the moments of momenta of all the electrons external to a com 
pleted shell. The simple character of the spectra of the alkali 
metals may be attributed to the fact that they each have but 
a single electron outside the completed shell which resembles 
that of the rare gases. 
In concluding this necessarily brief account of the modern 
theory of the structure of the atom some reference must be 
made to the important work carried out by Saha.* He has 
investigated how the equilibrium distribution of the various 
“ electronic isomers ” depends upon temperature. (By electronic 
isomers are meant ions or atoms having the same nuclear charge 
and the same number of electrons.) This theory of high tempera 
ture ionization has been of great service in the interpretation 
of solar and stellar spectra. The first effect of increasing tempera 
ture will be to tear off the outermost electron from the atomic 
system, with the formation of the ionized atom which has lost 
one electron. This corresponds to what Lockyer called the 
“ proto ” element, the spectral lines being known as “ enhanced ” 
lines. The energy of ionization can be calculated from the 
ionization potential, or from the convergence frequency of the 
principal series, and the fraction of ionized atoms at a given 
temperature can be found by applying Saha’s formula. The 
second stage in the process of ionization, in which the atom 
loses another electron, is then considered. The work of Saha 
corroborates the view of H. N. Russell that the continuous 
variation of stellar spectral types is mainly due to the varying 
values of the temperature of the stellar atmosphere. 
* M. N. Saha., Phil. Mag., voi. 40, pp. 472,809, 1920; voi. 41, p. 267, 
1921 ; Proc. Roy. Soc., voi. 99, p. 135, 1921.