In the last number of the London Artisan, we find the report of a lecture on the above subject, by Prof. Grove, delivered before the Royal Society, from which we condense a few interesting extracts. He employed the term "molscular " as signifying particles of bodies smaller than those having sensible magnitude. The effects of light and electricity depend upon the molecular structure of bodies subjected to their influence. Carbon, in the form of the diamond, transmits light, but stops electricity ; carbon in the form of coke or graphite, into which the diamond may be transformed by heat, transmits electricity bat stops heat. All solid bodies which transmit light (being transparent) are non-conductors of electricity, while all the best conductors of electricity are opaque to light. The converse of this class of interesting facts was the principal subject of the lecture, namely, the changes produced in the molecular structure of matter by light and electricity. Euler conceived that light may be regarded as the undulation of ordinary matter, but Dr. Young, in answer to this theory, asserted that if this were the case, all bodies should be thrown into a state of molecular vibration, by the impact of light, and this was considered a formidable argument against Euler. Recent experiments, however, especially those, of M. Niepce de St. Victor, go to prove that bodies are thrown into a state of molecular undulation by light falling on them. The following experiment tests the truth of this theory: — Take an engraving which has been kept in a dark room, and expose one-half of it to the sunlight, the other half being covered with an opaque screen ; then take it again into a dark room, remove the opaque screen and place the whole surface in close proximity to a sheet of sensitive photographic paper, and allow it to remain thus for some hours. It will now be found that the portion of the engraving which has been exposed to the light will have reproduced itself upon the photographic paper, while no effect has been produced by the part which was covered with the screen. Paper exposed to sunlight, then quickly placed in a covered tin case, will, when set in the dark, radiate phosphorescent force through a round aperture in the lid, and produce a circular mark on photographic paper—even impressing upon it the lines of an interposed engraving. Last autumn, while Prof. Grove was fishing at Fontenay, he observed some patches on the skin of a trout, which he was sure were not there when it was taken out of the water. The thought struck him that the cause was exposure of some parts of the fish to the sun, other parts being covered. To determine this he took the first fresh-caught trout and placed it on the ground with a green serrated leaf on each side. After an hour's exposure the fish was examined, when the well defined image of the leaf was found on the upper exposed side, but no effect was observable on the under or sheltered side of the trout. The effect of light is not so well understood, nor so generally recognized as it should be. Light is required for the healthy growth of animals and plants. Dark rooms are not so healthy as those exposed to light. There is an invisible phosphorescence which radiates from walls and furniture, exerting a powerful tendency to produce chemical changes greatly affecting the animate world. Electricity also produces molecular changes in bodies exposed to its action, the most familiar of which is the conversion of atmospheric air into ozone, by a succession of electrical discharges passed through it. This is a branch of science, however, regarding which much has yet to be learned—a boundless field is still open for investigation.
This article was originally published with the title "Molecular Impressions by Light and Electricity" in Scientific American 13, 33, 262 (April 1858)