SOME CONTEMrOK.INY .-(/)r.-l,VC£.V IN rUVSICS nil Jl" 



entirt'ly coiirfak-tl. It is not altogftluT inarci'ssihU'; for llu- di-llec- 

 tions sufferetl liy alpha-particles ami electrons living throuKli atoms 

 are iiithienced !>>• the electrons of the atom, not by the nucleus ex- 

 clusiveK ; anti from the decree in which the observed deflections difTcr 

 from what the nucleus alone would compel, it is possible to draw- 

 some conclusions about the way in which the electrons are arranged. 

 The mathematical ililViculties, as the reader will readily admit, are 

 tremenilous; the problem of determiniiiK the path of a n\'in>; electron 

 through a cloud of electrons, probably themselves in motion, is 

 enough to make the best of mathematicians despair; yet some progress 

 in this direction has already been achieved, as I narrated in the second 

 article of this series. Again, the scattering of X-rays by atoms 

 should depend on the manner in which their electrons are arranged; 

 and some measurements and some deductions have already been 

 made, although the researches have been in abeyance for some years, 

 probably because the newest disco\eries about X-ray scattering make 

 it extremely doubtful what the mechanism of the effect really is. 



The study of deflections of alpha-particles by atoms has thus 

 brought precious guidance to the atom-builder, and imposed severe 

 limitations upim him, yet only partial ones. He is constrained to erect 

 his atom according to certain fundamental rules, and yet has an ex- 

 tremely free hand in arranging the details. He is practically com- 

 pelled to build the atom of an element which occupies the iVth place 

 in the periodic system, out of N electrons and a much more massive 

 nucleus with a positive charge Ne. The data which I have cited do 

 not absolutely enforce these numerical values; but there is no other 

 model which they permit which could possibly rival this one in 

 respect of convincing simplicity. He may not make the electrons 

 go more than a few times 10~' cm. from the nucleus; he is constrained 

 to leave a small vacant space around the nucleus, and within this 

 space he may not tamper with the inverse-square law of force (a 

 restriction which has eliminated several favored atom-models of the 

 decade before 1910)." Having conformed to these restrictions he 



" Except that he may and must alter the inverse-square law of force to just the 

 extent that further and more delicate exjieriments of this type require. Thus 

 Bieler (I.e. supra) concludes, from a study of tletlections of alpha-particles passing 

 close to the nuclei of aluminium atoms, that within about 10"'' cm. of the aluminium 

 nucleus the inverse-square repulsion which it exerts upon an alpha-particle is 

 supplemented by an attractive force — perhaps an inverse-fourth-powcr attraction, 

 just balancing the repulsion at a distance of .^.44- lO^'-" cm. from the centre of the 

 nucleus. Rutherford earlier found anomalies in the encounters between hydrogen 

 nuclei and alpha-particles, which suggested to Uarwin that the latter might be 

 considere<l as a disc-shapc<l hard particle, or an oblate spheroid of semi-axes 4.10 " 

 and 8.10" cm.; this would repel hydrogen nuclei according to the inverse-square 

 law so long as it did not actually strike them. 



