.vo.u/i coNTEMroN.iRy .■inr.ixci.s ix physics riii 411 



In awaiting that eventual atom-model, it is best to regard the 

 atoms of the present day as mutable anti transitory. Like railway 

 time-tables, atom-nuxicls should be inscribed "sui)jert to change 

 without notice." Nothing is irrevocable in plusics, except the 

 record of past events; and we who have seen the undulatory theory 

 of light assailed and shaken may well hesitate to put uniiuestionirfg 

 faith in any atom-m(xlel. K\en if there is no daiij^er of chanjje, it 

 is a virtue to keep data and theories sharply sejiarated in one's mind. 

 In no field is this more diflicult and important than in the field of 

 this article, where the ver>' language used to describe the data is 

 saturated with the spirit of a particular conception of the atom, and 

 it is customary to expoimd the theory before the facts. For these 

 reasons I shall go to the opposite extreme, and treat the contemporary 

 atomic theory with an exaggerated reserve which in many places will 

 seem excessive to the reader and in some to the writer himself. 



The favorite atom-model of the physicists of toda>- is a structure of 

 electrons, congregated about a positiveK'-charged nucleus. The 

 data which this atom is designed primarily to interpret were discovered 

 before 1913, or else since 1913 by methods developed before that 

 time. These discoveries are due largely to Rutherford, whose name 

 the model often bears. The sections of this article which are labelled 

 B, C and I) are de\oted to these data, and to the inferences from 

 them. In 1913 a great change in the situation was wrought by a 

 brilliant idea of Niels Bohr. Bohr did not discover new data; he 

 taught a new way of interpreting old ones, he showed men how to 

 read spectra. Through this interpretation of spectra, and through 

 data which were discovered by men inspired with his idea, a previ- 

 ously-unknown property of matter was disclosed. This is expressed 

 by saying that each atom possesses many distinct Stationary States. 

 The largest section of this First Part of the article, the section E, 

 is devoted to the knowledge of these Stationary States. Had these 

 been discovered earlier, an atom-model might have been devised to 

 explain them and them alone. Rutherford's atom-model was already 

 in the field, and it was modified so that it might interpret the new 

 knowledge. To these modifications, of which some are of a remark- 

 able simplicity and beauty, the .Second Part of this article will be 



devoted. 



B. TiiK Electron^ 



The electron is the atom of negative electricity. An individual 



electron can be captured upon a droplet of oil or mercury, or a minute 



• This section is rlrastirnlly curtailed, for the chief facts about the electron should 

 by this time be common knowledge. .Millikan's book "The Electron" (now in its 

 aecond edition) may be consulted. 



