which we started. Possibly the entire
galactic system may, in an inconceivably remote future, remodel itself
in this way; and possibly the nebula from which our own group of planets
has been formed may have owed its origin to the disintegration of
systems which had accomplished their career in the depths of the bygone
eternity.

When the problem is extended to these huge dimensions, the prospect of
an ultimate cessation of cosmical work is indefinitely postponed, but
at the same time it becomes impossible for us to deal very securely
with the questions we have raised. The magnitudes and periods we have
introduced are so nearly infinite as to baffle speculation itself: One
point, however, we seem dimly to discern. Supposing the stellar universe
not to be absolutely infinite in extent, we may hold that the day of
doom, so often postponed, must come at last. The concentration of matter
and dissipation of energy, so often checked, must in the end prevail,
so that, as the final outcome of things, the entire universe will be
reduced to a single enormous ball, dead and frozen, solid and black,
its potential energy of motion having been all transformed into heat
and radiated away. Such a conclusion has been suggested by Sir William
Thomson, and it is quite forcibly stated by the authors of "The Unseen
Universe." They remind us that "if there be any one form of energy
less readily or less completely transformable than the others, and if
transformations constantly go on, more and more of the whole energy
of the universe will inevitably sink into this lower grade as time
advances." Now radiant heat, as we have seen, is such a lower grade
of energy. "At each transformation of heat-energy into work, a large
portion is degraded, while only a small portion is transformed into
work. So that while it is very easy to change all of our mechanical or
useful energy into heat, it is only possible to transform a portion of
this heat-energy back again into work. After each change, too, the
heat becomes more and more dissipated or degraded, and less and less
available for any future transformation. In other words," our authors
continue, "the tendency of heat is towards equalization; heat is
par excellence the communist of our universe, and it will no doubt
ultimately bring the system to an end..... It is absolutely certain that
life, so far as it is physical, depends essentially upon transformations
of energy; it is also absolutely certain tha