the Corpuscular Hypothesis of the 7 and X Rays. 415 



Crowther supposes that secondary radiation must be present 

 and be responsible for the want of agreement ; but there 

 does not seem to be any good reason for selecting secondary 

 radiation as the cause of the error. On the other hand, the 

 entity hypothesis leads naturally to a simple explanation of 

 the general form of the curves both of aluminium and of 

 platinum. 



In the case of /3 and cathode rays there is very little 

 accurate knowledge of the third of the phenomena which I 

 have tried to distinguish above, viz. the conversion of form. 

 The conversion of ft rays into 7 rays is often doubted 

 altogether ; but it can hardly be safe to deny it, for if the 

 number of 7 rays produced by a given number of /3 rays 

 were relatively as few as the X rays produced by a stream 

 of cathode rays, the effect produced by the 7 rays would be 

 almost imperceptible. The conversion of cathode rays into 

 X rays is, however, a very obvious and common process, and 

 it is rather striking that so little work has been done to 

 discover the laws of it. It would be a great help to know 

 whether there is a critical speed or more than one critical 

 speed at which an electron should strike an atom in order to 

 get an X ray effect. Let us suppose that there is a speed 

 which it is necessary for a cathode ray falling on a given 

 atom to possess in order that the conversion may take place, 

 which does not seem at all unlikely considering the general 

 behaviour of X ray tubes. Let us suppose, further, that the 

 critical speed increases with the atomic weight, for which 

 also there is something already to be said. Then we seem 

 to have a reasonable chance of explaining the very remarkable 

 phenomena of the homogeneous secondary X radiations which 

 Barkla has discovered. The explanation given by Barkla 

 himself is not at all in accord with the arguments which I 

 have tried to state above. He supposes the primary pulse to 

 shake an atom in passing and make it give out its own cha- 

 racteristic quivers. But this suggests that a single primary 

 X ray is the cause of many secondary X rays. 



We have to explain why one single primary entity — an 

 X ray — is replaced by one secondary X ray entity after 

 collision with a certain atom, the energy of the secondary 

 being characteristic of the atom not of the primary, and its 

 direction of motion being also independent of the primary, 

 i. e. of the direction of motion of the primary. We have to 

 explain further why the X ray emitted by zinc can excite 

 the copper atom to emit its own characteristic X ray, and 

 why the reverse does not take place, the copper X ray is not 

 able to excite the zinc X ray. Let X rays from zinc, that is 



