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. There is room for much speculation in this field, but no
certainty appears to be attainable.

The ancients believed that light and heat were emanations which were
given off from the bodies that yielded them substantially as odours
are given forth by many substances. Since the days of Newton inquiry
has forced us to the conviction that these effects of temperature are
produced by vibrations having the general character of waves, which
are sent through the spaces with great celerity. When a ray of light
departs from the sun or other luminous body, it does not convey any
part of the mass; it transmits only motion. A conception of the action
can perhaps best be formed by suspending a number of balls of ivory,