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Solar radiation dissociates water vapor into hydrogen atoms and hydroxyl radicles. Hydrogen and hydrogen peroxide molecules, and perhydroxyl radicles, are also produced as a result of subsequent chemical reactions with the allotropic forms of oxygen. The rate of the oxidizing processes falls off more rapidly with increase of altitude than does that of the reducing processes, and the hydrogen compounds are almost completely broken down at about the 90-km level (or even lower). There is a continual escape of the hydrogen atoms into interplanetary space; but the liberated oxygen atoms remain in the atmosphere, and the number that must thus have been added in geological time seems to be comparable with the number now present. Consideration of the general equilibrium reveals several features of interest, such as, for example, the existence of a thin layer of molecular hydrogen. In spite of the prominence of the Meinel bands, the concentration of hydroxyl radicles is quite small. It is thought that these radicles are excited during, rather than after, their formation. The mechanism proposed is two body collisions between hydrogen atoms and ozone molecules.