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
various planets would be beautifully disposed of if we could accept the
theory urged by Mr. Cope Whitehouse, to the effect that the sun is not
really a hot body at all, and that what we call solar light and heat are
only local manifestations produced in our atmosphere by the
transformation of some other form of energy transmitted from the sun;
very much as the electric impulses carried by a wire from the
transmitting to the receiving station on a telephone line are translated
by the receiver into waves of sound. According to this theory, which is