34 SCIENCE PROGRESS 



than is the case generally, this means a change in velocity on 

 i or 2 per cent. only. Next we may ask ourselves — Did the 

 deflection have any action on the atom in which it took place, 

 such as ionisation ? It is really very hard to answer this 

 question : it may be that ionisation occurs in cases where the 

 atom is simply traversed without a deflection ensuing ; and, on 

 the other hand, that deflection occurs without ionisation. Again, 

 it may be that ionisation occurs only when the action and 

 reaction exceed a certain limit ; when this is passed the ionisa- 

 tion of the atom occurs and the ft particle is deflected through 

 a certain angle at least. We do not really know whether there 

 is any connection between ionisation and deflection. 



So the ft particle experiences one deflection after another 

 and whether it loses energy only at the moment of encounter 

 with the centres of force within the atom or whether it loses 

 energy during rectilinear portions of its path as the a particle 

 does, it certainly loses energy somehow ; for several experiments 

 have lately made this clear. Does it then gradually slow down 

 or does it ever become incorporated within an atom while still 

 going at a high speed ? It is so hard to trace the slowing down 

 completely that many workers have supposed that the ray often 

 disappears within an atom while still going fast and never 

 emerges again. For my part I am inclined to think that this 

 last effect does not occur and that an atom has no power to 

 pull up and retain a fast-moving electron : it may deflect it or it 

 may rob it of some fraction of its energy, perhaps only a small 

 fraction ; and that is all it can do. On the other hand, the 

 slower an electron goes, the more rapidly does it spend energy 

 on ionisation, just as the a particle does ; moreover, there can 

 be no doubt that the slower it goes the more likely it is to be 

 deflected. Once it starts on the downhill path, its end is very 

 precipitate ; and it might seem as if it suddenly disappeared. 

 In reality its track would be a variant of something like that 

 shown in fig. 12. If this be the case, every ft ray is a cathode ray 

 before it is done ; the distinction between the two is only a matter 

 of speed ; slowing down still more, it reaches the speed at which 

 it cannot emerge from the atom once it has gone inside ; this is 

 the critical speed of about io 8 cm. per second or that which a 

 few volts of electromotive force, say ten, can generate in an 

 electron. This last point has been made clear by experiment ; 

 it has been shown that unless the speed exceeds this limit no 



