44:4 Prof. W. H. Bragg on tlie Properties and 



with a velocity which is independent o£ the intensity of the 

 pulse, and why it should be able to exercise ionizing powers 

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

 of a sphere of say 10 or 20 feet radius. All these pheno- 

 mena are more simply explained if we suppose the ray to 

 be a neutral pair which has only a local action ; i. e. can only 

 affect the molecules on its path, which can penetrate to 

 great distances in air, losing little speed as it goes, and which 

 gives rise to a cathode ray when it is broken by impact. 



It seems to me that the material-nature hypothesis shows 

 to advantage when we consider the secondary radiation of 

 the X rays. The rays cause the emission of cathode rays 

 whose speed averages about 5xl0 9 (Dorn). We have no 

 experience of any aether wave causing the emission of any 

 but 8 rays, i. e., electrons with a speed of about 10 8 . It can 

 hardly be said that differences in intensity of the aether pulse 

 can account for this remarkable contrast. For the speed of 

 the S rays caused by ultra-violet light has been shown by 

 Lenard to be independent of the intensity of the light ; and 

 the velocity of the X-ray secondary radiation does not depend 

 on the intensity of the X rays. It may be argued that the 

 breadth of the pulse is the prime factor, on the grounds that 

 Lenard found the velocity of the 8 rays due to ultra-violet 

 light to depend somewhat on the nature of the light ; but it 

 is hard to believe that a diminution of the width of the pulse, 

 no matter how extreme, can increase the energy of the 

 ejected electron about a thousand times. 



But if we regard the secondary radiation as the result of 

 the break-up of a neutral pair, the high velocity of the ejected 

 electron (5 x 10 9 ) may be more readily explained. The action 

 must be entirely different from that of ultra-violet light. 



It is difficult to found any arguments for or against either 

 theory on considerations of the relative energies of the 

 original cathode stream, the X rays, and the secondary rays. 

 For if the energies of any transformation do not balance, it 

 is easy to square the account by postulating either some 

 release of the internal energy of the atom, or the reverse, 

 viz. the absorption of energy by the atom involving a dis- 

 appearance of the visible energy. On the neutral-pair hypo- 

 thesis the cathode rays would probably have a trigger action, 

 and the pairs would draw their energy from that internal to 

 the atom: it might not be necessary to invoke the aid of 

 internal atomic energy in order to account for the energy 

 of the secondary radiation. In the case of the aether-pulse 

 theory it is necessary to suppose that the secondary radiation 

 derives its energy from the atom's store (' Conduction of 



