state. Now, from the existence of the surrounding polar sea, we remark that in the substance composing the polar caps of Mars this does not occur. A considerable portion of it is always in the transition state of a liquid. Carbonic dioxide would not thus tarry: water would.
Third: from the curve of metamorphosis, it is evident that the temperature necessary to freeze the gas under the pressure of one seventh of an atmosphere must lie between -100° C. and -200° C., if not lower. -200° C. is, so far as we can judge, about the temperature of inter-planetary space, or what would be the temperature of the night side of Mars were the planet destitute of atmosphere. But there is an atmosphere on Mars, and, even if there were not, on melting the carbonic dioxide would itself make an atmosphere. This would instantly raise the temperature, and under any rise in temperature the congealing of the gas at once becomes an impossibility. The gas itself thus suggests its own refutation.
There is no such apparent objection to water. With an atmosphere properly constituted (and there is nothing to show that the Martian atmosphere is not so constituted), the temperature might easily rise high enough to melt ice. We may therefore conclude water to be the most probable solution of the question.
With such more or less solid ground to