92 



of Electricity through Gases, pp. 294-297), why the spreading 

 pulse should only affect a few of the atoms passed over, why 

 the secondary cathode rays are ejected with a velocity which 

 is independent of the intensity of the pulse which weakens as 

 it spreads, and why it should be able to exercise ionising 

 power when its energy is distributed over so wide a surface as 

 that of a sphere of, say, ten or twenty feet radius. All these 

 phenomena are capable of quite simple explanation if we sup- 

 pose the ray to be a neutral pair which has only a local action, 

 i.e., can only affect the molecules which it traverses, which 

 can penetrate to great distances, which loses very little speed 

 as it goes, and gives rise to a cathode ray when it is broken 

 by impact. 



The hypothesis can readily be developed so as to furnish 

 an explanation of the polarisation effects which Bark) a (loc. 

 clt.) has worked out with so much success. If the cathode 

 particles so affect the motion of an atom as to make it throw 

 off a pair, then the plane of rotation of the pair will contain 

 the direction of its translatory motion. And if such a pair 

 fell upon a reflecting surface, it might naturally be taken up 

 only by an atom revolving in the same plane, and, if ejected 

 again, continue moving and revolving in that plane, thus 

 exhibiting exactly the effects which Barkla has found. 



The sudden arrest of a cathode particle m.ust originate 

 an ether pulse. But, as a matter of fact, it has never been 

 shown that such sudden stoppages do take place. Wien has 

 calculated (Ann. d. Phys., December 28, 1905) that the 

 particles must come to rest in a space of about 10~'° cm., if 

 the energy of the X-rays is to be accounted for in this way, 

 even supposing the whole of the cathode rays to be effective; 

 and the appearances of the bundles of reflected ji and cathode 

 rays rather point to gradual deviations with a very slow dimi- 

 nution of energy, and therefore very little radiation. On the 

 other hand, we know that atoms are set in motion with very 

 high speeds inside the vacuum tube, and that the gas in the 

 X-ray tube is largel}'- driven into the walls (Campbell 

 Swinton, Chemical News, March 22, 1907) in a manner which 

 reminds us of a rays penetrating a solid. 



It is possible that the example of the ". particle shows 

 that a pair cannot possess a velocity greater than 10^, since 

 at a higher speed it M'ould be stripped of an electron, and 

 become an a particle. J. J. Thomson has suggested that at 

 this critical speed the c particle becomes electrically neu- 

 tralized by the attachment of an electron. Presumabl}^ such 

 a pair would then go on as a y ray. No such consequence 

 has been observed ; and on the present hypothesis it would 

 be better to su])pose that the a particle ends its career by 



