d be the case if it
possessed an atmosphere. Such a lack of atmosphere is, indeed, only to
be expected from what is known as the _Kinetic Theory of Gases_.
According to this theory, which is based upon the behaviour of various
kinds of gas, it is found that these elements tend to escape into space
from the surface of bodies whose force of gravitation is weak. Hydrogen
gas, for example, tends to fly away from our earth, as any one may see
for himself when a balloon rises into the air. The gravitation of the
earth seems, however, powerful enough to hold down other gases, as, for
instance, those of which the air is chiefly composed, namely, oxygen and
nitrogen. In due accordance with the Kinetic theory, we find the moon
and Mercury, which are much about the same size, destitute of
atmospheres. Mars, too, whose diameter is only about double that of the
moon, has very little atmosphere. We find, on the other hand, that
Venus, which is about the same size as our earth, clearly possesses an
atmosphere, as just before the planet is in transit across the sun, the
outline of its dark body is seen to be surrounded by a bright ring of
light.

The results of telescopic observation show that more markings are
visible on Mercury than on Venus. The intense brilliancy of Venus is,
indeed, about the same as that of our white clouds when the sun is
shining directly upon them. It has, therefore, been supposed that the
planet is thickly enveloped in cloud, and that we do not ever see any
part of its surface, except perchance the summit of some lofty mountain
projecting through the fleecy mass.

With regard to the great brilliancy of Venus, it may be mentioned that
she has frequently been seen in England, with the naked eye in full
sunshine, when at the time of her greatest brightness. The writer has
seen her thus at noonday. Needless to say, the sky at the moment was
intensely blue and clear.

The orbit of Mercury is very oval, and much more so than that of any
other planet. The consequence is that, when Mercury is nearest to the
sun, the heat which it receives is twice as great as when it is farthest
away. The orbit of Venus, on the other hand, is in marked contrast with
that of Mercury, and is, besides, more nearly of a circular shape than
that of any of the other planets. Venus, therefore, always keeps about
the same distance from the sun, and so the heat which she receives
during the course of her year can only be subject to very sli