t age after age the
possibility of such transformations is becoming less and less; and, so
far as we yet know, the final state of the present universe must be an
aggregation (into one mass) of all the matter it contains, i. e. the
potential energy gone, and a practically useless state of kinetic
energy, i. e. uniform temperature throughout that mass." Thus our
authors conclude that the visible universe began in time and will in
time come to an end; and they add that under the physical conditions of
such a universe "immortality is impossible."

Concerning the latter inference we shall by and by have something to
say. Meanwhile this whole speculation as to the final cessation of
cosmical work seems to me--as it does to my friend, Professor Clifford
[3]--by no means trustworthy. The conditions of the problem so
far transcend our grasp that any such speculation must remain an
unverifiable guess. I do not go with Professor Clifford in doubting
whether the laws of mechanics are absolutely the same throughout
eternity; I cannot quite reconcile such a doubt with faith in the
principle of continuity. But it does seem to me needful, before we
conclude that radiated energy is absolutely and forever wasted, that we
should find out what becomes of it. What we call radiant heat is simply
transverse wave-motion, propagated with enormous velocity through an
ocean of subtle ethereal matter which bathes the atoms of all visible
or palpable bodies and fills the whole of space, extending beyond the
remotest star which the telescope can reach. Whether there are any
bounds at all to this ethereal ocean, or whether it is as infinite
as space itself, we cannot surmise. If it be limited, the possible
dispersion of radiant energy is limited by its extent. Heat and light
cannot travel through emptiness. If the ether is bounded by surrounding
emptiness, then a ray of heat, on arriving at this limiting emptiness,
would be reflected back as surely as a ball is sent back when thrown
against a solid wall. If this be the case, it will not affect our
conclusions concerning such a tiny region of space as is occupied by
the solar system, but it will seriously modify Sir William Thomson's
suggestion as to the fate of the universe as a whole. The radiance
thrown away by the sun is indeed lost so far as the future of our system
is concerned, but not a single unit of it is lost from the universe.
Sooner or later, reflected back in all directions, it must do w