ue of their similar manner of
rotation. This is not the only reason for looking upon Jupiter as being,
in some respects, almost as much a solar as a planetary body. Its
exceptional brightness rather favors the view that a small part of the
light by which it shines comes from its own incandescence. In size and
mass it is half-way between the earth and the sun. Jupiter is eleven
times greater than the earth in diameter and thirteen hundred times
greater in volume; the sun is ten times greater than Jupiter in diameter
and a thousand times greater in volume. The mean density of Jupiter, as
we have seen, is almost exactly the same as the sun's.

Now, the history of the solar system, according to the nebular
hypothesis, is a history of cooling and condensation. The sun, a
thousand times larger than Jupiter, has not yet sufficiently cooled and
contracted to become incrusted, except with a shell of incandescent
metallic clouds; Jupiter, a thousand times smaller than the sun, has
cooled and contracted until it is but slightly, if at all, incandescent
at its surface, while its thickening shell, although still composed of
vapor and smoke, and still probably hot, has grown so dense that it
entirely cuts off the luminous radiation from within; the earth, to
carry the comparison one step further, being more than a thousand times
smaller than Jupiter, has progressed so far in the process of cooling
that its original shell of vapor has given place to one of solid rock.

A sudden outburst of light from Jupiter, such as occurs occasionally in
a star that is losing its radiance through the condensation of
absorbing vapors around it, would furnish strong corroboration of the
theory that Jupiter is really an extinguished sun which is now on the
way to become a planet in the terrestrial sense.

Not very long ago, as time is reckoned in astronomy, our sun, viewed
from the distance of the nearer fixed stars, may have appeared as a
binary star, the brighter component of the pair being the sun itself and
the fainter one the body now called the planet Jupiter. Supposing the
latter to have had the same intrinsic brilliance, surface for surface,
as the sun, it would have radiated one hundred times less light than the
sun. A difference of one hundredfold between the light of two stars
means that they are six magnitudes apart; or, in other words, from a
point in space where the sun appeared as bright as what we call a
first-magnitude star, its comp