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.