as we now see it; that it is a self-sustaining system, able to go
on forever with only such cycles of transformation as may repeat
themselves indefinitely, and may, therefore, have repeated
themselves indefinitely in the past. Ordinary observation does not
make anything known to us which would seem to invalidate this
hypothesis. In looking upon the operations of the universe, we may
liken ourselves to a visitor to the earth from another sphere who
has to draw conclusions about the life of an individual man from
observations extending through a few days. During that time, he
would see no reason why the life of the man should have either a
beginning or an end. He sees a daily round of change, activity and
rest, nutrition and waste; but, at the end of the round, the
individual is seemingly restored to his state of the day before.
Why may not this round have been going on forever, and continue in
the future without end? It would take a profounder course of
observation and a longer time to show that, notwithstanding this
seeming restoration, an imperceptible residual of vital energy,
necessary to the continuance of life, has not been restored, and
that the loss of this residuum day by day must finally result in
death.

The case is much the same with the great bodies of the universe.
Although, to superficial observation, it might seem that they
could radiate their light forever, the modern generalizations of
physics show that such cannot be the case. The radiation of light
necessarily involves a corresponding loss of heat and with it the
expenditure of some form of energy. The amount of energy within
any body is necessarily limited. The supply must be exhausted
unless the energy of the light sent out into infinite space is, in
some way, restored to the body which expended it. The possibility