, 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 ordinary ponderable matter, is
yet like it in exerting friction. This friction is almost infinitely
little, yet it has a wellnigh infinite length of time to work in, and
during all this wellnigh infinite length of time it is slowly eating up
the momentum of the planets and diminishing their ability to maintain
their distances from the sun. Hence in course of time the planets will
all fall into the sun, one after another, so that the solar system will
end, as it began, by consisting of a single mass of matter.

But this is by no means the end of the story. When two bodies rush
together, each parts with some of its energy of motion, and this lost
energy of motion reappears as heat. In the concussion of two cosmical
bodies, like the sun and the earth, an enormous quantity of motion is
thus converted into heat. Now heat, when not allowed to radiate, or when
generated faster than it can be radiated, is transformed into motion of
expansion. Hence the shock of sun and planet would at once result in the
vaporization of both bodies; and there can be no doubt that by the time
the sun has absorbed the outermost of his attendant planets, he will
have resumed something like his original nebulous condition. He will
have been dilated into a huge mass of vapour, and will have become
fit for a new process of contraction and for a new production of
life-bearing planets.

We are now, however, confronted by an interesting but difficult
question. Throughout all this grand past and future career of the solar
system which we have just briefly traced, we have been witnessing a
most prodigal dissipation of energy in the shape of radiant heat. At
the outset we had an enormous quantity of what i