The four planets which lie beyond the asteroids give us relatively
little information concerning their physical condition, though they
afford a wide field for the philosophic imagination. From this point
of view the reader is advised to consult the writings of the late R.A.
Proctor, who has brought to the task of interpreting the planetary
conditions the skill of a well-trained astronomer and a remarkable
constructive imagination.

The planet Jupiter, by far the largest of the children of the sun,
appears to be still in a state where its internal heat has not so far
escaped that the surface has cooled down in the manner of our earth.
What appear to be good observations show that the equatorial part of
its area, at least, still glows from its own heat. The sphere is
cloud-wrapped, but it is doubtful whether the envelope be of watery
vapour; it is, indeed, quite possible that besides such vapour it may
contain some part of the many substances which occupy the atmosphere
of the sun. If the Jovian sphere were no larger than the earth, it
would, on account of its greater age, long ago have parted with its
heat; but on account of its great size it has been able,
notwithstanding its antiquity, to retain a measure of temperature
which has long since passed away from our earth.

In the case of Saturn, the cloud bands are somewhat less visible than
on Jupiter, but there is reason to suppose in this, as in the
last-named planet, that we do not behold the more solid surface of the
sphere, but see only a cloud wrap, which is probably due rather to the
heat of the sphere itself than to that which comes to it from the sun.
At the distance of Saturn from the centre of the solar system a given
area of surface receives less than one ninetieth of the sun's heat as
compared with the earth; therefore we can not conceive that any