generate even a grander nebula than the one with 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