cold to support life
upon our earth.

Since the discovery of radium it has, on the other hand, been suggested,
and not unreasonably, that radio-active matter may possibly play an
important part in keeping up the heat of the sun. But the body of
scientific opinion appears to consider the theory of contraction as a
result of gravitation, which has been outlined above, to be of itself
quite a sound explanation. Indeed, the late Lord Kelvin is said to have
held to the last that it was amply sufficient to account for the
underground heat of the earth, the heat of the sun, and that of all the
stars in the universe.

One great difficulty in forming theories with regard to the sun, is the
fact that the temperature and gravitation there are enormously in excess
of anything we meet with upon our earth. The force of gravity at the
sun's surface is, indeed, about twenty-seven times that at the surface
of our globe.

The earth's atmosphere appears to absorb about one-half of the
radiations which come to us from the sun. This absorptive effect is very
noticeable when the solar orb is low down in our sky, for its light and
heat are then clearly much reduced. Of the light rays, the blue ones are
the most easily absorbed in this way; which explains why the sun looks
red when near the horizon. It has then, of course, to shine through a
much greater thickness of atmosphere than when high up in the heavens.

What astonishes one most about the solar radiation, is the immense
amount of it that is apparently wasted into space in comparison with
what falls directly upon the bodies of the solar system. Only about the
one-hundred-millionth is caught by all the planets together. What
becomes of the rest we cannot tell.

That brilliant white body of the sun, which we see, is enveloped by
several layers of gases and vaporous matter, in the same manner as our
globe is enveloped by its atmosphere (see Fig. 10, p. 131). These are
transparent, just as our atmosphere is transparent; and so we see the
white bright body of the sun right through them.

This white bright portion is called the _Photosphere_. From it comes
most of that light and heat which we see and feel. We do not know what
lies under the photosphere, but, no doubt, the more solid portions of
the sun are there situated. Just above the photosphere, and lying close
upon it, is a veil of smoke-like haze.

Next upon this is what is known as the _Reversing Layer_, which is
between 500