r. The same force of gravitation, or the same
pressure of the surrounding ether, which compresses the central mass
into a fiery globe, will act upon the loose material of the arms and
compress it into smaller globes. But there is an interesting and acute
difference of opinion amongst modern experts as to whether these smaller
globes, the early planets, would become white-hot bodies.

The general opinion, especially among astronomers, is that the
compression of the nebulous material of the arms into globes would
generate enormous heat, as in the case of the sun. On that view the
various planets would begin their careers as small suns, and would pass
through those stages of cooling and shrinking which we have traced in
the story of the stars. A glance at the photograph of one of the spiral
nebulae strongly confirms this. Great luminous knots, or nuclei, are
seen at intervals in the arms. Smaller suns seem to be forming in them,
each gathering into its body the neighbouring material of the arm,
and rising in temperature as the mass is compressed into a globe. The
spectroscope shows that these knots are condensing masses of white-hot
liquid or solid matter. It therefore seems plain that each planet will
first become a liquid globe of fire, coursing round the central sun, and
will gradually, as its heat is dissipated and the supply begins to fail,
form a solid crust.

This familiar view is challenged by the new "planetesimal hypothesis,"
which has been adopted by many distinguished geologists (Chamberlin,
Gregory, Coleman, etc.). In their view the particles in the arms of
the nebula are all moving in the same direction round the sun. They
therefore quietly overtake the nucleus to which they are attracted,
instead of violently colliding with each other, and much less heat
is generated at the surface. In that case the planets would not pass
through a white-hot, or even red-hot, stage at all. They are formed by
a slow ingathering of the scattered particles, which are called
"planetesimals" round the larger or denser masses of stuff which were
discharged by the exploding sun. Possibly these masses were prevented
from falling back into the sun by the attraction of the colliding body,
or the body which caused the eruption. They would revolve round the
parent body, and the shoals of smaller particles would gather about them
by gravitation. If there were any large region in the arm of the nebula
which had no single massive nucleus,