the density of the atmosphere and even the nature of its gaseous
constituents, are affected by the force of gravity, and if we could
journey from world to world, in our bodily form, it would make a great
difference to us to find gravity considerably greater or less upon other
planets than it is upon our own. This alone might suffice to render some
of the planets impossible places of abode for us, unless a decided
change were effected in our present physical organization.

One of the first questions that we should ask about a foreign world to
which we proposed to pay a visit, would relate to its atmosphere. We
should wish to know in advance if it had air and water, and in what
proportions and quantities. However its own peculiar inhabitants might
be supposed able to dispense with these things, to _us_ their presence
would be essential, and if we did not find them, even a planet that
blazed with gold and diamonds only waiting to be seized would remain
perfectly safe from our invasion. Now, in the case of Mercury, some
doubt on this point exists.

Messrs. Huggins, Vogel, and others have believed that they found
spectroscopic proof of the existence of both air and the vapor of water
on Mercury. But the necessary observations are of a very delicate
nature, and difficult to make, and some astronomers doubt whether we
possess sufficient proof that Mercury has an atmosphere. At any rate,
its atmosphere is very rare as compared with the earth's, but we need
not, on that account, conclude that Mercury is lifeless. Possibly, in
view of certain other peculiarities soon to be explained, a rare
atmosphere would be decidedly advantageous.

Being much nearer the sun than the earth is, Mercury can be seen by us
only in the same quarter of the sky where the sun itself appears. As it