On the a Particles of Radium. 601 



consisting of simple curves overlapping. The entire absence 

 of any effect of this kind shows that the critical speed is 

 invariable. This seems a very important result ; and 

 strengthens the argument that the act of ionization is exactly 

 the same in all gases. 



It might possibly be argued that the ionization curve of a 

 mixture is simple because the a particle is absolutely stopped 

 at a certain velocity by one of the gases of the mixture. Apart 

 from other difficulties, this explanation is clearly impossible, 

 because, if it were true, the range in the mixture would 

 depend on that gas only. 



I may add that we have now examined the range of the a. 

 particle in a number of substances besides those given in our 

 first list, viz. : lead, iron, nickel, ethyl iodide, chloroform, 

 oxygen, carbon dioxide, carbon bisulphide, pentane, and 

 benzene ; with results at least as good as those previously 

 obtained. 



As regards our attempt to discover the relation between 

 velocity and ionization we are not so fortunate; and our 

 original calculation fails because we assumed the a. particle 

 of Ra to have lost its speed at 7 cms. from the start. It is 

 easy, however, to make the proper alteration ; but it is pos- 

 sible that the extra complication reduces the chance of 

 obtaining a correct interpretation of the result. 



Let s be the distance traversed by the particle from the 

 point where it ceases ionizing to the point where its velocity 

 becomes negligible. Then in the case of the a particle of 

 Ra C, the energy at a distance s Q from this latter point is 

 40 per cent, of the energy at a distance 7-fs . If distance 

 from this point be denoted generally by s, and if ionization 

 varies inversely as the nth. power of the velocity, then the 

 energy cc s 2 /( n + 2 > (see previous paper), 



and .-. 5_ = («o+J)* +i ; 



so that 7 



71 + 2 



(2-5pr-l 



Having thus found s , we see that the ionization is proportional 

 to 



(* + *„) 2l(n+2) ~ + * - d) 2/(n+2 ), 



when a particles from the highest stratum of the radium layer 



