416 Tlie Corpuscular Hypothesis of the y and X Mays. 



to say secondary X rays coming off a plate of zinc on which 

 sufficiently hard X rays are falling, be made to strike a plate 

 of copper. Their energy is gradually converted into that of 

 cathode rays, which possess a certain definite power of pene- 

 tration, i. e. a certain definite speed (or perhaps average 

 speed) as Sadler has shown. These cathode rays possess 

 more than the critical speed for copper ; we may imagine 

 them to scatter in the zinc, losing all sense of original direc- 

 tion very quickly and falling in speed. When they reach 

 the critical speed for copper and the maximum conversion of 

 form takes place, the characteristic X rays of copper will 

 flash out in all directions. If they pass this speed without 

 conversion their energy is spent merely on the copper atoms, 

 transforming itself in the usual ways into heat. But if X 

 rays produced by some means in a copper plate are allowed 

 to fall on a zinc plate, and there form cathode rays, the 

 speed of these latter rays is below the critical speed for zinc, 

 and no X rays characteristic of zinc are produced. Thus all 

 Barkla's effects are qualitatively explained. Until the con- 

 version of cathode ray energy into X ray energy has been 

 more fully investigated, such an hypothesis can be no more 

 than a provisional one, but it seems simple and reasonable, 

 and suggests promising lines of research. 



In the foregoing pages I have tried to follow out the con- 

 sequences of adopting the " entity hypothesis " of X and 

 7 rays, and to show how we are led to modify our views of 

 well known theories and our interpretations of well known 

 experiments. Since there is so much to consider, the dis- 

 cussion has, I fear, been rather lengthy; but I think the 

 result is simple. We are to think of each entity as possessing 

 initially a certain store of energy which it spends gradually 

 as it goes along, the result being ionization of the material 

 through which it passes ; there are no sudden accessions or 

 withdrawals of energy ; the path is not necessarily straight, 

 but made up of a number of small pieces more or less straight, 

 the deflexions or turnings being the results of intra-atomic 

 collisions ; the /3 rays are very liable to such deflexions, and 

 the cathode rays even more so. Certain conversions of form 

 may take place, y into /3, X into cathode ray, and so on; but 

 in such cases the energy is handed on, and in some cases at 

 least the momentum. The essence of it all is the recognition 

 of the individuality of each entity which is to be followed by 

 itself from its origin through all its changes of direction and 

 sometimes its changes of form, until its gradually diminishing 

 energy becomes too small to render it distinguishable. 



