poured off, and the precipitate is washed with water. Before throwing away the liquid, fresh lime-water should be added to it; and if the dark hue recurs, the precipitate must be allowed to subside again; if no change takes place, it may be inferred that the silver is all extracted. The oxide of silver should not be dried, but be kept in bottles with water. * * * * * POTASSIUM. CYANIDE OR SIMPLE CYANURET OF POTASSIUM. Before entering into the preparation of this ingredient, I may mention that the _prussiate of potash_ of commerce has, by many young experimentalists, been mistaken for the above article, because it is often sold, by those unacquainted with chemical technicalities, under the name of _cyanuret of potassium_. It is, in chemical parlance, termed _ferro-cyanuret_, from its containing a certain portion of iron; and differs in its properties, very materially, from the _simple_ salt. It is of a bright yellow color, and is converted into the colorless, simple cyanuret in the following manner: Take 4 ounces of the yellow prussiate, break it in small pieces, and well dry it on a plate of iron; then reduce it in a mortar to exceedingly fine powder. Dry and pound in like manner one and a-half oz. of carbonate of potash. Incorporate the two ingredients thoroughly. Place a Hessian crucible in the fire; and when it attains a red heat, throw into it the prepared mixture, and closely cover the crucible. Keep up the heat, and the contents of the crucible will soon fuse; and the fluid mass will become red-hot. After this, immerse in it, from time to time, a hot glass rod; the mass which adheres, is in the early stages of the process brown on cooling; as the heat is continued, it appears yellowish, and finally, colorless and transparent. The operation is then complete: the crucible must be removed; and after its contents have been allowed to settle, the fused mass may be poured off; the greater portion of which consists of the _simple cyanuret of potassium_. This salt is very deliquescent, and must therefore be retained in close bottles: it will readily be recognized by its powerful odor,--that noticed in peach blossoms. The mere mention of prussic acid entering largely into its composition, will be sufficient to induce my reader to exercise common caution in handling it. Having these ingredients prepared, take one pint of pure rain or distilled water; add to it two ounces of the cyanuret of potassium, shake them together occasionally, until the latter is entirely dissolved; and allow the liquid to become clear. Then add a quarter of an ounce of oxide of silver, which will very speedily dissolve; the dissolution may be hastened by heat, and after a short time, a clear transparent solution will be obtained.--_Walker._ * * * * * STEREO-CHROMIC PAINTING.