This restoration is
believed to be a function of the living choroid, and probably of the
retinal epithelium, though it is independent of the black pigment which
this epithelium contains. This vision purple is the latest discovery in
optical physiology, and it cannot fail to be a most important one. How
far it will alter the received views upon the subject of changes in the
strength of vision, which are now attributed to alterations in the
distance of the crystalline lens, cannot be foretold. But it may be
found possible to stimulate by drugs the restorative action of the
choroid, and thus by gaining increased "definition," improve weak
sight. As to the detection of murderers by photographing the last
retinal picture from their victims' eyes, while these discoveries do
not leave this an impossibility, they do not much improve the
probability of its ever being done. Very often the sight of the
assassin is not the last which comes within the victim's vision. Too
long a time also usually elapses before discovery. These and similar
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 add