Density of a Luminous Gas and the Emission of Light by Atoms in Metastable States

Abstract

MR. I. S. BOWBN points out in an interesting letter to NATURE (Oct. 1, 1927, p. 473) that the low density prevailing in nebulæ favours the emission of light by atoms in metastable states, and thus he accounts for a number of hitherto unexplained nebular lines as the result of transitions from initial metastable states of ionised atoms of oxygen and nitrogen. There is no doubt, as Prof. Fowler, who has adduced further spectroscopic evidence in support of the suggestion, puts it, that “a satisfactory explanation of some of the most important nebular lines has at last been reached.” The suggestion, however, implies that we have to give up the position that the transition of an atom from a metastable state to one of lower energy level resulting in emission is necessarily conditioned by the action on the atom of an agent external to it, an electric field, a magnetic field, or the impact of another atom. If, as is sometimes alleged, the influence of neighbouring atoms causes the emission of forbidden lines from initial metastable states, a high density, and not a low one, should be the condition favourable for such emission.