. A spherical shell of 

 darkened biotite is first produced and the inner 

 colouration is only effected as the more feeble 

 ionisation along the track of the ray in course of 

 ages gives rise to sufficient alteration of the 

 mineral. This more feeble ionisation is, near the 

 nucleus, enhanced in its effects by the fact that 

 there all the rays combine to increase the ionisa- 

 tion and, moreover, the several tracks are there 

 crowded by the convergency to the centre. 

 Hence the most elementary haloes seldom show 

 definite rings due to uranium, etc., but appear as 

 embryonic disc-like markings. The photographs 

 on the screen illustrate many of the phases of halo 

 development. Rutherford succeeded in making 

 a halo artificially by compressing into a capillary 

 glass tube a quantity of the emanation of radium. 

 As the emanation decayed the various derived 

 products came into existence and all the several 

 alpha rays penetrated the glass; darkening the 

 walls of the capillary out to the limit of the range 

 of radium C in glass. Fig. 8 is a magnified 

 view of the tube. The dark central part is the 

 capillary. The tubular halo surrounds it. This 

 experiment has, however, been anticipated by 



