32 SCIENCE PROGRESS 



his method turns out to be merely an electron moving faster 

 than any cathode ray. Taking it in with the others already 

 investigated, we have a complete series of electrons moving 

 with all speeds ranging from a certain important limit, which 

 will be discussed presently, up to something just less than the 

 velocity of light. There could be no point in a discussion here 

 of the beautiful experiments which go to the determination of 

 the electronic charge and mass : they are to be found in full and 

 logical statement in the original papers and in many resumes. 

 My purpose is rather to consider the significance which the fast- 

 moving electron has in relation to the new science. I shall 

 therefore assume that the electron of the cathode ray or the 

 $ ray, the electron set free by X rays, 7 rays, ultraviolet light 

 or any other agency, is always the same thing, having no 

 variable characteristic but that of speed ; and I shall limit these 

 considerations to the phenomena attending the passage of these 

 rays through matter and especially to the influence on them of 

 the speed of the electron. Even by confining the discussion to 

 these limits, it is not possible to consider the /S ray as syste- 

 matically as the a ray, for the experiments are more complicated 

 and more difficult to interpret. The /3 ray has always been a far 

 harder problem than the a ray. Even now every one is not 

 of the same mind as to the mechanism of the absorption ; but 

 certain main principles seem to stand out with some clearness 

 and I shall not blur their outlines by too much qualification but 

 prefer to run a little risk of inaccuracy as to details. 



The very difficulty and complexity of the /9 ray problem is due 

 to a most instructive characteristic of the ray — viz. its extreme 

 liability to deflection as compared with the a ray. The latter, 

 by virtue of its comparatively large mass, moves undeflected 

 through places where the fi ray is easily swung to one side. 

 The a particle, according to Rutherford, has twice the charge 

 of the /3 particle numerically (the signs are opposed but that 

 does not matter) and is several thousand times as massive, so 

 that the difference in their behaviour is easily explained. A 

 stream of /3 rays therefore very quickly loses its sharpness of 

 definition and becomes diffuse and irregular : moreover, every 

 single particle, though it sometimes possesses a higher speed 

 than the a particle, has far less energy and we must use large 

 numbers to get a measurable effect. Taking these points into 

 consideration, it is easily seen that the experimental difficulties 



