tructure or shade which could be
attributed to cities or other works of man. To all appearances the
whole surface of our satellite is as completely devoid of life as the
lava newly thrown from Vesuvius. We next pass to the planets. Mercury,
the nearest to the sun, is in a position very unfavorable for
observation from the earth, because when nearest to us it is between us
and the sun, so that its dark hemisphere is presented to us. Nothing
satisfactory has yet been made out as to its condition. We cannot say
with certainty whether it has an atmosphere or not. What seems very
probable is that the temperature on its surface is higher than any of
our earthly animals could sustain. But this proves nothing.

We know that Venus has an atmosphere. This was very conclusively shown
during the transits of Venus in 1874 and 1882. But this atmosphere is
so filled with clouds or vapor that it does not seem likely that we
ever get a view of the solid body of the planet through it. Some
observers have thought they could see spots on Venus day after day,
while others have disputed this view. On the whole, if intelligent
inhabitants live there, it is not likely that they ever see sun or
stars. Instead of the sun they see only an effulgence in the vapory sky
which disappears and reappears at regular intervals.

When we come to Mars, we have more definite knowledge, and there seems
to be greater possibilities for life there than in the case of any
other planet besides the earth. The main reason for denying that life
such as ours could exist there is that the atmosphere of Mars is so
rare that, in the light of the most recent researches, we cannot be
fully assured that it exists at all. The very careful comparisons of
the spectra of Mars and of the moon made by Campbell at the Lick
Observatory failed to show the slightest difference in the two. If Mars
had an atmosphere as dense as ours, the result could be seen in the
darkening of the lines of the spectrum produced by the double passage
of the light through it. There were no lines in the spectrum of Mars
that were not seen with equal distinctness in that of the moon. But
this does not prove the entire absence of an atmosphere. It only shows
a limit to its density. It may be one-fifth or one-fourth the density
of that on the earth, but probably no more.

That there must be something in the nature of vapor at least seems to
be shown by the formation and disappearance of the white polar ca