difficulties must prevent the utilization of these discoveries in this direction, even if they should prove to be in themselves all that is hoped. The retinal picture has not yet been photographed, but it seems probable, from the above recounted experiments, that it can be. ACTION OF ORGANIC ACIDS ON MINERALS. DR. H. C. BOLTON of the New York School of Mines has made the interesting discovery that minerals may be decomposed by boiling with organic acids, just as they are by treatment with the strong mineral acids. He has tried the action of such acids as citric, tartaric, oxalic, acetic, malic, and other acids, on finely powdered carbonates, silicates, sulphides, and other classes of mineral. All the carbonates examined (fourteen in number) dissolved with effervescence, sulphides were decomposed with evolution of sulphuretted hydrogen, and silicates with formation of gelatinous silica. This important discovery will greatly add to the resources of the mineralogist, who is compelled to do much of his work in the field. Hitherto he has been debarred from using the mineral acids (the action of which sometimes forms a decisive test) by the impossibility of carrying them in the pocket or wallet without danger. The organic acids are solid, and can be conveniently stowed away. Their action, however, is not so decided as that of the mineral acids, but this is not always a defect, but offers additional means of determination. For example, all the specimens of bornite and pyrrhotite examined yielded sulphuretted hydrogen with tartaric, citric, and oxalic acids, but chalcopyrite and pyrite do not. On the other hand, the use of the organic acids may give rise in some cases to the formation of nitric acid, which in its nascent condition will afford a very powerful agent of decomposition. Thus all the sulphides examined (seventeen), with the exception of molybdenite and cinnabar, were quickly attacked by citric or tartaric acid, to which a little potassium nitrate had been added. Potassium chlorate produces a similar though slower action. These examples are sufficient to show that Dr. Bolton has found a promising field of inquiry, and, singular to say, considering the attention which the action of organic acids has received, it is a field believed to be entirely new. He is continuing his researches. SCIENTIFIC ORCHESTRATION.