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