Various Gases by the a. Particle* or Radium. 319 



the ionization produced. It will be seen later that this effect 

 is now clearly shown, but I was unfortunate in using a com- 

 parison of benzene with acetylene as an illustration. 



§ 3 *. 



Though our knowledge of the process o£ ionization by 

 the a particle is as yet only small and imperfect, it does not 

 seem out of place to draw together what facts we do know, 

 and to endeavour to connect them by some thread of argument, 

 which may be useful for a time. 



In the first place, there is the fact that the ionization 

 produced by the a particle increases as its velocity diminishes. 

 Now, Rutherford has recently shown (Phil. Mag., Aug. 

 1906) that the particle spends energy at a uniform rate along 

 its path. It follows, therefore, that the ionization produced 

 is not proportional to the energy spent. In my preliminary 

 paper I have already given a reason for supposing that the 

 energy spent and the ionization produced are not directly 

 connected, viz.. that the former is related to the atomic weight 

 by a simple law and The latter is not. 



As a temporary hypothesis let us suppose that there is 

 an intervening link ; that the a particle produces a primary 

 effect A. which in turn produces a secondary effect B. The 

 latter consists of ionization, the former may or may not do 

 so. It is in the production of the primary effect that the 

 energy of the particle is spent. 



Since the energy spent is related to the atomic weight 

 by a simple law. since it is independent of velocity, and since 

 there is a critical speed at which all ionization ceases, which 

 speed is the same for all atoms, it appears clear that A is 

 a sub-atomic effect. It consists in the performance of some 

 act which always involves the expenditure of the same 

 amount of energy : and the stopping-power of an atom is 

 proportional to the number of times that the act is performed 

 within it. The effect might consist, for example, in some 

 operation upon a common constituent of all atoms, such as an 

 a particle. The critical speed might be that at which the 

 moving a. particle failed to penetrate, or, more generally, act 

 upon the a particle of the atom. 



In the next place, consider the effect B. The proportion 

 of! ionization to energy spent varies from molecule to molecule, 

 and is dependent on the velocity of the a particle. The 

 results described in this paper show that, as already said, 

 di=kf(v)de. The nature of the function f(y) i- of great 



* The greater part of § 3 lias been written since the above was read 

 before the lioval Society of South Australia. 



