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.