PHOSPHORESCENCE The researches of T. A. Vaughton and others seem to have demonstrated that the sensitive silver salts, such as the bromide, iodide and chloride, if precipitated and kept in the dark, have the property, under certain conditions, of emitting light in degrees proportionate to their sensitiveness. In a red light, an unexposed bromide plate is placed in an ordinary pyro-soda developing solution for ten minutes, removed and washed. Next, in total darkness, plunge it suddenly into a dish containing a saturated solution of aluminium sulphate, and the plate and the solution will immediately become phosphorescent, the light dying away in the course of a minute or two. On pouring the solution into d. bottle, the whole body of the liquid becomes luminous and remains so for several minutes, the light being increased by shaking. If half the plate is exposed to the action of white light for a second before treating with the pyro-soda solution, that half remains dark and emits no light when the plate is put into the aluminium sulphate. If the plate is given a short exposure in the camera, and developed and put into the aluminium sul phate solution, the image will appear dark on a phosphorescent background. Precipitated silver bromide (which has been kept a few days in a corked test-tube in the dark), contained in a porcelain dish and exposed to a bright-red light while adding the pyro-soda solution, appears black, but on pouring off the solution the precipitate gradually assumes a bright green appearance under the red light, while in white light it appears dark grey or black.
As the result of a series of supplementary experiments, H. Edwards has stated that not only the plate itself, but also the used developer, will give phosphorescence with alum solution. Quinine sulphate or hydrochloride is not luminous when the used developer is added, but becomes so if a few drops of sulphuric acid are subsequently introduced. The experiment may be still more
easily made by mixing potassium bromide and silver nitrate solutions in dim gaslight, decanting, and shaking np the resulting silver bromide with pyro-soda. A red liquid results which gives a luminous effect when poured into alum solution or dilute sulphuric acid.
Dr. J. Precht explains the phenomenon of phosphorescence as follows : (1) The alkaline pyrogallic acid solution liberates oxygen on the addition of acids, by which the sodium sulphite is oxidised to sulphate, this oxidation being accompanied by phosphorescence. (2) The acid pyro solution suddenly takes up oxygen on the addition of sodium sulphite and soda, and also this oxidation is accompanied by phosphores cence. Thus easily reducible substances pro duce phosphorescence with an alkaline pyro solu tion.
Potassium permanganate, for instance, is phosphorescent when a mixture of pyrogallic acid and soda is added to it. It is assumed that an intermediary product is formed of alkaline pyro solution and oxygen, which gives off again the oxygen only on the addition of acid, and that the then liberated oxygen gives rise to phos phorescence by combining itself with the sulphite to form sulphate. Phosphorescence is liable to fog gelatine plates should these retain traces of the pyro developer, and therefore, in practice, such plates should not pass from the pyro developer into a solution of alum or of citric acid, or, indeed, to any other solution, before the last traces of the developer have been removed from it by careful washing. Exceedingly small quantities of pyro, less than -oos per cent., are sufficient in some cases to cause a bright phosphorescence of the film.