mall, 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
say nearly all of it--slips by the few and relatively small planets
and disappears in the great void.

The destiny of all the celestial spheres seems in time to be that
they shall become cooled down to a temperature far below anything
which is now experienced on this earth. Even the sun, though its heat
will doubtless endure for millions of years to come, must in time, so
far as we can see, become dark and cold. So far as we know, we can
perceive no certain method by which the life of the slowly decaying
suns can be restored. It has, however, been suggested that in many
cases a planetary system which has attained the lifeless and lightless
stage may by collision with some other association of spheres be by
the blow restored to its previous state of vapour, the joint mass of
the colliding systems once again to resume the process of
concentration through which it had gone before. Now and then stars
have been seen to flash suddenly into great brilliancy in a way which
suggests that possibly their heat had been refreshed by a collision
with some great mass which had fallen into them from the celestial
spaces. The