A Manual Of Photography

THE AGENT PRODUCING CHEMICAL CHANGE. 123 some consequence to attend to this fact. We are not yet in a position to record more than the fact,—the cause of the diffe- rence is not yet determined ; probably it may be found to exist in a greater absorptive action of the atmosphere, caused by the elevation of aqueous vapour from the earth. But the experiments of M. Malaguti seem to imply the contrary, this philosopher having found that the chemical rays permeate water more readily than they do air : some experiments of my own, however, are not in accordance with M. Malaguti’s results. In the neigh- bourhood of large towns it might be accounted for by the circum- stance of the air becoming, during the day, more and more impreg- nated with coal smoke, &c., which offers very powerful interruption to the free passage of the chemical rays. This will, however, scarcely account for the same interference being found to exist in the open country, some miles from any town. Until our meteorological observers adopt a system of registering the variations of light and actinic power by means of some well- devised instrument, we cannot expect to arrive at any very defi- nite results. The subject involves some matters of the first importance in photometry and meteorology, and it is to be desired that our public observatories should be furnished with the required instruments for carrying out a series of observa- tions on the diurnal and monthly changes in the relative condi- tions of the solar radiations. We have now evidence which proves that changes, almost inappreciable, in the condition of the atmo- spheric media, through which the solar rays traverse, are capable of producing a most remarkable influence upon the colours of the spectrum and their chemical power. Many of the phenomena of vegetable life will be found to be directly dependent upon the operation of these principles ; and it would be important to mark any abnormal states of growth— such as not unfrequently occur—and to be enabled to refer them to peculiar solar conditions.