M. E. Becquerel assures us that almost all substances are phosphorescent after being exposed to the sun if instantly withdrawn into darkness, and that it depends upon the arrangement of the particles and not upon chemical action. The salts of uranium give the same kind of phosphorescent light as that produced by the violet rays of the solar spectrum. A solution of the bisulphate of quinine emits a yellow phosphorescent light, whereas the fluorescent light of that liquid is blue. The colours of these two kinds of light are generally complementary to one another.
Phosphorescence is probably more or less concerned in some, at least, of a series of very curious experiments made by M. Niepcé de Saint-Victor, on what he calls the saturation of substances with light. It has long been known that, if a person in an intensely dark room should expose his arm to the sun through a hole in a window-shutter, it will shine on being drawn into the darkness. Now, M. de Saint-Victor found that if an engraving be exposed for a certain time to the sun, and instantly brought into darkness, it will make a photographic impression on a collodion or argentine surface, and that anything written or drawn with tartaric acid, or a solution of the salts of uranium, in large characters, is reproduced even at a small distance from a sensitive surface. It may be presumed that the light communicates its vibrations to the surfaces exposed to it with sufficient force to enable them to disturb the unstable equilibrium of such sensitive substances as collodion or the argentine salts. M. de Saint-Victor has shown that tartaric acid, which is readily impressed by sunlight, is neither fluorescent nor phosphorescent, whence he concludes that his experiments are independent of both of these modes of action. Uranium appears to have very peculiar properties: its salts are strongly luminous when exposed to the sun; they are very fluorescent; and the crystallized azitote of uranium becomes phosphorescent by percussion.
Voltaic Electricity—The Voltaic Battery—Intensity—Quantity—Static Electricity, and Electricity in Motion—Luminous Effects—Mr. Grove on the Electric Arc and Light—Decomposition of Water—Formation of Crystals by Voltaic Electricity—Photo-galvanic Engraving—Conduction—Heat of Voltaic Electricity—Electric Fish.