174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



speed of light. When we visualize the extraordinary complexity 

 of the electronic system we may be surprised that it has been pos- 

 sible to find any order in the apparent medley of motions. 



In reaching these conclusions, which we owe largely to Professor 

 Bohr and his coworkers, every available kind of data about the 

 different atoms has been taken into consideration. A study of the 

 X-ray spectra, in particular, affords information of great value as 

 to the arrangement of the various groups in the atom, while the 

 optical spectrum and general chemical properties are of great im- 

 portance in deciding the arrangements of the superficial electrons. 

 While the solution of the grouping of the electrons proposed by 

 Bohr has been assisted by considerations of this kind, it is not em- 

 pirical in character, but has been largely based on general theoretical 

 considerations of the orbits of electrons that are physically possible 

 on the generalized quantum theory. The real problem involved may 

 be illustrated in the following way. Suppose the gold nucleus be in 

 some way stripped of its attendant 79 electrons and that the atom 

 is reconstituted by the successive addition of electrons one by one. 

 According to Bohr the atom will be reorganized in one way only, 

 and one group after another will successively form and be filled 

 up in the manner outlined. The nucleus atom has often been likened 

 to a solar system where the sun corresponds to the nucleus and the 

 planets to the electrons. The analogy, however, must not be pressed 

 too far. Suppose, for example, we imagined that some large and 

 swift celestial visitor traverses and escapes from our solar system 

 without any catastrophe to itself or the planets. There will in- 

 evitably result permanent changes in the lengths of the month and 

 year, and our system will never return to its original state. Con- 

 trast this with the effect of shooting an electron or a particle through 

 the electronic structure of the atom. The motion of many of the 

 electrons will be disturbed by its passage, and in special cases an 

 electron may be removed from its orbit and hurled out of its atomic 

 system. In a short time another electron will fall into the vacant 

 place from one of the outer groups, and this vacant place in turn 

 will be filled up, and so on until the atom is again reorganized. In 

 all cases the final state of the electronic system is the same as in the 

 beginning. This illustration also serves to indicate the origin of 

 the X rays excited in the atom, for these arise in the process of 

 reformation of an atom from which an electron has been ejected, 

 and the radiation of highest frequency arises when the electron is 

 removed from the K group. 



It is possibly too soon to express a final opinion on the accuracy of 

 this theory which defines the outer structure of the atom, but there 

 can be no doubt that it constitutes a great advance. Not only does 

 it offer a general explanation of the optical and X-ray spectra of the 



