has occurred--Geological Evidence as to the Changes of the Sun's
Heat Doubtful--The Cooling of the Sun--The Sun cannot be merely an
Incandescent Solid Cooling--Combustion will not Explain the
Matter--Some Heat is obtained from Meteoric Matter, but this is not
Adequate to the Maintenance of the Sun's Heat--The Contraction of a
Heated Globe of Gas--An Apparent Paradox--The Doctrine of
Energy--The Nebular Theory--Evidence in Support of this
Theory--Sidereal Evidence of the Nebular Theory--Herschel's View of
Sidereal Aggregation--The Nebulae do not Exhibit Changes within the
Limits of our Observation.

That a portion of a work on astronomy should bear the title placed at
the head of this chapter will perhaps strike some of our readers as
unusual, if not actually inappropriate. Is not heat, it may be said, a
question merely of experimental physics? and how can it be legitimately
introduced into a treatise upon the heavenly bodies and their movements?
Whatever weight such objections might have once had need not now be
considered. The recent researches on heat have shown not only that heat
has important bearings on astronomy, but that it has really been one of
the chief agents by which the universe has been moulded into its actual
form. At the present time no work on astronomy could be complete without
some account of the remarkable connection between the laws of heat and
the astronomical consequences which follow from those laws.

In discussing the planetary motions and the laws of Kepler, or in
discussing the movements of the moon, the proper motions of the stars,
or the revolutions of the binary stars, we proceed on the supposition
that the bodies we are dealing with are rigid particles, and the
question as to whether these particles are hot or cold does not seem to
have any especial bearing. No doubt the ordinary periodic phenomena of
our system, such as the revolution of the planets in conformity with
Kepler's laws, will be observed for countless ages, whether the planets
be hot or cold, or whatever may be the heat of the sun. It must,
however, be admitted that the laws of heat introduce certain
modifications into the statement of these laws. The effects of heat may
not be immediately perceptible, but they exist--they are constantly
acting; and in the progress of time they are adequate to effecting the
mightiest changes throughout the universe.

Let us briefly recapitulat