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.
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