expands it will be very cold, because the heat of the
cubic inch, now reduced to the normal temperature of the surrounding
air, is distributed over a cubic foot of space.

This is precisely what takes place when heated air at the surface of
the earth (which is condensed to a certain extent) rises to the higher
regions of the atmosphere. There is a gradual expansion as it ascends,
and consequently a gradual cooling, because a given amount of heat is
being constantly distributed over a greater amount of space. At an
altitude of forty-five miles it will have expanded about 25,000 times,
which will bring the temperature down to between 200 and 300 degrees
below zero.

When we get beyond the limits of the atmosphere we get into the region
of absolute cold, because heat is atomic motion, and there can be no
atomic motion where there are no atoms.

We have now traced the atmosphere up to the point where it shades off
into the ether that is supposed to fill all interplanetary space. As
Dryden says:

There fields of light and liquid ether flow,
Purg'd from the pond'rous dregs of earth below.

By interplanetary space we mean all space between the planets not
occupied by sensible material. It is the same as interatomic space, or
the space between atoms, except in degree, as the same substance that
fills interplanetary space also fills interatomic space, so that all the
atoms of matter float in it and are held together from flying off into
space by the attraction of cohesion. What this ether is, has been the
subject of much speculation among philosophers, without, however,
arriving at any definite conclusion, further than that it is a substance
possessing almost infinite elasticity, and whose ultimate particles, if
particles there be, are so small that no sensible substance can be made
sufficiently dense to resist it or confine it. It is easy to see that a
substance possessing such qualities cannot be weighed or in any way made
appreciable to our senses. But from the fact that radiant energy can be
transmitted through it, with vibrations amounting to billions per
second, we know that it must be a substance with elastic qualities that
approach the infinite. Assuming that the ether is a substance, the
question arises how is it related to other forms of substance? This is a
question more easily asked than answered. The longer one dwells upon the
subject, however, the more one is impressed with the thought that after
all