Note on fluorescence and absorption

In a, paper “On the Change of Absorption produced by Fluorescence,” I described the existence of a very remarkable difference in the absorption of the fluorescent light of uranium glass, when in the luminous and non-luminous states. This difference I have attributed to a temporary change in the structure or chemical composition of the body when exposed to the influence of the exciting light; and I have been led to suppose that it is due to the formation of new atomic connections which give rise to new frequencies during the period of luminosity. These I regarded as the result of the formation of unstable molecular aggregates by the more refrangible or exciting rays; and the luminosity or fluorescence, as the radiation which results from the breaking down of such unstable molecular groups. As in the case of photographic action, some chemical change appears to be produced by the blue and violet rays; the two cases differing, from the physical point of view, merely so far as the molecular aggregations, instead of remaining fixed, rapidly disintegrate, radiating intensely at the same time the energy which was stored up in their formation. Thus the luminosity itself is but the visible manifestation of the building up and breaking down of what are probably complicated molecular agglomerations. In their very interesting and important paper on this subject, Messrs. Nichols and Merritt have shown that the phenomenon of the change of absorption depends upon the intensity of the fluorescence. They find that a saturation effect takes place as the intensity of the luminosity increases, so that the change of absorption reaches a maximum with a certain intensity of the fluorescent light.