Our attention should first be directed to the enormous waste of
energy which has accompanied this contraction of the solar
nebula. The first result of such a contraction is the generation
of a great quantity of heat, and when the heat thus generated has
been lost by radiation into surrounding space it becomes possible
for the contraction to continue. Thus, as concentration goes on,
heat is incessantly generated and incessantly dissipated. How
long this process is to endure depends chiefly on the size of the
contracting mass, as small bodies radiate heat much faster than
large ones. The moon seems to be already thoroughly refrigerated,
while Jupiter and Saturn are very much hotter than the earth, as
is shown by the tremendous atmospheric phenomena which occur on
their surfaces. The sun, again, generates heat so rapidly, owing
to his great energy of contraction, and loses it so slowly, owing
to his great size, that his surface is always kept in a state of
incandescence. His surface-temperature is estimated at some three
million degrees of Fahrenheit, and a diminution of his diameter
far too small to be detected by the finest existing instruments
would suffice to maintain the present supply of heat for more
than fifty centuries. These facts point to a very long future
during which the sun will continue to warm the earth and its
companion planets, but at the same time they carry on their face
the story of inevitable ultimate doom. If things continue to go
on as they have all along gone on, the sun must by and by grow
black and cold, and all life whatever throughout the solar system
must come to an end. Long before this consummation, however, life
will probably have become extinct through the refrigeration of
each of the planets into a state like the present state of the
moon, in which the atmosphere and oceans have disappeared from