Mr. Matteucci has observed that the black deposit obtained on the cathode, in the electrolysis of this salt, instantaneously becomes white when the current ceases; that it does not occur except when the solution is weak, because from strong solutions, crystalline silver is at once deposited; that if a portion of the deposit, after it has became white, be suspended between the electrodes while the current passes, those portions towards the anode again become _black_, if the experiment is made in a solution of the nitrate, but not if in mere acid water; it will, however, occur if a mere drop of nitrate is added to the solution, if a plate of a glass is interposed between the electrodes, the phenomenon does not occur; this is attributed to the great reduction of intensity. It is suggested "that the black deposit is formed, of oxide of silver, which is preserved by the passage of the current, and which, when the current ceases, passes immediately to the metallic condition." * * * * * OXIDE OF SILVER. Place pieces of silver in a glass vessel,, and pour on them about equal parts of water and strong nitric acid; the metal will soon dissolve, giving off fumes of nitric oxide. Should the solution have a green hue, which is invariably the case, unless the metal has been obtained fine from the refiners, it indicates the presence of copper, in which case immerse some pieces of copper in the solution, and the nitric acid, by elective affinity, will combine with the copper; and a precipitate of pure silver, in the form of grayish powder, will take place. Throw away the liquid and wash the silver precipitate several times in sulphuric acid and water, and afterwards in water alone. Then re-dissolve it as before, in nitric acid and water; and a _solution of pure nitrate of silver_ will be obtained. Place this in an evaporating dish, or a saucer, and apply the heat of a spirit-lamp, or place the saucer by the fire-side, till some portion of the liquid is driven off in vapor. Allow the residue to cool, and it will shoot out into long colorless transparent crystals, which are _nitrate of silver_. They must be handled with care, as they possess the property of staining animal and vegetable substances with an almost indelible black;--fused nitrate of silver being the lunar caustic of surgery, and the main ingredient of marking-ink. Next prepare some _lime-water_, by stirring lime into water and filtering the solution. As lime is very sparingly soluble in water, requiring, at 60° Fahrenheit, 750 times its weight, it is necessary to make an abundant supply. Place the lime-water in a glass or other vessel, and drop in it a few crystals of nitrate of silver; the colorless solution will instantly assume an unsightly brown hue; and, after remaining quiescent for a time, the _oxide of silver_ will subside in the form of a dark brown precipitate. The liquid is then