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
of such a restoration completely transcends our science. How can
the little vibration which strikes our eye from some distant star,
and which has been perhaps thousands of years in reaching us, find
its way back to its origin? The light emitted by the sun 10,000
years ago is to-day pursuing its way in a sphere whose surface is
10,000 light-years distant on all sides. Science has nothing even
to suggest the possibility of its restoration, and the most
delicate observations fail to show any return from the
unfathomable abyss.
Up to the time when radium was discovered, the most careful
investigations of all conceivable sources of supply had shown only
one which could possibly be of long duration.
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