PHYSICS IN 1913 619 



the most stable system consequent on the emission of the 



maximum amount of energy the angular momentum is — . 



This is probably closely connected with Weiss' magneton, or 

 elementary unit of magnetism. Balmer's and Rydberg's laws 

 follow from these assumptions, and, considering the case of 

 hydrogen as represented by a central nucleus with one rotating 

 electron, that of helium as a nucleus with two electrons, Bohr 

 has calculated a value for Rydberg's constant in close agree- 

 ment with the empirical value. His theory gives all the 

 hydrogen spectra observed ; in connection with the spectral 

 lines calculated by him for helium the question has arisen 

 whether certain lines observed from hydrogen contaminated 

 with helium, and previously attributed to hydrogen, are not 

 due to helium. Fowler attributed the lines to hydrogen ; but 

 the recent work of Evans points to these lines being part 

 of the helium spectrum as required by Bohr's theory. It may 

 be noted that Bohr finds that the charge on the nucleus, or 

 the number of electrons present in the neutral atom, is equal 

 to the atomic number of the element; this has also been sug- 

 gested by van der Broek, and agrees with Moseley's X-ray 

 spectrum results, as already mentioned. 



Bohr's atom is satisfactory as agreeing with Rutherford's 

 postulates required by the scattering experiments, and giving 

 a good account of the spectral series in a way much more in 

 accord with modern ideas on radiation than Ritz's atom ; the 

 numerical agreement is especially good. It remains to be 

 seen if the atom will give the Zeeman effect, and a new effect 

 discovered by Stark — the resolution of a spectral line into 

 components in an electric field — which will be described later. 

 Warburg has very recently worked out that such an atom 

 model will not give the required resolution into lines, but 

 merely a broadening of the spectral line ; however, these results 

 obviously depend upon the nature of the special assumptions 

 made, as further assumptions are necessary, and we await 

 further work from Bohr himself upon this point. 



The new effect discovered by Stark, just mentioned, is the 

 resolution of the chief hydrogen and helium lined into com- 

 ponents by means of a strong electric field, corresponding to 

 the Zeeman effect in the magnetic field. The great difficulty 

 in such experiments is to obtain a strong electric field in a 



