84 



have here a critical speed for the electron. Below 

 this, it is not able to leave the parent atom. If its 

 velocity exceeds the critical amount it possesses powers of 

 penetration and of causing ionisation, the extent of these 

 powers depending on the excess. 



The existence of a common speed for all S rays may, of 

 course, imply that the ejection is not directly effected by the 

 ionising agent, but that tlie latter simply precipitates the 

 discharge. A man running through a battery might pull the 

 triggers of some or all of the guns which it contained, and the 

 velocity of the shot would not depend on the strength of the 

 man, nor the rate at which he ran, nor how much energy 

 he spent m the transit. And so it may be understood why 

 8 rays are projected at a speed which is independent of the 

 nature of the agent, as has been said above. 

 So also it appears to be independent of the 

 intensity of the agent's action. Fuchtbauer found 

 the velocity of the 8 rays produced by canal rays to be 

 independent of the intensity of the primary rays; Lenard 

 found the same for ultraviolet light. In my own experiments 

 on the a rays (Phil. Mag., March, 1907), I have brought for- 

 ward evidence to show that the amount of ionisation produced 

 in an atom is proportional to the volume of the atom approxi- 

 mately. Taking this in conjunction with the rule that the 

 ionisation produced in a gas is nearly proportional to the 

 inverse of the speed, we have the very simple, if approximate, 

 law, that the ionisation produced by an a particle in any 

 atom under any circumstances is inversely proportional to 

 the time spent inside the atom. This appears to point to the 

 ionisation as purely a trigger effect. Not that the a particle 

 spends no energy in the atom ; it is clear it must do so, since 

 its speed is gradually reduced, but there is not a direct con- 

 nection between the energy spent and the number of ions 

 produced. But whatever energy the ionising agent may 

 spend, or in whatever way it spends it, it seems likely that 

 the issue of the 8 particle is the result of some disruption in 

 the atom, or sub-atom, which is the same for all atoms and 

 under all circumstances. 



If we turn our attention now to all secondary radiation 

 other than the 8 rays, it seems to be, in general, a rough 

 reflection or scattering of the primary. Allen has shown that 

 there is only a little less velocity in the secondary rays than in 

 the primary /3 rays, or in the tertiary than in the secondary. 

 McClelland has measured the total ionisation produced by the 

 secondary as compared with the primary /i radiation; and 

 since he used a small ionisation-cliamber with which he ex- 



