through the stages of a transition in which a continually
concentrating vapour, drawn together by gravitation, became
progressively hotter and more dense until it assumed the condition of
a fluid. This fluid gradually parted with its heat to the cold spaces
of the heavens, and became more and more concentrated and of a lower
temperature until in the end, as in the case of our earth and of other
planets, it ceased to glow on the outside, though it remained
intensely heated in the inner parts. It is easy to see that the rate
of this cooling would be in some proportion to the size of the sphere.
Thus the earth, which is relatively small, has become relatively cold,
while the sun itself, because of its vastly greater mass, still
retains an exceedingly high temperature. The reason for this can
readily be conceived by making a comparison of the rate of cooling
which occurs in many of our ordinary experiences. Thus a vial of hot
water will quickly come down to the temperature of the air, while a
large jug filled with the fluid at the same temperature will retain
its heat many times as long. The reason for this rests upon the simple
principle that the contents of a sphere increase with its enlargement
more rapidly than the surface through which the cooling takes place.

The modern studies on the physical history of the sun and other
celestial bodies show that their original store of heat is constantly
flowing away into the empty realms of space. The rate at which this
form of energy goes away from the sun is vast beyond the powers of the
imagination to conceive; thus, in the case of our earth, which viewed
from the sun would appear no more than a small star, the amount of
heat which falls upon it from the great centre is enough each day to
melt, if it all could be put to such work, about eight thousand cubic
miles of ice. Yet the earth receives only 1/2,170,000,000 part of the
solar radiation. The greater part of this solar heat--in fact, we may