90 



atoms in this way; and there can be little doubt that its 

 charge, that is to say, the field which is about it, is a main 

 cause of this loss of energy. But if a /i particle is asso- 

 ciated with the a particle so that the tubes of induction pass 

 from one particle to the other, and the field is greatly con- 

 tracted, it would seem that the chief cause of the stopping of 

 the a particle has been removed. ^2) The penetrating power of 

 a pair might be very great indeed, and its ionising power cor- 

 respondingly reduced ; for, although there does not seem to 

 be a direct connection between energy spent and ionisation 

 produced, there can be no doubt that the two are simul- 

 taneous. The limitation of the field of the pair would depend 

 on its moment; if the latter was small, that is to say, if the 

 positive and negative were close together, the field would be 

 more circumscribed. It is, therefore, possible to provide for 

 pairs to have varying penetrating and ionising powers; a 

 pair of small moment being a good penetrater but a bad 

 ioniser. Such a pair would be incapable of deflection by mag- 

 netic or electric fields, and would show no refraction. It is 

 conceivable that it might show a one-sided or polarisation 

 effect, for if it were ejected from a rotating atom it would 

 itself possess an axis of rotation. 



When X-rays were first investigated, and again when y 

 rays were discovered, it was often suggested, in each case, that 

 the radiation might consist of material particles. Rontgen 

 himself proposed in the third of his memoirs a theory of 

 this nature. But it was always felt that the difficulty of 

 accounting for the great penetration of these radiations was 

 insuperable. It seems now that this difficulty was quite 

 exaggerated, and even imaginary. It does not appear out of 

 place, therefore, to reconsider the position in the light of the 

 more recent knowledge. 



Assuming, then, that the neutral pair has great pene- 

 trating, but weak ionising, powers, is uninfluenced by mag- 

 netic or electric fields, and shows no refraction, it does so 

 far conform to the properties of the y ray. And, further, if 

 it has any moment at all, and therefore any external field, it 

 may at last suffer some violent encounter which will resolve 

 it into a positive and a negative, an a and a /i particle. Of 

 these the ft particle would be the one possessed of much the 

 greater velocity, and would appear as a secondary ray. Thus, 

 in the neighbourhood of the point of impact, an ionisation 

 would appear of much greater intensity than anything pro- 

 duced along the track of the pair itself. So Becquerel has 

 found the action of the y rays on a photographic plate to be 

 almost entirely due to the secondary rays which they produce. 



(2) See also Kutlierford's l^adioactive Transformations, p. 272. 



