arely
one and a half times as heavy as the water. Of course, the actual mass
of the sun is very enormous; it is no less than 330,000 times as great
as that of the earth. The solar material itself is, however, relatively
light, so that the sun is four times as big as it would have to be if,
while its weight remained the same, its density equalled that of the
earth. Bearing in mind this lightness of the sun, and also the
exceedingly high temperature which we know to prevail there, no other
conclusion seems possible than that the body of the sun must be in a
gaseous state. The conditions under which such gases exist in the sun
are, no doubt, altogether different from those with which we are
acquainted on the earth. At the surface of the sun the force of gravity
is more than twenty-seven times as great as it is on the earth. A person
who on the earth could just lift twenty-seven equal pieces of metal
would, if he were transferred to the sun, only be able to lift one of
the pieces at a time. The pressure of the gases below the surface must
therefore be very great, and it might be supposed that they would become
liquefied in consequence. It was, however, discovered by Andrews that so
long as a gas is kept at a temperature higher than a certain point,
known as the "critical temperature" (which is different for different
gases), the gas will not be turned into a liquid however great be the
pressure to which it is submitted. The temperature on the sun cannot be
lower than the critical temperatures of the gases there existing; so it
would seem that even the enormous pressure can hardly reduce the gases
in the great luminary to the liquid form.

Of the interior of the sun we can, of course, expect to learn little or
nothing. What we observe is the surface-layer, the so-called
photosphere, in which the cold of space produces the condensation of the
gases into those luminous clouds which we see in our drawings and
photographs as "rice grains" or "willow leaves." It has been suggested
by Dr. Johnstone Stoney (and afterwards by Professor Hastings, of
Baltimore) that these luminous clouds are mainly composed of carbon with
those of the related elements silicon and boron, the boiling points of
which are much higher than those of other elements which might be
considered likely to form the photospheric clouds. The low atomic weight
of carbon must also have the effect of giving the molecules of this
element a very high velocity, and thereb