INAUGURAL ADDRESS. 17 



can also take place. For, when we come to consider the life history 

 of the Rontgen or gamma rays, we find that, like the cathode or beta 

 rays, from which they may be transfoi-med, they move in straight 

 lines through many atoms, and yet may be deflected at last. The 

 penetration is, however, enormously greater. Also, as in the case of 

 the flying electron, these I'ays may be modified by the act of deflec- 

 tion, and become less penetrating than before. The changes have been 

 studied by Barkla in the case of X rays, and by Madsen in the case of 

 the gamma rays. And, again, as in the case of the electron, the rays 

 may disappear altogether; the reversed transformation then takes 

 place, the X ray being replaced by a cathode ray, the gamma by a 

 beta. It is to be remarked that when cathode rays ai'e replaced by X 

 rays, and in turn these disappear to be replaced by cathode rays, the 

 last have nearly the velocity of the original cathode rays. Putting all 

 these facts together, we have some idea of the life histoiy of these 

 rays, which we see cannot be considered separately. Of an original 

 bundle of cathode rays some, after a number of deflections, lose their 

 energ}^ altogether, some are for a time replaced by X rays, but pre- 

 sently cathode rays reappear of j-ather less energy than before ; and, 

 finally, we have no longer a stream of cathode particles flying at high 

 speed, but a number of negativeh' chai'ged molecules scattered through 

 the gas. 



This very curious alternation of forms leads us to ask of what 

 nature the Rontgen and gamma rays must be to make the transforma- 

 tion possible? 



The orthodox view, due to Sir Georg-e Stokes, is, that they consist 

 of aether pulses; that they are the disturbances of the aether which 

 spread away from the places where the motion of electrons is altered. 

 In the original form of the theory the disturbance was supposed to 

 disperse its energy over widening surfaces, as the energy of the splash 

 where a stone enters the water distributes itself in spreading ripples. 

 But this view has necessarily been abandoned. It could hold only so 

 long as the appearance of cathode rays from atoms struck by X rays 

 was supposed to be due to radio-active explosions of the atoms, pre- 

 cipitated by the X rays. This was imagined in order to explain why 

 the cathode rays, which arise in this way, have precisely the same 

 speed, no matter how much the pulses have spread before they origi- 

 nate the rays. But it is found (1) that the rays have the same speed 

 no matter from what atoms they arise, and we could not expect such 

 uniformity as the result of explosions of widely different atoms; (2) 

 that the rays, at least in the case of the gamma and beta sequence, 

 move off at first in continuation of the line of movement of the gamma 



B 



