488 nni.L SYSTEAf TECHNICAL JOURXAL 



as certain as any principle not dealing with things which can be seen 

 and handled. I shall mention them briefly, in a nearly chronological 

 order. 



The direct measurement of the charge of the helium nucleus. Ruther- 

 ford' and Regener independently measured the charge on the alpha- 

 particle in the simplest, most direct and most incontrovertible way; 

 they counted the alpha-particles emitted from a sample of a radio- 

 active element in a given time, and measured the total charge they 

 carried away from it, and divided the one datum by the other. Ruther- 

 ford obtained twice 4.65-10^"' (electrostatic units) for the charge of 

 the individual particles; Regener obtained twice 4.79-10""'. The 

 agreement of the latter value with twice Millikan's standard value of 

 the electron-charge (4.774-10""') is magical; the agreement of the 

 former value is also good. Though this is an average value for a 

 great number of particles, the fact that a beam of alpha-particles is 

 not spread or split by a magnetic field proves that each has the same 

 charge (at least, to be perfectly precise, the same charge-to-mass 

 ratio). It is established that the alpha-particle is the bare helium 

 nucleus.'" 



The determination of nuclear charges by the scattering of alpha- 

 particles. When a beam of alpha-particles is played against a sheet 

 of metal foil, the nuclei of the metal atoms deflect the alpha-particles 

 passing very close to them, by virtue of the electrostatic repulsion 

 between the charge +2e on the alpha-particle and the charge -\-Ne 

 on the nucleus of the metal atom. The distribution-in-anglc of the 

 scattered alpha-particles can be calculated, assuming that the action 

 of the metal nuclei is not complicated by any forces due to the electrons 

 surrounding them. The distribution-in-anglc actually ob.served 

 agrees in form with the calculated niic; this pro\es ncii that ijicre 

 are no electrons surrounding tin- metal nuclei, but tiiai llieie is a 

 vacant space around each nucleus, wide enough so that llie majtn- pari 

 of the deflection of an alpha-particle takes place within it. .\ii this 

 was discussed in the second article of this series. The form of the 

 distribution-in-angle and its variation with the speed of the alpha- 

 particles pro\-e the existence of the atom-nuclei, of their positive 



" l-"or heliiiiii i»as, evimi'd by its spL-ilruni, .ippcars in a tiil)L' into which alpha- 

 particles are fired through the wall, and is exuded from a piece of metal which is 

 melted after alpha-particles have heen shot into it (Rutherford's e.\|xTimentsl; 

 furthermore impacts between alpha-particles and atoms of helium gas show them 

 to be of the same mass (Blackett's experiments), and the value of the e/m ratio is 

 correct for doubly-ionized helium atoms but not for any other admissible variety 

 of atom. .-\nd the radius of the alpha-particles, calculated from the experiments on 

 scattering, is smaller by several orders of magnitude, than the effective radius of 

 any known atom having electrons in addition to its nucleus. 



