Mr. Johnson proposes to treat the alloy first with acetic acid to lessen the quan tity of lead, and then either melt it or subject it to cupellation.
The treatment of the compound ores of silver is usually accomplished either by roasting or amalgamation. A consid erable quantity of silver is obtained from argentiferous galena. In fact, almost every specimen of native sulphuret of lead will be found to contain traces of this metal. When the proportion rises to a certain amount it is worth extracting. The ore is reduced in the usual manner, the whole of the silver remaining with the lead ; the latter is then remelted in a large vessel, and slowly allowed to cool until solidification commences ; the por tion which first crystallizes is nearly pure lead, the alloy with silver being more fusible than lead itself. By particular management this is drained away, and is found to contain nearly the whole of the silver. This rich mass is next exposed to a red heat on the shallow hearth of a furnace, while the stream of air is allowed to infringe upon its surface. Oxidation takes place with great rapidity : the fused oxide, or litharge, being constantly swept from the metal by the blast. When the greater part of the lead has been thus re moved, the residue is transferred to shal low dishes made of bone ash, and again heated. The last of the lead is now oxi dated, and the oxide sinks in a melted state into the porous vessel, while the silver, almost chemically pure and having a brilliant surface, remains behind. (See CimnimArioN.) When silver is melted in open vessels it has the curious property of absorbing oxygen, which it gives out when it con geals. This is the cause of the appear ance of granular crystallization which sil ver assumes when hastily cooled : a small per centage of copper entirelyprevents the effect. The only pure which act upon silver are the nitric and sulphuric. The nitric acid dissolves silver without the aid of heat, nitrous gas is evolved, and a dense colorless solution obtained, from which tabultir crystals of nitrate of silver may be produced by evaporation. These crystals are anhydrous, and consist of 118 oxide of silver and 54 nitric acid, the equivalent of silver being 110. When melted, and cast into sticks or quills, they form the lunar caustic of the surgeons. Chloride of silver is readily made by adding common salt to a solution of ni trate of silver. It is a white curdy pow der which becomes violet when exposed to sunshine, all( is soluble in water of ammonia and alkaline hyposulphites. SILVERING. The covering the sur face of metal, or other body, with a bright coating of silver. (See PLATE, MANUFAC TURE or.) The dial-plates of clocks, the scales of barometers, and other similar articles, are silvered by rubbing upon them a mixture of muriate of silver, sea-salt, and tartar, and afterwards washing off the saline matter with water. In this operation, the silver is precipitated from the muria tie acid, which unites with part of the coppery surface. Silvering if pins is
effected by boiling them with tin filings and tartar.
To make shell-silver, silver-leaf is ground with gum-water, or honey: the gum, or honey, is washed away, and the powder which remains is used with gum-water, or white of eggs, laid on with a hair pencil.
Silveringfor glasses are silvered by an extemporaneous amalgamation of tin and quicksilver. Tin-foil is placed on the back of the glass, and some quicksilver is poured upon it, and spread over the surface with a hare's foot. Another glass is then slid over the tin, to drive off part of the quicksilver ; and paper and a board being laid on the tin, it is strongly pressed with a number of weights, to expel, by degrees, the superfluous quicksilver, and leave only a crystallized amalgam on the back of the glass.
Mr. Drayton, some years back, invent ed a new mode of silvering, by using a solution of nitrate of silver in ammonia, and then adding a small quantity of essential oil, as that of cloves, cajeput, &c. By applying gentle heat, in a short time the silver is reduced in a beautiful bright mirror upon the surface of the glass ; if the latter surface be flat, walls of putty are necessary. Deep vessels can have the solution poured in and al lowed to lay till deposition takes place.
Silvering for globes.—This amalgam is made by dissolving one pound of tin, glass, or bismuth, in four pounds of quicksilver. The globes to be silvered are thoroughly cleaned on the inside, and warmed ; then the above amalgam being heated, so as to be perfectly liquid, is poured in by a paper funnel, and the globe inclined in various directions, that, as the amalgam crystarn-i-s by cooling, it may adhere to all par the globe ; the superfluous amalgam is then poured out.
Silvering ivory.—Immerse a slip of ivory in a weak solution of nitrate of sil ver, and let it remain till the solution has given it a deep yellow color : then take it out and immerse it in a tumbler of clear water, and expose it in water to the rays of the sun. In about three hours the ivory acquires a black color, but the black surface, on being rubbed, is soon changed to a brilliant silver.
Silvering Daguerreotype plates.—Preci pitate oxide of silver from the nitrate by potass ; filter, wash, and dry it. Dis solve this oxide in pare liquid ammonia, the solution will be of a yellow color. Immerse a slip of polished copper in it, and let the moisture evaporate. When the copper is quite dry, hold it over a charcoal fire ; the oxide will be decom posed, and the metal reduced on the cop per in the form of a complete coating. This may be made beautifully bright by polishing with leather. It otters a much more brilliant and smooth surface than that of the last experiment, and is a ready method of silvering copper-plates for the Daguerreotype pictures. It is not, how ever, equally well performed as by the gal vanic process. (See ELECTRO METAL LURGY.)