water, no rain, and tropical regions which are almost entirely
desert. Many writers have therefore doubted the existence of water in
any form upon this planet, supposing that the snow-caps are not formed
of frozen water but of carbon-dioxide, or some other heavy gas, in a
frozen state; and Mr. Lowell evidently feels this to be a difficulty,
since the only fact he is able to adduce in favour of the melting snows
of the polar caps producing water is, that at the time they are melting
a marginal blue band appears which accompanies them in their retreat,
and this blue colour is said to prove conclusively that the liquid is
not carbonic acid but water. This point he dwells upon repeatedly,
stating, of these blue borders: "This excludes the possibility of their
being formed by carbon-dioxide, and shows that of all the substances we
know the material composing them must be water."

[Footnote 7: In a paper written since the book appeared the density of
air at the surface of Mars is said to be 1/12 of the earth's.]

This is the only proof of the existence of _water_ he adduces, and it is
certainly a most extraordinary and futile one. For it is perfectly well
known that although water, in large masses and by transmitted light, is
of a blue colour, yet shallow water by reflected light is not so; and in
the case of the liquid produced by the snow-caps of Mars, which the
whole conditions of the planet show must be shallow, and also be more or
less turbid, it cannot possibly be the cause of the 'deep blue' tint
said to result from the melting of the snow.

But there is a very weighty argument depending on the molecular theory
of gases against the polar caps of Mars being composed of frozen water
at all. The mass and elastic force of the several gases is due to the
greater or less rapidity of the vibratory motion of their molecules
under identical conditions. The speed of these molecular motions has
been ascertained for all the chief gases, and it is found to be so great
as in certain cases to enable them to overcome the force of gravity and
escape from a planet's surface into space. Dr. G. Johnstone Stoney has
specially investigated this subject, and he finds that the force of
gravity on the earth is sufficient to retain all the gases composing its
atmosphere, but not sufficient to retain hydrogen; and as a consequence,
although this gas is produced in small quantities by volcanoes and by
decomposing vegetation, yet no trace of it i