for electro-deposits of copper or brass, for ageing freshly made silver-baths, for precipitating fulminating gold from the chloride of gold, and for dissolving the film of copper deposited at the beginning of galvanoplastic operations with silver, etc.

Silver.

We use silver in different forms, in plates, foil, powder, or granulated, according to the purpose for which it is

intended.

We should, as far as practicable, employ it pure; but the silver found in the trade, even under the name of virgin silver, retains traces of copper. Coin silver contains one-tenth of copper, and stamped silverware a greater or less proportion according to the standard.

Silver is purified by several methods:

1. The impure metal is dissolved in nitric acid, and the solution after being largely diluted with water, is treated with an excess of a filtered solution of common salt. An abundant white precipitate of chloride of silver is produced, which rapidly settles to the bottom of the vessel. All the silver salt is decomposed, when the clear supernatant liquid is not rendered turbid by a further addition of common salt. The chloride of silver is collected, and washed several times, until the wash liquors are no longer colored brown by ferro-cyanide of potassium. This is the proof that all the copper has been washed out. The washed chloride of silver is mixed with two or three times its weight of carbonate of sodium, dried, and the mixture fused in a crucible at a high temperature. After cooling, the metal is found in the shape of a button at the bottom of the crucible. To granulate it, the molten silver is poured in a thin stream from a moderate height into a large volume of water.

2. The chloride of silver is prepared as in the first method; then, after having washed out the copper salt, a considerable volume of water is added, then an excess of scraps of sheet zinc and some sulphuric or hydrochloric acid. Hydrogen gas is immediately produced in abundance, and, combining with the chlorine of the chloride of silver, reduces the metal to the form of a fine powder, which is separated when all the zinc is dissolved. As it very rarely happens that commercial zinc is free from lead or carbon, this method is seldom employed. The substitution of iron for zinc presents no advantages.

Silver is easily dissolved in pure nitric acid, but not so rapidly when the acid is contaminated by chlorine or hydrochloric acid, which forms an envelope of chloride of silver around the metal, thus hindering its solution.

Sulphuric acid also combines with silver, and the resulting salt is only slightly soluble in water. As for hydrochloric acid, the trace of silver which may be dissolved is immediately transformed into chloride, preventing the further action of the acid.

Cyanide of potassium dissolves metallic silver slowly, and forms a double salt with it. On the other hand, the sulphites dissolve only the salts of silver and have a tendency to reduce them to the metallic state.

Pure silver is employed for the preparation of the nitrate and other silver salts, and as the soluble anode in silver-baths. In the form of impalpable powder, and mixed with cream of tartar, it is used for silvering with the brush, and for the paste with which watch-works are grained before being gilded. In this case, the silver powder is prepared by reducing a very diluted solution of the nitrate of this metal by the means. of strips of copper. This powder should be washed several times with ammo

nia, which will remove all the

copper

which may

adhere

to it.

Nitrate of Silver (poisonous).

(Argentic nitrate. Lunar caustic. Lapis infernalis.) This salt is found in the trade in three forms: either as crystallized nitrate of silver in thin, rhombic, and transparent plates; or in amorphous, opaque, and white plates of fused nitrate; or in small cylinders, of white, or gray, or black color, according to the nature of the mould employed, in which form it constitutes the lunar caustic for surgical

uses.

The crystallized nitrate of silver still retains a small proportion of nitric acid and water between the lamina of its crystals; the white fused salt should be pure, though it is often fraudulently adulterated with nitrate of potassium or sodium. The third variety, or lunar caustic, generally has its surface coated with a film of reduced silver and of oxide of copper from the moulds into which it is run: or its color may simply be due to the inferior quality of the silver employed.

Nitrate of silver is very soluble in distilled water,* but with ordinary water gives a precipitate which is the more or less copious according to the quantity of chlorides the water may contain. This precipitate at first is white, but soon darkens by the action of solar light, but this transformation does not take place when it contains even traces of chloride of mercury.

The solution of nitrate of silver forms yellow precipitates with the iodides, phosphates, and bromides; and white

*However, when the nitrate of silver has been kept too long in igneous fusion, it is less soluble in distilled water, and the white turbidity of the liquid is probably due to the formation of a subnitrate

of silver.

precipitates with the soluble chlorides, sulphites, cyanides, hyposulphites, and pyrophosphates. With the exception of the phosphates and pyrophosphates, the other reagents, when employed in greater or less excess, will redissolve the precipitate with the formation of double salts.

It is precisely this property of the cyanides, sulphites, and hyposulphites, which is the basis of the preparation of silver-baths by simple immersion, or with the battery. Ammonia dissolves even the most insoluble silver salts except the iodides; but as this solution attacks copper, Roseleur remarks that it cannot be used as a silvering bath, notwithstanding the advice of Boettger.

Nitrate of silver is prepared by dissolving pure silver, granulated or laminated, in double its weight of pure nitric acid of 40° Baumé. This operation is conducted in a glass flask, or in a porcelain capsule. Nitrous vapors are copiously disengaged, and the metal soon disappears to form a colorless liquid (blue or green if copper be present). After cooling, and a rest of a few hours, a mass of crystals of nitrate of silver is found, which is drained, and to remove the excess of acid, washed with a little distilled water saturated with nitrate of silver. The crystals are dried in a stove, and kept away from solar light. If, instead of cooling the liquid after the silver has been dissolved, the evaporation be continued, the mass will become spongy, and then fuse by a greater heat to a grayish liquid, which may be run into moulds and forms the so-called lunar caustic. The mass obtained by the fusion of the crystals of nitrate of silver is whiter, and is known as white lunar caustic. It may be either run into moulds or allowed to solidify by cooling on the sides of the capsule.

This salt, whatever be its mode of preparation, should be kept in black or blue bottles, and free from contact with organic substances, which would reduce the metal.

It is employed for preparing silvering baths, metallizing moulds, and for many other purposes.

Nitrate of Mercury (poisonous).

(Mercuric nitrate. Acid nitrate of mercury. "Quicking" solution.) This salt is generally sold in the form of an oily, colorless liquid, which is the denser as it contains more free acid. It stains the skin a violet-red, and in contact with water produces a pulverulent and yellowish-white precipitate, of a subnitrate. This precipitate disappears upon the addition of a few drops of sulphuric or nitric acid, and the liquid becomes clear. Should the solution of nitrate of mercury be concentrated, sulphuric acid, instead of dissolving the precipitate, increases it, from the formation of insoluble subsulphate of mercury.

This salt is used in our art for slightly amalgamating (coating with quicksilver) the pieces which are to be silvered or gilded. This amalgam forms a kind of solder between the metallic surface operated on, and the deposited metal.

Nitrate of mercury is obtained by dissolving at a temperature of about 212° Fah. a certain quantity of mer cury in double its weight of nitric acid of 40° Baumé, and continuing the heat until yellow fumes no longer appear. If, instead of operating with the aid of heat, the re action be effected in the cold; or, if the proportion of mercury be in excess of the necessary quantity of nitric acid, we obtain a salt of suboxide which is not so suit able for our purpose.

Nitrate of Potassium.

(Potassium nitrate. Saltpetre. Nitre.)

This salt is sometimes produced by the action of car bonate of potassium upon nitrate of calcium; the latter