on of
energy is that it is due to shrinkage of the sun's bulk under the force
of gravity. Gravity is one of the most mysterious forces of nature, but
it is an obvious fact that bodies behave as if they attracted one
another, and Newton worked out the law of this attraction. We may say,
without trying to go too deeply into things, that every particle of
matter attracts every other throughout the universe. If the diameter of
the sun were to shrink by one mile all round, this would mean that all
the millions of tons in the outer one-mile thickness would have a
straight drop of one mile towards the centre. And that is not all,
because obviously the layers below this outer mile would also drop
inwards, each to a less degree than the one above it. What a tremendous
movement of matter, however slowly it might take place! And what a
tremendous energy would be involved! Astronomers calculate that the
above shrinkage of one mile all round would require fifty years for its
completion, assuming, reasonably, that there is close and continuous
relationship between loss of heat by radiation and shrinkage. Even if
this were true we need not feel over-anxious on this theory; before the
sun became too cold to support life many millions of years would be
required.

It was suggested at one time that falls of meteoric matter into the sun
would account for the sun's heat. This position is hardly tenable now.
The mere bulk of the meteoric matter required by the hypothesis, apart
from other reasons, is against it. There is undoubtedly an enormous
amount of meteoric matter moving about within the bounds of the solar
system, but most of it seems to be following definite routes round the
sun like the planets. The stray erratic quantities destined to meet
their doom by collision with the sun can hardly be sufficient to account
for the sun's heat.

Recent study of radio-active bodies has suggested another factor that
may be working powerfully along with the force of gravitation to
maintain the sun's store of heat. In radio-active bodies certain atoms
seem to be undergoing disintegration. These atoms appear to be splitting
up into very minute and primitive constituents. But since matter may be
split up into such constituents, may it not be built up from them?

The question is whether these "radio-active" elements are undergoing
disintegration, or formation, in the sun. If they are undergoing
disintegration--and the sun itself is undoubtedly radio