s's atmosphere is unparalleled elsewhere in the
solar system, it may be worth while to give a graphic illustration of
it. In the accompanying figure the planet is represented at three
successive points in its advance toward inferior conjunction. As it
approaches conjunction it slowly draws nearer the earth, and its
apparent diameter consequently increases. At _A_ a large part of the
luminous crescent is composed of the planet's surface reflecting the
sunshine; at _B_ the ratio of the reflecting surface to the illuminated
atmosphere has diminished, and the latter has extended, like the curved
arms of a pair of calipers, far around the unilluminated side of the
disk; at _C_ the atmosphere is illuminated all around by the sunlight
coming through it from behind, while the surface of the planet has
passed entirely out of the light--that is to say, Venus has become an
invisible globe embraced by a circle of refracted sunshine.

We return to the question of life. With almost twice as much solar heat
and light as we have, and with a deeper and denser atmosphere than ours,
it is evident, without seeking other causes of variation, that the
conditions of life upon Venus are notably different from those with
which we are acquainted. At first sight it would seem that a dense
atmosphere, together with a more copious supply of heat, might render
the surface temperature of Venus unsuitable for organic life as we
understand it. But so much depends upon the precise composition of the
atmosphere and upon the relative quantities of its constituents, that it
will not do to pronounce a positive judgment in such a case, because we
lack information on too many essential points.

Experiment has shown that the temperature of the air varies with changes
in the amount of carbonic acid and of water vapor that it contains. It
has been suggested that in past geologic ages the earth's atmosphere was
denser and more heavily charged with vapors than it is at present; yet
even then forms of life suited to their environment existed, and from
those forms the present inhabitants of our globe have been developed.
There are several lines of reasoning which may be followed to the
conclusion that Venus, as a life-bearing world, is younger than the
earth, and, according to that view, we are at liberty to imagine our
beautiful sister planet as now passing through some such period in its
history as that at which the earth had arrived in the age of the
carbonifero