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.