Abstract
The properties of the upper atmosphere and in particular of the ionosphere and of the night-sky emission are outlined. The fundamental processes that might be involved are discussed. Various suggestions regarding the production of the layers are considered in detail and the main difficulties indicated. The theory of the equilibrium of the layers is built up and the outstanding problem of explaining the rate of disappearance of electrons is emphasized. Current views on the origin of the night-sky light are studied critically and serious discrepancies are shown to exist. Other possibilities are examined.

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