s found in our atmosphere.
The moon however, having only one-eightieth the mass of the earth,
cannot retain any gas, hence its airless and waterless condition.

_Water Vapour cannot exist on Mars._

Now, Dr. Stoney finds that in order to retain water vapour permanently a
planet must have a mass at least a quarter that of the earth. But the
mass of Mars is only one-ninth that of the earth; therefore, unless
there are some special conditions that prevent its loss, this gas cannot
be present in the atmosphere. Mr. Lowell does not refer to this argument
against his view, neither does he claim the evidence of spectroscopy in
his favour. This was alleged more than thirty years ago to show the
existence of water-vapour in the atmosphere of Mars, but of late years
it has been doubted, and Mr. Lowell's complete silence on the subject,
while laying stress on such a very weak and inconclusive argument as
that from the tinge of colour that is observed a little distance from
the edge of the diminishing snow-caps, shows that he himself does not
think the fact to be thus proved. If he did he would hardly adduce such
an argument for its presence as the following: "The melting of the caps
on the one hand and their re-forming on the other affirm the presence of
water-vapour in the Martian atmosphere, of whatever else that air
consists" (p. 162). Yet absolutely the only proof he gives that the caps
are frozen water is the almost frivolous colour-argument above referred
to!

_No Spectroscopic Evidence of Water Vapour._

As Sir William Huggins is the chief authority quoted for this fact, and
is referred to as being almost conclusive in the third edition of Miss
Clerke's _History of Astronomy_ in 1893, I have ascertained that his
opinion at the present time is that "there is no conclusive proof of the
presence of aqueous vapour in the atmosphere of Mars, and that
observations at the Lick Observatory (in 1895), where the conditions and
instruments are of the highest order, were negative." He also informs me
that Marchand at the Pic du Midi Observatory was unable to obtain lines
of aqueous vapour in the spectrum of Mars; and that in 1905, Slipher, at
Mr. Lowell's observatory, was unable to detect any indications of
aqueous vapour in the spectrum of Mars.

It thus appears that spectroscopic observations are quite accordant with
the calculations founded on the molecular theory of gases as to the
absence of aqueous vapour, and therefore pres