ELECTRICAL STRUCTURE OF MATTER RUTHERFORD 175 



atom but it accounts in detail for many of the most characteristic 

 features of the periodic law of Mendeleeff. It gives us for the first 

 time a clear idea of the reason for the appearance in the family of 

 elements of groups of consecutive elements with similar chemical 

 properties, such as the groups analogous to the iron group and the 

 unique group of rare earths. The theory of Bohr, like all living 

 theories, has not only correlated a multitude of isolated facts known 

 about the atom, but has shown its power to predict new relations 

 which can be verified by experiment. For example, the theory 

 predicted the relations which must subsist between the Rydberg 

 constants of the arc and spark spectra, and generally between all 

 the successive optical spectra of an element, a prediction so strik- 

 ingly confirmed by Paschen's work on the spectrum of doubly 

 ionized aluminium and Fowler's work on the spectrum of trebly 

 ionized silicon. Finally it predicted with such great confidence the 

 chemical properties of the missing element, number 72, that it gave 

 the necessary incentive for its recent discovery. 



Wliile the progress of our knowledge of the outer structure of 

 atoms has been much more rapid than could have been anticipated, 

 we clearly see that only a beginning has been made on this great 

 problem, and that an enormous amount of work is still required 

 before we can hope to form anything like a complete picture even 

 of the outer structure of the atom. We may be confident that the 

 main features of the structure are clear, but in a problem of such 

 great complexity progress in detail must of necessity be difficult 

 and slow. 



"We have not so far referred to the very difficult question of the 

 explanation on this theory of the chemical combination of atoms. 

 In fact, as yet the theory has hardly concerned itself with mole- 

 cular structure. On the chemical side, however, certain advances 

 have already been made, notably by G. N. Lewis, Kossel, and Lang- 

 muir, in the interpretation of the chemical evidence by the idea of 

 shared electrons, which play a part in the electronic structure of 

 two combined atoms. There can be little doubt that the next decade 

 will see an intensified attack by physicists and chemists on this very 

 important but undoubtedly very complicated question. 



Before leaving this subject, it may be of interest to refer to cer- 

 tain points in Bohr's theory of a more philosophical nature. It is 

 seen that the orbits and energies of the various groups of electrons 

 can be specified by certain quantum numbers and the nature of the 

 radiation associated with a change of orbit can be defined. But at the 

 same time we can not explain why these orbits are alone permissible 

 under normal conditions, or understand the mechanism by which ra- 

 diation is emitted. It may be quite possible to formulate accurately 

 the energy relation of the electrons in the atom on a simple theory, 



