Slipher, of Flagstaff Observatory, has made many experiments with
specially sensitised photographic plates. He has taken several
photographs of the spectrum of the moon and others of the spectrum of
the planet Mars. The plates of the lunar spectrum show a darkening of
the 'a' band, which indicates the presence of water vapour, and we know
that is due to the water vapour in our own atmosphere. The plates of the
spectrum of Mars show a much more definite darkening of the 'a' band,
and Professor Lowell contends that this can only be due to water vapour
in the atmosphere of Mars.

"Professor Campbell has, however, made similar experiments, and is of
opinion that Professor Lowell has been deceived by the water vapour in
our own atmosphere. Thus the matter stands at the present time, and we
must await the result of further investigation before we can consider
the matter settled.

"I, however, regard it as a certainty that improved means will
definitely show that water vapour undoubtedly exists in the Martian
atmosphere, and it is not unlikely that other constituents of that
atmosphere may also be identified, and possibly even the relative
quantities may be ascertained."

John here remarked that he had read of it being contended that life
could not exist on Mars because as water would boil at a temperature a
hundred degrees lower than it did on the earth, it would be impossible
to boil a potato properly, or make a good cup of tea. He thought,
however, that if water boiled at such a low temperature, then the
proportion of water vapour in the air would be increased, as evaporation
would be more rapid than on the earth.

"Undoubtedly so," I replied. "The first argument, however, is very weak.
For many thousands of years the people on the earth not only managed to
live, but attained a high state of civilisation, yet we have no reason
to believe that they ever ate potatoes or drank tea! Even in England we
have only known and used these articles for about three hundred years!
The inhabitants of any world would be suited to their environments.

"The polar-caps on Mars are shown on very early drawings of the planet;
but, up to the year 1877, little was known of the general surface
details beyond the fact that the general colour was orange-red,
diversified by dark patches of blue-green in some parts, and some
narrow, serpentine markings here and there. All these markings are now
much more accurately drawn, as the result of more c