The Unseen World and Other Essays | Page 7

the atmosphere and oceans have disappeared from 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 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 is called "energy of

position," that is, the outer parts of our primitive nebula had a very long
distance through which to travel towards one another in the slow
process of concentration; and this distance was the measure of the
quantity of work possible to our system. As the particles of our nebula
drew nearer and nearer together, the energy of position continually lost
reappeared continually as heat, of which the greater part was radiated
off, but of which a certain amount was retained. All the gigantic
amount of work achieved in the geologic development of our earth and
its companion planets, and in the development of life wherever life may
exist in our system, has been the product of this retained heat. At the
present day the same wasteful process is going on. Each moment the
sun's particles are losing energy of position as they draw closer and
closer together,