_Water and Air essential to Life._

Here, fortunately, the issue is rendered very simple, because Mr. Lowell
fully recognises the identity of the constitution of matter and of
physical laws throughout the solar-system, and that for any high form of
organic life certain conditions which are absolutely essential on our
earth must also exist in Mars. He admits, for example, that water is
essential, that an atmosphere containing oxygen, nitrogen, aqueous
vapour, and carbonic acid gas is essential, and that an abundant
vegetation is essential; and these of course involve a
surface-temperature through a considerable portion of the year that
renders the existence of these--especially of water--possible and
available for the purposes of a high and abundant animal life.

_Blue Colour the only Evidence of Water._

In attempting to show that these essentials actually exist on Mars he is
not very successful. He adduces evidence of an atmosphere, but of an
exceedingly scanty one, since the greatest amount he can give to it is--
"not more than about four inches of barometric pressure as we reckon
it";[7] and he assumes, as he has a fair right to do till disproved,
that it consists of oxygen and nitrogen, with carbon-dioxide and
water-vapour, in approximately the same proportions as with us. With
regard to the last item--the water-vapour--there are however many
serious difficulties. The water-vapour of our atmosphere is derived from
the enormous area of our seas, oceans, lakes, and rivers, as well as
from the evaporation from heated lands and tropical forests of much of
the moisture produced by frequent and abundant rains. All these sources
of supply are admittedly absent from Mars, which has no permanent bodies