to the long list of familiar
substances, including several chemical compounds, in the red stars,
may prove to be fundamentally significant when adequately studied
from the standpoint of the investigator of atomic structure. The
existing evidence seems to favor the view, recently expressed by
Saha, that many of these differences are due to varying degrees
of ionization, the outer electrons of the atoms being split off
by high temperature or electrical excitation. It is even possible
that cosmic crucibles, unrivalled by terrestrial ones, may help
materially to reveal the secret of the formation of complex elements
from simpler ones. Physicists now believe that all of the elements are
compounded of hydrogen atoms, bound together by negative electrons.
Thus helium is made up of four hydrogen atoms, yet the atomic weight
of helium (4) is less than four times that of hydrogen (1.008).
The difference may represent the mass of the electrical energy
released when the transmutation occurred.

[Illustration: Fig. 39. The Trifid Nebula in Sagittarius (Ritchey).

The gas "nebulium," not yet found on the earth, is the most
characteristic constituent of irregular nebulae. Nebulium is recognized
by two green lines in its spectrum, which cause the green color of
nebulae of the gaseous type.]

Eddington has speculated in a most interesting way on this possible
source of stellar heat in his recent presidential address before the
British Association for the Advancement of Science (see _Nature_,
September 2, 1920). He points out that the old contraction hypothesis,
according to which the source of solar and stellar heat was supposed
to reside in the slow condensation of a radiating mass of gas under
the action of gravity, is wholly inadequate to explain the observed
phenomena. If the old view were correct, the earlier history of
a star, from the giant stage of a cool and diaphanous gas to the
period of highest temperature, would be run through within eighty
thousand years, whereas we have the best of evidence that many
thousands of centuries would not suffice. Some other source of energy
is imperatively needed. If 5 per cent of a star's mass consists
originally of hydrogen atoms, which gradually combine in the slow
process of time to form more complex elements, the total heat thus
liberated would more than suffice to account for all demands, and
it would be unnecessary to assume the existence of any other source
of heat.

[Illustrati