uch meteors
doubtless fall incessantly upon the sun. Acted on by this force, the
earth, were it stopped in its orbit to-morrow, would rush towards, and
finally combine with, the sun. Heat would also be developed by this
collision. Mayer first, and Helmholtz and Thomson afterwards, have
calculated its amount. It would equal that produced by the combustion
of more than 5,000 worlds of solid coal, all this heat being generated
at the instant of collision. In the attraction of gravity, therefore,
acting upon non-luminous matter, we have a source of heat more
powerful than could be derived from any terrestrial combustion. And
were the matter of the universe thrown in cold detached fragments into
space, and there abandoned to the mutual gravitation of its own parts,
the collision of the fragments would in the end produce the fires of
the stars.

The action of gravity upon matter originally cold may, in fact, be the
origin of all light and heat, and also the proximate source of such
other powers as are generated by light and heat. But we have now to
enquire what is the light and what is the heat thus produced? This
question has already been answered in a general way. Both light and
heat are modes of motion. Two planets clash and come to rest; their
motion, considered as that of masses, is destroyed, but it is in great
part continued as a motion of their ultimate particles. It is this
latter motion, taken up by the rather, and propagated through it with
a velocity of 186,000 miles a second, that comes to its as the light
and heat of suns and stars. The atoms of a hot body swing with
inconceivable rapidity--billions of times in a second--but this power
of vibration necessarily implies the operation of forces between the
atoms themselves. It reveals to us that while they are held together
by one force, they are kept asunder by another, their position at any
moment depending on the equilibrium of attraction and repulsion. The
atoms behave as if connected by elastic springs, which oppose at the
same time their approach and their retreat, but which tolerate the
vibration called heat. The molecular vibration once set up is
instantly shared with the aether, and diffused by it throughout space.

We on the earth's surface live night and day in the midst of aethereal
commotion. The medium is never still. The cloud canopy above us may
be thick enough to shut out the light of the stars; but this canopy is
itself a warm b