ns. It has its winter aspect and its summer aspect, perfectly
distinct and recognizable, in each hemisphere by turns, and whether the
polar caps be snow or carbon dioxide, at any rate they melt and
disappear under a high sun, thus proving that an accumulation of heat
takes place.

Professor Young says: "As to the temperature of Mars we have no certain
knowledge. On the one hand, we know that on account of the planet's
distance from the sun the intensity of solar radiation upon its surface
must be less than here in the ratio of 1 to (1.524)^2--i.e., only about
43 per cent as great as with us; its 'solar constant' must be less than
13 calories against our 30. Then, too, the low density of its
atmosphere, probably less at the planet's surface than on the tops of
our highest mountains, would naturally assist to keep down the
temperature to a point far below the freezing-point of water. But, on
the other hand, things certainly _look_ as if the polar caps were really
masses of _snow_ and _ice_ deposited from vapor in the planet's
atmosphere, and as if these actually melted during the Martian summer,
sending floods of water through the channels provided for them, and
causing the growth of vegetation along their banks. We are driven,
therefore, to suppose either that the planet has sources of heat
internal or external which are not yet explained, or else, as long ago
suggested, that the polar 'snow' may possibly be composed of something
else than frozen _water_."[4]

[Footnote 4: General Astronomy, by Charles A. Young. Revised edition,
1898, p. 363.]

Even while granting the worst that can be said for the low temperature
of Mars, the persistent believer in its habitability could take refuge
in the results of recent experiments which have proved that bacterial
life is able to resist the utmost degree of cold that can be applied,
microscopic organisms perfectly retaining their vitality--or at least
their power to resume it--when subjected to the fearfully low
temperature of liquid air. But then he would be open to the reply that
the organisms thus treated are in a torpid condition and deprived of all
activity until revived by the application of heat; and the picture of a
world in a state of perpetual sleep is not particularly attractive,
unless the fortunate prince who is destined to awake the slumbering
beauty can also be introduced into the romance.[5]

[Footnote 5: Many of the present difficulties about temperatures on the