Most of the planets in the solar system have atmospheres, but that of the Earth is unique. It's composition is quite different from all other planetary atmospheres as it not only supports but is also strongly influenced by living oganisms. Assuming no prior scientific knowledge this volume examines, dissects and explains the phenomena, air - which many scientists believe we are now seriously polluting. It attempts to answer the question What is air? by defining its composition, its evolution and structure and the way it is influenced by living organisms.