The question of the sun’s heat is one which concerns us in a very vital
manner, since all life upon the earth is directly dependent upon the
sun’s radiations, and, were these cut off, all forms of life would soon
become extinct.
Until about a hundred years ago, it was thought that the sun was merely
an enormous body of fire, and that the heat emitted from it was of
the same character as that emitted by a grate of burning coals. This
was the _chemical_ theory of the sun’s heat, and it was thought to be
no more mysterious than the combustion noted in any furnace--only of
course, on a vaster scale!
This purely chemical theory had to be given up when it was shown that,
at any ordinary rate of combustion, the heat emitted by even so large
a body as the sun would be exhausted in a few tens of thousands of
years--whereas the course of evolution on our earth shows us that life
has been continuous here for many millions of years, with practically
no variation of heat upon the earth surface.
Certainly, therefore, the purely chemical theory of combustion does not
suffice, and other theories were accordingly advanced, in attempts to
explain the maintenance of the sun’s heat.
It was first of all suggested that, as the sun cools, it _contracts_,
and that this contraction tended to raise the heat, and consequently
its temperature would be maintained for a long period of time. But
again, it was proved that, if such were the case, the heat of the sun
would have been expended millions of years ago.
The theory was then advanced that the sun’s heat was maintained by the
_impact_ upon its surface of quantities of meteors and other small
bodies which crashed into it--being attracted by the enormous pull
of gravitation exerted by the sun. We know that heat is generated by
friction or rapid impact, and it was suggested that this might be one
of the long period of time. But again, it was proved mathematically
that this would entirely fail to account for the continuous and steady
heat emitted by our central luminary.
One of the latest theories is that the sun’s heat is maintained by the
activity of _radium_, and that the constant flow of energy which has
been noted uninterruptedly for so many millions of years is due to the
intra-atomic energy of the atoms composing it. We know that radium can
continue to give-off energy for thousands of years, and that even a
minute speck of it has a life of extraordinary length. (It has been
argued that the heat of our earth is also to some extent maintained
by the quantities of radium hidden beneath its surface, in the deeper
layers of the earth’s crust.)
Of late, however, even this theory has been questioned, and it must be
admitted that, today the heat of the sun remains an unsolved problem.
It remains for the future to determine its exact nature and causation.