for Production of Luminosity in Atmospheric Neon. 939 



anore than one ionization velocity has been detected. In 

 employing the method which we used for the investigation 

 of critical electron velocities in argon and other gases, 

 the detection o£ a second or third critical velocity for the 

 removal of a first electron from the atom, if more than 

 one critical velocity existed, would depend upon the relative 

 probabilities of ionization resulting from encounters between 

 atoms and electrons having velocities in excess of the critical 

 value, in each case. It is doubtful whether the higher 

 critical velocities would always be detected, unless the 

 corresponding radiation velocities were intermediate to the 

 ionization velocities. 



Of the theories of atomic structure which have been put 

 forward, that proposed by Lewis and by Langmuir * affords 

 an explanation of the greatest number of different phenomena. 

 This theory, which was originally evolved to account for the 

 facts of chemical combination and valency, has since obtained 

 considerable support from the recent work on X-rays and 

 crystal structure f, and has been found to be consistent 

 with the results of the investigations on magnetism and 

 atomic structure J. So far, however, it has not been made 

 to supply any explanation of the excitation of spectrum 

 lines and the series relations in spectra. In this direction 

 the Bohr-Sommerfeld theory of the existence within the 

 atom of a series of non-radiating orbits, any of which may 

 be temporarily occupied by an electron, lias been found 

 the most satisfactory, but it has not yet adequately explained 

 the existence of the several series of non-radiating orbits 

 necessary to account for all the lines of the complicated 

 spectra of elements whose atoms contain several electrons. 

 The fact that in neon principal and corresponding sub- 

 ordinate series appear to be associated with the transitions 

 of electrons differently situated in the normal atom, seems 

 likely to be of importance in this connexion. 



According to the Lewis-Langmuir theory, the electrons 

 in the atom of neon exist in two concentric shells, the inner 

 shell containing two electrons and the outer shell the re- 

 maining eight electrons. In this model the electrons are 

 either stationary or execute small oscillations about mean 

 positions. Since the two electrons in the inner shell are 

 closer to the nucleus than the eight outer electrons, it seems 

 reasonable to assume that a much larger quantum of energy 



* I. Langmuir, Pliys. Rev. vol. xiii. p. 300 (1019). 



t W. L. Bragg-, Phil. Mag-, vol. xl. p. 169 (3920). 



\ A. E, Gxley, Proc. Roy. Soc. A. vol. xcviii. p. 264 (1921). 



