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The action of gravity lupon matter originally cold may,
in fact, be the originof all light and heat, and the proxi-
mate source of such other powers as are generated by
light and heat. But we have now to inquire 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 masses, is de-
stroyed, but it is really continued as a motion of their
ultimate particles. It is this motion, taken up by the
ether, and propagated through it with a velocity of one
hundred and eighty-five thousand miles a second, that comes
to us as the light and. heat of suns and stars. The atoms
of a hot body swing with inconceivable rapidity, 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 de-
pending on the equilibrium of attraction and repulsion. The
atoms are virtually connected by elastic springs, which op-
pose at the same time their approach and their retreat, but
which tolerate the Vibration called heat. When two bod-
ies drawn together by the force of gravity strike each other,
the intensity of the ultimate vibration, or, in other words,
the amount of heat generated, is proportional to the @229
viva destroyed by the collision. The molecular motion
once set up is instantly shared with the ether, and diffused
by it throughout space.

We on the earth’s surface live night and day in the
midst of ethereal 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
body, which radiates its motion through the ether. The
earth also is warm, and sends its heat-pulses incessantly
forth. It is the waste of its molecular motion in space

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