Page:Is Mars habitable - Wallace 1907.djvu/52

 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"; 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