the voyage would have been
through space more void than that to be found in the exhausted receiver
of an air-pump.

Such would be the case if the moon were coated with an atmosphere like
that surrounding our earth. But what are the facts? The traveller as he
drew near the moon would seek in vain for air to breathe at all
resembling ours. It is possible that close to the surface there are
faint traces of some gaseous material surrounding the moon, but it can
only be equal to a very small fractional part of the ample clothing
which the earth now enjoys. For all purposes of respiration, as we
understand the term, we may say that there is no air on the moon, and an
inhabitant of our earth transferred thereto would be as certainly
suffocated as he would be in the middle of space.

It may, however, be asked how we learn this. Is not air transparent, and
how, therefore, could our telescopes be expected to show whether the
moon really possessed such an envelope? It is by indirect, but
thoroughly reliable, methods of observation that we learn the destitute
condition of our satellite. There are various arguments to be adduced;
but the most conclusive is that obtained on the occurrence of what is
called an "occultation." It sometimes happens that the moon comes
directly between the earth and a star, and the temporary extinction of
the latter is an "occultation." We can observe the moment when the
phenomenon takes place, and the suddenness of the disappearance of the
star is generally remarked. If the moon were enveloped in a copious
atmosphere, the interposition of this gaseous mass by the movement of
the moon would produce a gradual evanescence of the star wholly wanting
the abruptness which marks the obscuration.[9]

Let us consider how we can account for the absence of an atmosphere from
the moon. What we call a gas has been found by modern research to be a
collection of an immense number of molecules, each of which is in
exceedingly rapid motion. This motion is only pursued for a short
distance in one direction before a molecule comes into collision with
some other molecule, whereby the directions and velocities of the
individual molecules are continually changed. There is a certain average
speed for each gas which is peculiar to the molecules of that gas at a
certain temperature. When several gases are mixed, as oxygen and
nitrogen are in our atmosphere, the molecules of each gas continue to
move with their own characteris