440 Prof. W. H. Bragg on the Properties and 



The absorption of 7 and X rays appears to follow a 

 purely exponential law so far as experiment has been made. 

 The 8 rays are absorbed by molecules immediately on their 

 production. 



Having thus discussed certain properties of the various 

 rays which do exist, it seems interesting to make an attempt 

 at the estimation of the properties of some rays which might 

 exist, though the fact has not been proved as yet. Radio- 

 active substances emit both positive and negative particles. 

 It does not seem at all out of place to consider the possibility 

 of the emission of neutral particles, such as, for example, a 

 pair consisting of one a or positive particle and one B or 

 negative particle. The recent additions to our knowledge 

 of the laws of absorption of ol and /5 particles give us some 

 grounds on which we may attempt to found an estimate of 

 the properties of such pairs. 



We know that the a particle moves in a rectilinear course 

 throughout its whole range, and passes through the atoms 

 which it encounters without deflexion. It does not pursue 

 a course which is straight on the whole, but zigzag in 

 detail ; the direction and amount of a particle in motion are 

 the whole characteristics of that motion at any instant, and 

 no memory of any previous motion exists. If, therefore, a 

 particle pursues a straight line in its motion as a whole, 

 it must keep to that line entirely and make no excursions 

 from side to side. We must, therefore, suppose that an 

 atom, or at least, an ol particle, endowed with sufficient speed, 

 can pass directly through another atom without appreciable 

 deflexion. The ol particle loses speed as it penetrates 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 /3 particle is associated with 

 the ol particle so that the tubes of induction pass from one 

 particle to the other, and the field is greatly contracted, 

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

 a particle has been removed *. The penetrating power of 

 a pair might be very great indeed, and its ionizing power 

 correspondingly reduced ; for, although there does not seem 

 to be a direct connexion between energy spent and ionization 

 produced, there can be no doubt that the two are simultaneous. 

 The limitation of the field of the pair would depend on its 

 moment ; if the latter were 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 

 * See also Rutherford's ' Radioactive Transformations,' p. 272. 



