416 Prof. J. J. Thomson on the Cathode Rays. 



above and the electron when the atom is charged? The charged 

 atom cannot behave as if the charge were spread over its 

 surface ; for if it did it would require a potential fall of about 

 a million volts to separate the electron from the atom. Again, 

 the value of m/e as determined by the Zeeman effect is of the 

 samo order as that deduced from the deflexion of the cathode 

 rays, so that the charge must move independently of the body 

 charged. The electron thus appears to act as a satellite to 

 the atom. 



A difficulty in the way of supposing that mass is entirely 

 an electrical effect, and that in the impact of cathode rays we 

 have electrons striking against much larger masses, is the 

 large proportion of the energy converted into heat when the 

 cathode rays strike against a solid. AVhen an electron is 

 stopped, theory shows that the energy travels off in a pulse of 

 electromagnetic disturbance, and this energy would only 

 appear as heat at the place struck if the waves were absorbed 

 by the target close to the point of impact : if these targets 

 were made of a substance like aluminium, which is trans- 

 parent to these waves, we should expect much of the energy 

 to escape in the pulse. As far as I can see the only ad- 

 vantage of the electron view is that it avoids the necessity of 

 supposing the atoms to be split up : it has the disadvantage 

 that to explain any property of the cathode rays such as 

 Lenard's law of absorption, which follows directly from the 

 other view, hypothesis after hypothesis has to be made : it 

 supposes that a charge of electricity can exist apart from 

 matter, of which there is as little direct evidence as of the 

 divisibility of the atom ; and it leads to the view that cathode 

 rays can be produced without the interposition of matter at 

 ail by splitting up neutrons into electrons : it has no ad- 

 vantage over the other view in explaining the penetration of 

 solids by the rays, this on both views is due to the smallness 

 of the particles. Until we know something about the vibra- 

 tions of electrons, it does not seem to throw much light on 

 Eontgen rays to say that these are vibrations of the electrons. 

 The direct experimental investigation of the chemical 

 nature (so to speak) of the cathode rays is very difficult, and 

 though I have for some time past been engaged on experi- 

 ments with this object, they have not so far given any decisive 

 result. 



Yours very sincerely, 



J. J. Thomson. 

 Cavendish Laboratory Cambridge, 

 March 11th, 1899. 



