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