RECENT ADVANCES IN SCIENCE 123 



positive part is not diffused throughout the volume of the atom, 

 but is highly concentrated in a nucleus which acts as a centre 

 of force, attracting, according to the inverse square law, the 

 electrons which are rotating around it in rings. The advantage 

 of the Thomson atom is that its stability is assured on the basis 

 of ordinary dynamical theory. As regards the Rutherford atom, 

 it can be shown that, although the electrons are in stable 

 rotation so long as disturbances are at right angles to the 

 orbital plane, this is not in general the case for disturbances in 

 the plane of the orbits. However, the difficulty of reconciling 

 the Thomson model with large-angle scattering of a-rays has led 

 to a considerable discussion of the Rutherford atom on account 

 of the ease with which its structure accounts for such large- 

 angle scattering. The question of its dynamical stability has 

 recently been treated from a new point of view by Dr. Bohr, 

 who, by an application of the Quantum Theory of Radiation, 

 has indicated an avenue of discussion and research along which 

 workers may proceed, and possibly succeed in formulating 

 dynamical principles still more fundamental than the Newtonian, 

 and capable of summarising not only our knowledge concerning 

 the large-scale motions treated in ordinary mechanics, but also 

 our gradually increasing information about such small-scale 

 motions as are involved in the electronic orbits of atoms. On 

 this account a good deal of attention is being paid at present to 

 the Rutherford-Bohr model. 



Both models, of course, have rings of electrons as a common 

 postulate ; the number of electrons is closely connected with the 

 atomic weight ; on one view it is identified with the atomic 

 number of the element in the periodic series. Many ingenious 

 explanations have been forthcoming seeking to correlate the 

 numbers of the electrons, the size and number of the rings, and 

 their rate of rotation with chemical properties and with the line 

 spectra and characteristic X-ray radiation of the element con- 

 sidered. In a recent paper (Proc. P/iys. Soc, December 19 14) 

 Prof. Thomson points out that the characteristic Rontgen 

 radiations with which we are now experimentally acquainted 

 (usually called K and L radiations) may be identified as due to 

 vibration of the two rings of electrons nearest the centre of the 

 atom, whilst light of the visible spectrum is due to the outer- 

 most ring. The properties of these rings may therefore be 

 studied by making use of Rontgen radiation ranging from the 



