432 Sir J. J. Thomson on 



the molecule has not lost the energy acquired by one collision 

 before it is again in collision with the cathode particle when 

 the fresh energy it acquires is added to the store it already 

 possesses. Thus the energy of a molecule exposed to the 

 bombardment of the cathode rays goes on increasing until it 

 reaches a critical value, when the molecule explodes and a 

 positively charged particle, or it may be a neutral doublet, is 

 shot out with a velocity greater than 2 x10 s cm./sec. On 

 the theory of the Canalstrahlen given in my paper (Phil. 

 Mag. Dec. 1909) the velocity of the positive particles when 

 they have travelled some distance from the cathode is deter- 

 mined, not by the strength of the electric field nor even by 

 the properties of the molecules from which they are ejected y 

 but by the properties of a system of dissociating neutral 

 doublets. That this is the nature of the Canalstrahlen, or at 

 any rate of those of them which are deflected by electric or 

 magnetic forces, is suggested by many experiments. We 

 know that a particle with a positive charge does not remain 

 in this state indefinitely : after a time it unites with a 

 negatively electrified corpuscle and forms a neutral doublet :. 

 and again we know that constituents of the "positive rays" 

 may be at one time without a charge, and then subsequently 

 become charged, presumably by breaking up into a positively 

 charged particle and a corpuscle. 



Thus a stream of positive rays may be regarded as a gas 

 which is continually dissociating and recombining. The 

 molecules of the gas correspond to neutral doublets, the 

 atoms into which the molecules dissociate to the positively 

 charged particles and negatively charged corpuscles. 



Now it is only within certain limits of temperature, that is- 

 it is only when the kinetic energy of the gas is between 

 certain limits, that there can be appreciable amounts of the 

 gas in both the dissociated and undissociated states. For if 

 the temperature is above a certain value practically all the 

 gas will be dissociated, while if it is below another value 

 there will practically be no dissociation. The temperature 

 of the gas corresponds to the kinetic energy of the particles 

 in the positive rays : hence we see that it is only wdien the 

 velocity of these particles is within certain limits, which do 

 not depend at all upon the strength of the electric field in 

 the discharge- tube, that the positive rays could possess the 

 properties disclosed by the experiments made upon them. 

 We could not measure the velocity of much slower rays, for 

 these would not dissociate and could not therefore be deflected 

 either by electric or magnetic forces ; and if the rays whilst 

 approaching the cathode could not exist as neutral doublets, 



