The planet should also have a mean density greater than that of water, otherwise the seas would possess no stability, and destructive waves would quickly destroy all life on its surface. All these conditions are fulfilled in the case of Mars as well as on the earth. In the planet Saturn, however, the density is less than that of water, and in Uranus and Neptune only slightly greater. The planet must also possess a suitable atmosphere. This is an all-important condition for the support of animal life—at least for the existence of man and the higher orders of animals. This atmosphere must consist—so far as we know—of oxygen and nitrogen gases mechanically mixed in proper proportions, and with a small quantity of carbonic acid gas. Were the oxygen in smaller quantity than it exists in the earth’s atmosphere, life could not be supported. On the other hand, were it much in excess of its present amount, a fever would be produced in the blood which would very soon put an end to animal life. The presence of other gases in excessive quantities would also render the air unfit for breathing. We see, therefore, that a comparatively slight change in the composition of a planet’s atmosphere would—so far as our experience goes—render the planet uninhabitable by any of the higher forms of life with which we are familiar. For the support of life on a planet, water is also absolutely necessary. Without this useful fluid the world would soon become a desert, and life and vegetation would speedily vanish from its surface. Geological conditions must also be considered. It is clearly necessary for the welfare of human beings at least that the surface soil and rocks should contain coal, iron, lime, and other minerals, substances almost indispensable for the ordinary wants of civilized existence. [Illustration: Nine Views of the Hour-Glass Sea on Mars 1, Nov. 26, 1864; 2, June 29, 1873; 3, Oct. 28, 1879; 4, June 2, 1888; 5, June 20, 1890; 6, Aug. 6, 1892; 7, Oct., 1894; 8, Dec. 3, 1896; 9, Dec. 7, 1896]