320 SCIENCE PROGRESS 



of secondary cathode rays in the direction of propagation of the 

 X or 7 rays. 



That the velocity of the secondary corpuscles should increase 

 with harder rays follows at once ; for the faster the doublet, the 

 greater the initial speed of the negative component. 



The discrepancy between the number of molecules ionised 

 and the whole number present is explained by the fact that the 

 neutral pair is local in its action, and affects only those molecules 

 which it encounters. Its speed is maintained with little loss 

 during its flight : and we thus arrive at the independence of 

 the velocity of the secondary corpuscle on the ray intensity. 



The production, sometimes of true secondary rays and 

 sometimes of scattered rays, when X rays encounter matter, 

 occurs when the pairs striking the atoms are respectively 

 shattered or merely ejected unchanged. But it has also to be 

 noticed that when X rays are incident on a plate the cathode 

 rays leaving the near side of the plate are derived in part from 

 the stream of such particles which was travelling with the X rays 

 before incidence, and which had been formed from the molecular 

 encounters en route. This portion is scattered to an extent 

 which depends on the atomic weight of the plate. The other 

 part, which originates in the plate itself, also varies in amount 

 with the atomic weight. Now the intensity of the "incidence" 

 secondary cathode radiation in the case of X rays, /3 rays, and 

 7 rays has been shown by J. J. Thomson, McClelland and 

 Kleeman respectively, to follow the order of the atomic weights. 

 The neutral pair theory throws in each case the responsibility 

 for the resemblance largely on the presence of negative electrons 

 in the incident beam. 



Prof. Bragg has developed the theory to furnish an explana- 

 tion of polarisation. If in a Rontgen tube the effect of a cathode 

 particle on the motion of an atom in the anticathode is such 

 that a pair is thrown off, the pair will rotate in the plane which 

 contains the directions of propagation of both the incident 

 cathode ray and the ejected pair. If such a pair falls upon a 

 reflecting surface it will be liable to be taken up only by an 

 atom revolving in the same plane, and if ejected again will 

 continue to move and rotate in that plane. Thus the effects 

 will be very much like those which experiment has disclosed. 



The suggestiveness of the fact that the secondary cathode 

 rays due to X rays have velocities of the same order as those of 



