the surface. No doubt the sun will continue to give out heat a
long time after heat has ceased to be needed for the support of
living organisms. For the final refrigeration of the sun will
long be postponed by the fate of the planets themselves. The
separation of the planets from their parent solar mass seems to
be after all but a temporary separation. So nicely balanced are
they now in their orbits that they may well seem capable of
rolling on in their present courses forever. But this is not the
case. Two sets of circumstances are all the while striving, the
one to drive the planets farther away from the sun, the other to
draw them all into it. On the one hand, every body in our system
which contains fluid matter has tides raised upon its surface by
the attraction of neighbouring bodies. All the planets raise
tides upon the surface of the sun and the periodicity of
sun-spots (or solar cyclones) depends upon this fact. These tidal
waves act as a drag or brake upon the rotation of the sun,
somewhat diminishing its rapidity. But, in conformity with a
principle of mechanics well known to astronomers, though not
familiar to the general reader, all the motion of rotation thus
lost by the sun is added to the planets in the shape of annual
motion of revolution, and thus their orbits all tend to
enlarge,--they all tend to recede somewhat from the sun. But this
state of things, though long-enduring enough, is after all only
temporary, and will at any rate come to an end when the sun and
planets have become solid. Meanwhile another set of circumstances
is all the time tending to bring the planets nearer to the sun,
and in the long run must gain the mastery. The space through
which the planets move is filled with a kind of matter which
serves as a medium for the transmission of heat and light, and
this kind of matter, though different in some respects from