A CENTURY >S PROGRESS IN PHYSICS 385 



siders it to be constituted of electrons revolving about a 

 positive nucleus either singly or grouped in concentric 

 rings, in much the same manner as the planets revolve 

 around the sun. Experiments on the scattering of alpha 

 rays, however, show that the nucleus, while it must have 

 a positive charge sufficient to neutralize the charges of 

 all the electrons moving around it, cannot have a volume 

 of an order of magnitude greater than that of the elec- 

 tron. The number of unit charges residing on it, except 

 in the case of hydrogen, which is supposed to consist of a 

 singly charged nucleus and only one electron, is found to 

 be approximately half the atomic weight. Thus helium, 

 with an atomic weight of about four, has a doubly 

 charged nucleus with two electrons revolving about it, 

 and lithium a triply charged nucleus and three electrons. 

 The number of unit charges on the nucleus is supposed to 

 correspond with the atomic number used by Moseley in 

 interpreting the results of his experiment on the X-ray 

 spectra of the elements. 



Now the electron which is revolving around the posi- 

 tive nucleus of a hydrogen atom, must, according to elec- 

 trodynamic laws, radiate energy. This radiation will 

 act as a resistance to its motion, causing its orbit to 

 become smaller and its frequency to increase. Hence 

 luminous hydrogen would be expected to give off a con- 

 tinuous spectrum. The very fine lines actually found 

 seem inexplicable on the classical dynamical and electro- 

 dynamical theories. These lines, and those of many 

 other spectra, may even be grouped into series, and the 

 relations between them expressed in mathematical form. 

 Formulae have been proposed by Balmer, Rydberg, Ritz 

 and others, all of which contain a universal constant N 

 as^well as certain parameters which must be varied by 

 unity in passing from one line of a series to the next. 



In 1913 Bohr 18 proposed anatomic theory which brings 

 to light a remarkable numerical relationship between 

 this quantity N and Planck's constant h. He postulated 

 that the electron in the hydrogen atom, for instance, can- 

 not revolve in a circle of any arbitrary radius, but is con- 

 fined to those orbits for which its kinetic energy is an 

 integral multiple of h n, n being its orbital frequency. 

 Now at times this electron is supposed to jump from an 



