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 revolves in its orbit about the sun it is visible, alternately, in the evening for a short time after sunset and in the morning for a short time before sunrise, but it can never be seen, as the outer planets are seen, in the mid-heaven or late at night. When seen low in the twilight, at evening or morning, it glows with the brilliance of a bright first-magnitude star, and is a beautiful object, though few casual watchers of the stars ever catch sight of it. When it is nearest the earth and is about to pass between the earth and the sun, it temporarily disappears in the glare of the sunlight; and likewise, when it it is farthest from the earth and passing around in its orbit on the opposite side of the sun, it is concealed by the blinding solar rays. Consequently, except with the instruments of an observatory, which are able to show it in broad day, Mercury is never visible save during the comparatively brief periods of time when it is near its greatest apparent distance east or west from the sun.