the Corpuscular Hypothesis of the y and X Rays. 395 



has a considerable speed, say 2 x 10 9 cm./sec, and as an atom 

 o£ helium it must contain several electrons, yet we never find 

 in the gas traversed any electrons moving with a speed o£ 

 more than a few volts : that is to say, we find only 8 rays. 

 Again, one « ray never gives rise to two a rays : nor one 

 X ray to two X rays, so far as we can see. The enquiry 

 really resolves itself into the difficult question of the way 

 in which ionization comes about. There are indications 

 that it is not a straightforward process in which the moving 

 entity drives out the electron from the atom by direct collision, 

 because, in the case of the a. particle at least, the energy 

 spent is not always proportional to the ionization produced — 

 there must be an intervening link ; and because, as already 

 said, the ejected electrons all seem to have speeds of the same 

 low order. There is indeed little certain information on these 

 points, and it can only be said that to all appearances ionization 

 is the result of the passage of entities through molecules, and 

 that Ihe observed facts can be expressed on the simple hypo- 

 thesis that there is a gradual drain on the energy of the 

 entity but no large change at any one encounter with an 

 atom. Of course it may well be asked, what then does 

 happen when one electron moves so directly upon another 

 that we may expect a collision such as occurs when one 

 billiard ball strikes another ? But then we have here pre- 

 conceived ideas of volumes, surface contact", and elasticities, 

 which we must not carry over to the case of electrons 

 encountering each other. There is really nothing to compel 

 us to handle such electrons as anything more than mere 

 centres of force : if we give them dimensions, it is only to 

 make them have the right amount of electromagnetic mass. 

 Even when we take this view we have no sure ground on 

 which to base a calculation as to the probable result of an 

 encounter, because the electrons in the atom cannot be con- 

 sidered separately ; each one is backed up by unknown 

 linkages with positive electricity and with the general frame- 

 work of ihe atom, as we know from the fact that the scattering 

 of /3 rays depends very greatly on the atomic weight of the 

 scattering material. To sum up, there is nothing to be said 

 against, and something to be said in favour of, the simple 

 hypothesis that the j3 particle gradually spends energy along 

 its track, but does not lose any material portion of its energy 

 on account of the violent deflexions to which it is frequently 

 subjected. Its career is like that of an a. particle with many 

 more deflexions in it, though there is nothing at present to 

 prove that if the track were straightened out the length of it 

 would be constant, as in the case of the larger entity, the 



