Structure of the Atom, 



491 



electrons. This assumption seems to be legitimate when we 

 remember that the mass and energy of the a particle are 

 very large compared with that of an electron even moving 

 with a velocity comparable with that of light. Simple con- 

 siderations show that the deflexions which an a. particle would 

 experience even in passing through the complex electronic 

 distribution of a heavy atom like gold, must be small com- 

 pared with the large deflexions actually observed. In fact, 

 the passage of swift u particles through matter affords the 

 most definite and straightforward method of throwing light 

 on the gross structure of the atom, for the a particle is able 

 to penetrate the atom without serious disturbance from the 

 electronic distribution, and thus is only affected by the 

 intense field associated with the nucleus of the atom. 



This independence of the large angle scattering on the 

 external distribution of electrons is only true for charged 

 particles whose kinetic energy is very large. It is not to be 

 expected that it will hold for particles moving at very much 

 lower speeds and with much less energy — such, for example, 

 as the ordinary cathode particles or the recoil atoms from 

 active matter. In such cases it is probable that the external 

 electronic distribution plays a far more prominent part in 

 governing the scattering than in the case under consideration. 



Scattering of ft particles* 



It is to be anticipated on the nucleus theory that swift 

 ft particles should suffer deflexions through large angles in 

 their passage close to the nucleus. There seems to be no 

 doubt that such large deflexions are actually produced, and 

 I showed in my previous paper that the results of scattering 

 of ft particles found by Crowther * could be generally 

 explained on the nucleus theory of atomic structure. It 

 should be borne in mind, however, that there are several 

 important points of distinction between the effects to be 

 expected for an a particle and a ft particle. Since the force 

 between the nucleus and ft particle is attractive, the ft par- 

 ticle increases rapidly in speed in approaching the nucleus. 

 On the ordinary electrodynamics, this entails a loss of energy 

 by radiation, and also an increase of the apparent mass of 

 the electron. Darwin t has worked out mathematical I}' the 

 result of these effects on the orbit of the electron, and has 

 shown that, under certain conditions, the ft particle does not 

 escape from the atom but describes a spiral orbit ultimately 



* Crowther, Proc. Roy. Soc. A. lxxxiv. p. 226 (1910). 

 t Darwin, Phil. Mag. xxv. p. 201 (1913). 



