not remain united with reguline metals, and as these latter sink to the bottom when in fusion, on account of their greater specific gravity, they may be easily parted from the scoriæ.

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The silver to be purified by nitre is to be first granulated, and then mixed with a fourth part of its weight of dry nitre, an eighth part of potash, and a little common glass, all in powder. This mixture is to be put into a good crucible, two-thirds of which only must be full. This crucible is to be covered with a smaller crucible inverted, in the bottom of which a small hole has been made, and luted to the former. The crucibles, thus disposed, are to be placed in a furnace capable of drawing air sufficiently to make the fire intense enough only to melt the silver. Then charcoal is to be put into the furnace to such a height, that only the top of the inverted crucible shall be uncovered. The coal is then to be kindled, and the vessels to be made moderately red; a hot coal ought to be put upon the small hole in the bottom of the inverted crucible. If a shining light be observed round this coal, and a slight hissing noise at the same time heard, we may know that the operation proceeds well. The fire must be sustained at the same degree till these appearances cease, when it must be increased, so that the silver be well melted, and then the crucibles are to be taken out of the furnace. The larger crucible is to be broken when it is cold, and the silver will be found at the bottom covered with green alcaline scoriæ. If the metal be not sufficiently ductile, the operation must be again repeated.

Some silver is apt to be lost in this operation, by the swelling and detonation of the nitre, which often forces it through the hole in the upper cru

cible, unless great care be used; nevertheless, this method has its advantages, being much more expeditious than cupellation.

Separating Silver from Copper by Eliquation.

When it is desired to separate, in the large way, a small quantity of silver from much copper with which it is alloyed, the process called eliquation is resorted to. This operation is grounded on the nearer affinity of silver with lead than with copper; in consequence of which it fuses, and combines with lead at a degree of heat in which copper continues unfused.

Whitening Silver by Boiling.

Whitening of silver by boiling is one of the methods of parting copper from silver in the humid way. For this purpose, silver wrought in any shape is first ignited to redness, and afterwards boiled in a ley of muriate of soda, and acidulous tartrite of potash. By so doing, the copper is removed from the surface, and the silver receives a better appearance.

Precipitating Silver by Copper.

Copper has a much greater affinity with oxygen than silver; consequently, the silver is precipitated from its solutions as a fine silver dust, by metallic copper. This likewise affords a means to discover what portion of silver may be contained in an alloy of silver and copper. A quantity of the mixture determined by weight is dissolved in nitric acid;

the solution is diluted with water, filtered, and a plate of copper hung in it, till no more precipitate falls down. Then the weight of the precipitate, when edulcorated, is compared with that of the whole alloyed metal put to trial.

This silver dust well washed, and mixed with gum water, serves as a pigment in water painting.

Separating Silver from Copper by an Alcaline
Sulphuret.

The affinity of copper with sulphur is stronger than that of silver. Upon this ground, liver of sulphur (sulphuret of potash) has been proposed as an expedient to free silver from copper; for if silver holding copper be fused with alcaline sulphuret, the base metal combines with the latter, and is converted into scoriæ floating on the silver.

Mr. Keir's Mode of separating Silver from Copper.

Chemists have long been acquainted with the compound acid, called aqua regia (nitro-muriatic acid), which has the exclusive property of dissolving gold. The discovery of a compound acid, acting exclusively upon silver, is owing to Mr. Keir.

This compound acid is made by dissolving one pound of nitrate of potash (common nitre or saltpetre), in eight or ten pounds of sulphuric acid (oil of vitriol), or by mixing together sulphuric and nitric acids. This acid dissolves silver easily, while it will not attack copper, iron, lead, gold, or platina.

These properties have rendered it capable of a very useful application in the arts. Among the manufactures at Birmingham, that of making ves

sels of silver, plated on copper, is a very considerable one. On cutting out the rolled plated metal into pieces of the required form and sizes, there are many shreds, or scraps, as they are called, unfit for any purpose, but the recovery of the metals, by separating them from each other. The easiest and most economical method of parting these two metals, so as not to lose either of them, is an object of some consequence to the manufacturers. For this purpose, two modes were practised; one, by melting the whole of the mixed metals with lead, and separating them by eliquation and testing; and the second, by dissolving both metals in sulphuric acid, with the help of heat, and by separating the sulphate of copper, by dissolving it in water, from the sulphate of silver, which is afterwards to be reduced and purified.

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In the first of these methods, there is a considerable waste of lead and copper; and in the second, the quantity of sulphuric acid employed is very great, as much more is dissipated in the form of sulphureous acid than remains in the composition of the two sulphates.

Some years ago, Mr. Keir communicated to an artist the method of effecting the separating of silver and copper, by means of the above-mentioned compound of sulphuric acid and nitre. It is now commonly practised by the manufacturers at Birmingham, and is much more economical, and much easier executed, that any of the above-mentioned methods; for nothing more is necessary than to put the pieces of plated metal into a glazed earthen pan, to pour upon them some of the acid liquor, to stir them about, that the surfaces may be frequently exposed to fresh liquor, and to assist the action by a gentle heat, from 100° to 200° Fahr.

When the liquor is nearly saturated, the silver is to be precipitated from it by common salt, which forms muriate of silver, or luna cornea, easily reducible to a metallic state, by melting in a crycible, with a sufficient quantity of potash; and lastly, by refining the melted silver, if necessary, with a little nitre thrown upon it. In this manner, the silver will be obtained sufficiently pure, and the copper will remain unchanged. Otherwise, the silver may be precipitated in its metallic state, by adding to the solution of silver a few of the pieces of copper, and a sufficient quantity of water to enable the liquor to act upon the copper.

Method of obtaining Gold in a pure State.

Perfectly pure gold may be obtained, by dissolving the gold of of commerce in nitro muriatic acid, and precipitating the metal, by adding a weak solution of sulphate of iron. The precipitate, after being well washed and dried, is pure gold.

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Method of obtaining Silver in a pure State.

Dissolve the silver of commerce in nitric acid, and add to it some muriatic acid; a white curdy precipitate will be formed, which is muriate of silver. To reduce muriate of silver to the metallic state, let one part of it be mixed with three of soda, and exposed to a white heat. When the mixture is well fused, suffer it to cool; then break the crucible, and separate the pure silver from the muriate of soda which has been formed.