On Friday evening, the 2d instant, Professor H. R Roscoe gave a lecture " On the Measurement of the Chemical Action of the Solar Rays," and explained the construction of the chemical photometer. After no- ticing the important influence of solar light on the earth, the lecturer stated that the slowly-vibrating red rays were those possessing the heat- ing property, and to their agency was due the production of both wind and rain. Chemical action was confined to the, violet rays of the spec- trum, which were alone concerned in the decomposition of carbonic acid by plants. The heating rays were measured by the thermometer, and he had then to describe the method of ascertaining the power of those rays which acted chemically. It had been found that, under exposure to light, chlorine and hydrogen combined to form hydrochloric acid, and this fact might be employed as a test of the measure of the chemical rays. To obtain the two gases in a perfect state of mechanical combination, hydrochloric acid was decomposed by means of the electric current, and • the gases thus produced were conveyed into a long glass tube terminating in a vessel of water. Exposure to light then recombined the gases into hydrochloric acid, and this was dissolved by the water which advanced in the tube as fast as the acid was formed. The tube was attached to a graduated scale, as in the thermometer, and the unit was fixed by ex- posing the instrument for a given time to a known quantity of light. When made use of, the photometer was exposed for a certain time to a solar ray admitted into a dark room, the progress of the water through the tube marking the extent of chemical action produced by the light Observations had been made in this manner in various parts of the Nor there hemisphere, and as the instrument was quite reliable, although re quiring great care and attention to prepare it for working, it was hoped some day to ascertain the chemical climate of the different countries of the globe, a result of no little importance considering the effect of solar light on the production of animal and vegetable life. Several experi- ments were performed in the course of the lecture, to illustrate the action of phosphoric light when transmitted through various coloured media to fhe commingled gases, also to show the relative effects of exposing these gases to different degrees of light.