inference that its heat is
intense. And it has been further shown that the rate at which the
temperature increases on descending below the surface, is such as would
be found in a mass which had been cooling for an indefinite period. The
Moon, too, shows us, by its corrugations and its conspicuous extinct
volcanoes, that in it there has been a process of refrigeration and
contraction, like that which has gone on in the Earth. There is no
teleological explanation of these facts. The frequent destructions of
life by earthquakes and volcanoes, imply, rather, that it would have
been better had the Earth been created with a low internal temperature.
But if we contemplate the facts in connexion with the Nebular
Hypothesis, we see that this still-continued high internal heat is one
of its corollaries. The Earth must have passed through the gaseous and
the molten conditions before it became solid, and must for an almost
infinite period by its internal heat continue to bear evidence of this
origin.

The group of giant planets furnishes remarkable evidence. The _a priori_
inference drawn above, that great size joined with relatively high ratio
of centrifugal force to gravity must greatly retard aggregation, and
must thus, by checking the genesis and dissipation of heat, make the
process of cooling a slow one, has of late years received verifications
from inferences drawn _a posteriori_; so that now the current conclusion
among astronomers is that in physical condition the great planets are in
stages midway between that of the Earth and that of the Sun. The fact
that the centre of Jupiter's disc is twice or thrice as bright as his
periphery, joined with the facts that he seems to radiate more light
than is accounted for by reflection of the Sun's rays, and that his
spectrum shows the "red-star line", are taken as evidences of
luminosity; while the immense and rapid perturbations in his atmosphere,
far greater than could be caused by heat received from the Sun, as well
as the formation of spots analogous to those of the Sun, which also,
like those of the Sun, show a higher rate of rotation near the equator
than further from it, are held to imply high internal temperature. Thus
in Jupiter, as also in Saturn, we find states which, not admitting of
any teleological explanations (for they manifestly exclude the
possibility of life), admit of explanations derived from the Nebular
Hypothesis.

But the argument from temperature does not