s
the disk, the red glow lighting the edges of the belts, and the
spectroscopic evidence of an emission of light. Once more it is
difficult to doubt that a highly heated body is wrapped in that thick
mantle of vapour. With its ten moons and its marvellous ring-system--an
enormous collection of fragments, which the influence of the planet
or of its nearer satellites seems to have prevented from
concentrating--Saturn has always been a beautiful object to observe; it
is not less interesting in those features which we faintly detect in its
disk.

The next planet, Uranus, 32,000 miles in diameter, seems to be another
cloud-wrapt, greatly heated globe, if not, as some think, a sheer mass
of vapours without a liquid core. Neptune is too dim and distant for
profitable examination. It may be added, however, that the dense masses
of gas which are found to surround the outer planets seem to confirm the
nebular theory, which assumes that they were developed in the outer and
lighter part of the material hurled from the sun.

From this encouraging survey of the sister-planets we return with more
confidence to the story of the earth. I will not attempt to follow
an imaginative scheme in regard to its early development. Take four
photographs--one of a spiral nebula without knots in its arms, one of
a nebula like that in Canes Venatici, one of the sun, and one of
Jupiter--and you have an excellent illustration of the chief stages in
its formation. In the first picture a section of the luminous arm of the
nebula stretches thinly across millions of miles of space. In the next
stage this material is largely collected in a luminous and hazy sphere,
as we find in the nebula in Canes Venatici. The sun serves to illustrate
a further stage in the condensation of this sphere. Jupiter represents
a later chapter, in which the cooler vapours are wrapped close about the
red-hot body of the planet. That seems to have been the early story of
the earth. Some 6,000,000,000 billion tons of the nebulous matter were
attracted to a common centre. As the particles pressed centreward, the
temperature rose, and for a time the generation of heat was greater than
its dissipation. Whether the earth ever shone as a small white star we
cannot say. We must not hastily conclude that such a relatively small
mass would behave like the far greater mass of a star, but we may,
without attempting to determine its temperature, assume that it runs an
analogous course.

O