816 On Ionization by Charged P article s» 



Bumstead to his secondary ft rays. I£ this be the true 

 nature of Bumstead's secondary ft rays, a large proportion 

 of, but certainly not all, the 8 rays ultimately emerging may 

 be due to their action. 



Some interesting considerations are suggested by estimates 

 of the energy which rays of various kinds spend in making 

 an ion, obtained by dividing the total energy lost by the ray 

 by the number of ions made. For a rays this energy turns 

 out to be a little less than 40 volts ; for ft rays the figures 

 given for the absorption and ionizing power by Kossel (6) 

 lead to values between 300 volts for the rays of 200-volt 

 velocity to 800 volts for the ft rays of RaC. It is clear that 

 this energy must be greater than that necessary to make an 

 ion (for some of the 8 rays are liberated with a finite 

 energy), and that it is less than* the average energy which 

 the ray communicates to those electrons which it liberates as 

 8 rays (for some of the 3 rays liberated collide with atoms 

 and so liberate fresh 8 rays). It is not surprising, therefore,, 

 that, though the least energy required to make an ion and 

 the velocities of the 8 rays are the same on whatever material 

 the rays act, the average energy spent in making an ion i& 

 not the same ; for this average energy depends in some 

 measure on the frequency with which an electron, liberated 

 directly by the primary rays as a 8 ray, subsequently makes 

 ions by collisions with other atoms. The remarkable fact 

 noted by Bragg that, whereas the stopping power of materials 

 for ol rays is an additive atomic property, the molecular ioni- 

 zation depends on the state of chemical combination, may 

 find some explanation in this direction. 



The fact that the average energy spent by an a ray in 

 producing an ion is less than that spent by any ft ray, shows 

 that the theory which leads to the conclusion that a rays 

 should behave in respect of ionization exactly like ft rays of 

 twice their speed cannot be wholly correct. But it is not 

 surprising that conclusions based on the hypothesis that an 

 a ray is a simple point charge should not agree in all respects- 

 with experiment. 



12. This discussion has been vague and tentative. The 

 conclusion which is intended to be drawn is that, while 

 Thomson's theory of ionization does not provide a complete 

 explanation of all the facts, and cannot do so until the intra- 

 atomic forces are further taken into account, there is nothing- 

 resulting from experiments on 8 rays which is necessarily 

 inconsistent with that theory. The most remarkable of 

 these results which formerly seemed to me inconsistent with 

 any simple theory of ionization, namely, that the velocities 



