On the side where eternal sunlight shines the sky of Mercury contains no stars.  Forever the pitiless blaze of day; forever,

    “All in a hot and copper sky
    The bloody sun at noon.”

As it is difficult to understand how water can exist on the night hemisphere, except in the shape of perpetual snow and ice, so it is hard to imagine that on the day hemisphere water can ever be precipitated from the vaporous form.  In truth, there can be very little water on Mercury even in the form of vapor, else the spectroscope would have given unquestionable evidence of its presence.  Those who think that Mercury is entirely waterless and almost, if not quite, airless may be right.  In these respects it would then resemble the moon, and, according to some observers, it possesses another characteristic lunar feature in the roughening of its surface by what seem to be innumerable volcanic craters.

But if we suppose Mercury to possess an atmosphere much rarer than that of the earth, we may perceive therein a possible provision against the excessive solar heat to which it is subjected, since, as we see on high mountains, a light air permits a ready radiation of heat, which does not become stored up as in a denser atmosphere.

As the sun pours its heat without cessation upon the day hemisphere the warmed air must rise and flow off on all sides into the night hemisphere, while cold air rushes in below, to take its place, from the region of frost and darkness.  The intermediate areas, which see the sun part of the time, as explained above, are perhaps the scene of contending winds and tempests, where the moisture, if there be any, is precipitated, through the rapid cooling of the air, in whelming floods and wild snow-storms driven by hurrying blasts from the realm of endless night.

Enough seems now to have been said to indicate clearly the hopelessness of looking for any analogies between Mercury and the earth which would warrant the conclusion that the former planet is capable of supporting inhabitants or forms of life resembling those that swarm upon the latter.  If we would still believe that Mercury is a habitable globe we must depend entirely upon the imagination for pictures of creatures able to endure its extremes of heat and cold, of light and darkness, of instability, swift vicissitude, and violent contrast.

In the next chapter we shall study a more peaceful and even-going world, yet one of great brilliancy, which possesses some remarkable resemblances to the earth, as well as some surprising divergences from it.

CHAPTER III

VENUS, THE TWIN OF THE EARTH