Outer envelope of the stars. The large planets. Spectra. Mars, Earth, Venus, Mercury. Atmosphere impossible on the moon. The light from the Earth. The atmosphere of Mercury. The atmosphere of Venus and its Clouds. Composition of air and its change with height. Forced circulation. Troposphere and Stratosphere. Hydrogen in the highest strata of the atmosphere. Water vapour and carbonic acid in the air. “Geokoronium.” Influence of gravity on composition of the atmosphere. The air on Venus and Mars. CHAPTER V THE CHEMISTRY OF THE ATMOSPHERE 155 Inhabited Worlds. Kinship of the stellar bodies. Presence of life. Importance of water and carbon. Importance of temperature. All life evolved from existence in water. Necessity of oxygen. Bacteria. Reducing substances preponderable in the World-matter. Volcanic gases and gases in solidified lava. Water, vapour, carbon dioxide, nitrogen gas, and sulphurous acid. Permanent gases and hydrogen. The poisonous character of the original air. Its purification. The importance of plant life and necessity of a solid crust in this process. Supply of carbonic dioxide and production of oxygen. The work of Koene. Silica. Cooling of the Earth and changing surface temperature. The ice-periods. Centres of collapse and lines of fissure in the crust. General survey of the gradual change of the air. CHAPTER VI THE PLANET MARS 180