SOME CONTEMPORARY ADVANCES IN PHYSICS— X 139 



secutive adhesions of electrons, each of which settles down into a 

 peculiar orbit and remains there more or less unperturbed as the later 

 comers immigrate one after the other into the system. May not 

 then the process of X-ray absorption consist in a powerful intruding 

 entity, electron or quantum, violently invading the interior regions of 

 the atom and casting out one or another of the deeper-lying earlier- 

 added electrons, while the later-added ones nearer or upon the frontier 

 remain attached? May not X-ray emission consist in the passage of 

 one of these latter electrons into the orbit formerly held by its prede- 

 cessor, now unexpectedly reft away and its place left empty? 



Although an affirmative answer to these questions involves a very 

 literal and concrete conception of electron-orbits, most physicists 

 make it, and would like to prove it. The chief difficulty lies in the 

 fact that all information about X-ray Stationary States is primarily 

 information, not about the prior condition of the electron which is 

 gone, but about the final condition of the residue which is left behind. 



The data show at all events that there are not nearly so many 

 conditions of the residue, as there are electrons of the completed atom; 

 from which it is fairly safe to conclude that the electrons of the atom 

 are so arranged, that any one of several different electrons may be 

 removed and the residue be left always in the same condition — • 

 therefore, that the electrons are arranged in groups, each electron 

 being situated essentially like every other of its group. In discussing 

 the formation of an atom by successive binding of electrons, it was 

 remarked that several electrons may be bound in orbits each char- 

 acterized by a common value of n and a common value of k. These 

 two ideas may coincide; and great efforts are being made to bring 

 them into entire coincidence. The evidence indicates, for example, 

 that the first ten electrons bound to a nucleus are divided into four 

 groups. Absence of an electron from one of these groups entails 

 that the atom is in the K state; absence of an electron from the second, 

 third, or fourth group brings it about that the atom is in the Lj, or 

 Ljj, or Ljji state, respectively. So much the X-ray data do show 

 rather definitely; although the actual number of electrons in each 

 of the four groups cannot yet be deduced. If one could prove a priori 

 that the first ten electrons annexed by a nucleus settle down into 

 orbits of four distinct kinds, the achievement would be a great one. 

 Intimations that something of this sort has been achieved are made 

 every now and then; but it is difficult to tell whether the assertions 

 which are made have been derived cogently from a principle or are 

 inspired guesswork. 



There is a remarkable numerical agreement in this field, the meaning 



