388 Prof. W. H. Bragg on the Consequences of 



We therefore conclude that the energy of the j3 ray is 

 derived from that of the 7 ray, and similarly the energy of 

 the cathode ray from that of the X ray. We are then in a 

 position to take into account another experimental result. 

 The velocity of the cathode particle ejected by the X ray is 

 found to be the same, or nearly the same, as that of the 

 cathode particle in the original X-ray tube. There is no 

 doubt as to the .approximate truth of this statement, though 

 accurate experiment is wanting. Now there can be no question 

 of the storage of X-ray energy in an atom until there is 

 enough to provide for the ejection of a cathode ray, for then 

 the nature of the atom would again be of influence, and we 

 should revert to all our previous difficulties. One X ray 

 must be enough to provide one cathode ray. Nor does it 

 seem possible to suppose that the energy of several cathode 

 particles can be stored up in an atom until there is enough 

 to produoe one X ray ; for amongst other considerations there 

 would then be no apparent reason why the speed of the cathode 

 ray should influence the quality of the X ray so directly as 

 it does. Hence the X ray cannot have more energy than 

 was possessed by the cathode particle in the X ray bulb. 

 Put the two statements together and we find that one cathode 

 ray impinging on an atom may produce one X-ray and no 

 more, and in its turn the X ray through impact on an atom 

 (not necessarily the first it meets) produces one cathode ray 

 and no more, handing on its energy and its direction of 

 motion. 



It is this conclusion which seems fatal to the spreading 

 pulse theory. The latter taught us that when an electron 

 was arrested the energy set free travelled out in all directions 

 through space on an ever enlarging surface. We now find 

 that we must have the energy of the X ray confined within 

 werj narrow bounds which are not to widen as the X ray 

 travels, so that when at last the transference of energy takes 

 place the energy is all in one spot ready for the sudden 

 change. The speed of the cathode ray caused by the X ray 

 is the same no matter where it comes into being. We cannot 

 allow the energy of the X ray to spread even a little. The 

 ray is to be considered as a minute entity of some sort, its 

 energy as it travels being always bound up in an unaltering 

 volume of atomic magnitude at the most. 

 " This is a brief statement of the case for the entity hypo- 

 thesis, containing only one main line of argument. Many 

 subsidiary considerations are omitted. It is worth observing 

 that it turns on questions regarding energy. 



We must of course ask what we lose by the adoption of 



