s disclosed a new source of energy within
the atoms themselves, and there are scientific men, like Professor
Arrhenius, who attach great importance to this source. But, although it
may prolong the limited term of life which physicists formerly allotted
to the sun and other stars, it is still felt that the condensation of a
nebula offers the best explanation of the origin of a sun, and we have
ample evidence for the connection. We must, therefore, see what the
nebula is, and how it develops.

"Nebula" is merely the Latin word for cloud. Whatever the nature of
these diffused stretches of matter may be, then, the name applies fitly
to them, and any theory of the development of a star from them is still
a "nebular hypothesis." But the three theories which divide astronomers
to-day differ as to the nature of the nebula. The older theory, pointing
to the gaseous nebulae as the first stage, holds that the nebula is a
cloud of extremely attenuated gas. The meteoritic hypothesis (Sir N.
Lockyer, Sir G. Darwin, etc.), observing that space seems to swarm
with meteors and that the greater part of the nebulae are not gaseous,
believes that the starting-point is a colossal swarm of meteors,
surrounded by the gases evolved and lit up by their collisions. The
planetesimal hypothesis, advanced in recent years by Professor Moulton
and Professor Chamberlin, contends that the nebula is a vast cloud of
liquid or solid (but not gaseous) particles. This theory is based mainly
on the dynamical difficulties of the other two, which we will notice
presently.

The truth often lies between conflicting theories, or they may apply to
different cases. It is not improbable that this will be our experience
in regard to the nature of the initial nebula. The gaseous nebulae,
and the formation of such nebulae from disrupted stars, are facts
that cannot be ignored. The nebulae with a continuous spectrum, and
therefore--in part, at least--in a liquid or solid condition, may very
well be regarded as a more advanced stage of condensation of the same;
their spiral shape and conspicuous nuclei are consistent with this.
Moreover, a condensing swarm of meteors would, owing to the heat
evolved, tend to pass into a gaseous condition. On the tether hand, a
huge expanse of gas stretched over billions of miles of space would be
a net for the wandering particles, meteors, and comets that roam
through space. If it be true, as is calculated, that our 24,000 miles
of atm