condition of Mars. Three points arise which we will take in the inverse order of their importance. First: the appearance of the planet shows conclusively that, if the polar caps be composed of solid carbonic acid gas, then either there is no water at all on Mars in any form whatsoever, or what there is is ice so overlaid with detritus as to be invisible. For if the two substances were there together, and the cold at the surface of the planet of so extreme a character as to congeal the carbon dioxide, the water must a fortiori be frozen, and would continue so long after the temperature rose above the melting point of the former substance. We should therefore still have snow-fields of snow after the melting of those formed of carbonic acid gas, either visible as white patches or so covered up with dirt as to pass for land. Now there are no such additional white patches to be seen, nor, so far as we can judge, does any part of the planet behave as if it were glacier-bound.
Second: carbonic dioxide passes, as we saw, almost simultaneously into the liquid and solid states, especially under slight pressure. Now, the pressure is certainly very slight on the surface of Mars; not probably more than, and probably less than, one seventh of an atmosphere. In consequence, on a rise of temperature the frozen carbonic acid gas would there pass practically straight from the solid into the gaseous
