PHYSICS 



21 



cases have been of the " ring " rather than the " shell " type 

 suggested by Lewis and Langmuir ; i.e. it is assumed that the 

 electrons circulate in a number of orbits lying in one plane, or 

 at least in a number of parallel planes, and so the electrons are 

 by no means confined even to the limits of a region comparable 

 in size with one of the cells of the Lewis-Langmuir theory. Now 

 the success of a number of researches based on such ideas is 

 sufficient to warrant the belief that there is some element of 

 truth in the postulated models. It is true that the particular 

 principle which is used to select the suitable orbits from the 

 infinite number permissible on older views is quite foreign of 

 the classical system of dynamics ; but, as stated above, that 

 cannot be regarded to-day as a fatal flaw. An interesting 

 feature of all these models is the abandonment of the idea that 

 the frequency of the light emitted by an atom is related to the 

 period of vibration of an electron in it about some state of rest 

 or steady motion, this latter period being of course determined 

 by the electron's environment. The present idea is rather this : 

 each electron can circulate in one of a finite number of " station- 

 ary " orbits (which are selected by Bohr's principle) ; while in 

 such an orbit its motion can be deduced by the laws of classical 

 dynamics (but other orbits infinite in number and equally valid 

 on the older views are ruled out because they do not conform 

 to Bohr's principle). From time to time under external influ- 

 ences it leaves one orbit and takes up another with a different 

 amount of energy. The difference of energy is radiated or 

 absorbed at a frequency determined by Planck's law— energy = 

 hv, where v is the frequency and h is Planck's constant. 



Now, whatever may be the contradictions between the models 

 suggested by the purely chemical and static data and by the 

 spectroscopic data, they have one quality in common — dis- 

 regard of traditional principles where these seem to cramp un- 

 duly the use of scientific imagination. It is not unnatural, 

 therefore, that there still remain distinguished physicists who, 

 while not denying the real advances made by such methods, 

 still seek for solutions of our difficulties along more beaten tracks. 

 Thus during the past few years a number of papers have been 

 published by Sir J. J. Thomson, which, while suggesting modifi- 

 cations of the analytical expressions for the forces between 

 electrified particles at very close quarters, yet retain the con- 

 cepts and equations of classical dynamical and electromagnetic 

 theory. The modifications proposed in the expressions for 

 electric and magnetic forces allow of the electrons in Thomson's 

 model to take up positions of rest at the corners of certain 

 well-known polyhedra, such as the tetrahedron, the cube, the 

 rhombic dodecahedron, the cubo-octahedron, and so in the 

 sharing of electrons between atoms and the consequent fitting 



