The electric discharge produces luminosity in any gas or vapour through which it passes. The question presents itself, Does the luminosity persist after the current has ceased, or does its top immediately ? A full answer is likely to be of great importance in unravelling the cause and mechanism of the luminosity. There are exiting observations bearing on the subject, but these are somewhat scattered in the literature, and, so far as I am aware, their mutual relations have not been pointed out. It is hoped in this paper to do something towards systematising and extending them. The most conspicuous phenomena in this connection are the various forms of afterglow which have been discussed in previous papers. But these are not really relevant to tire present subject, for they are due to secondary causes of a chemical nature. Some of them, produced in gaseous mixtures containing oxygen, are due to the interaction of ozone with other substances present. Others, again, are connected with the formation of an active modification of nitrogen. In none of these cases can the after-luminosity be considered continuous with the luminosity of tire discharge which produced it. For it is always much less brilliant, even at first, and always has a quite different spectrum.
X. On the electric discharge in gases
