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.