RONTGEN RAYS 319 



violet light. This is indicated by the respective velocities of 

 the secondary corpuscles, which with 7 rays have about two 

 million times, and with X rays about twenty thousand times the 

 energy with ultra-violet light. 



The greater the frequency of the light the greater is the 

 energy in each unit, and it may thus happen that a molecule 

 may set a superior limit to the range of wave lengths of light 

 which can ionise it. This would explain the superior ionising 

 ability of ultra-violet light over visible light. 



As the units or bundles possess momentum as well as 

 energy, there is no difficulty in accounting for the fact that, 

 when matter is encountered, the secondary electrons pre- 

 ponderate in the direction of propagation of the rays. 



It is to be noticed that, on this theory, light will have an 

 extremely coarse structure ; for Rontgen and 7 rays the coarse- 

 ness will be even more marked. 



The Neutral Pair Theory. — Prof. W. H. Bragg in May 1907 

 suggested for Rontgen and 7 rays a constitution obtained by the 

 close association of a positive and a negative particle which 

 rotate round each other. The negative particle consists of a 

 cathode or /3 ray; the positive was at first regarded as of atomic 

 dimensions, but Prof. Bragg's later work has led him to give 

 preference to the, as yet unisolated, positive electron. 



Such a pair would display at once some of the features of a 

 Rontgen ray, such as the absence of deviation in an electric or 

 magnetic field, and of refraction. The large penetrating power 

 of the rays is explained partly by the high velocity, and partly 

 by the weak external electric field of the pair. The closer the 

 two components are together, or in other words the smaller 

 the moment of the doublet, the more limited will be the field, 

 and the greater the penetrating power. The latter, of course, 

 also increases with the speed. The ionising properties depend 

 on the strength of the external field ; they will be small with 

 pairs of little moment. Those doublets in which the attachment 

 of the positive to the negative has become loosened, constitute 

 a softer and more ionising radiation. 



As to the method of ionising, we have to suppose that the 

 united positive and negative particles passing into an atom are 

 separated if the internal field of the atom is strong enough ; the 

 negative flies on unaffected in speed by the field, and the positive 

 is left behind in the atom. This is the explanation of the excess 



