in relation to the receiving surface, the bath becomes too rich in silver, and the proportion of free cyanide is relatively diminished; an excess of free cyanide will have a similar effect. If, on the other hand, the anode is relatively small as compared with the receiving surface, the bath will become impoverished in silver; and if the bath is deficient in free cyanide, the result will be the same; or the two causes combined will aggravate the evil.

When, after long usage, the density of the cyanide bath becomes too great and its electric conductivity diminishes, the silver is deposited, but the cyanide remains behind and is partly decomposed into carbonate of potassium, which retards the silvering process by preventing the free passage of the electric current. When this condition of things exists, it will be well to add to the bath, instead of the cyanide of potassium, hydrocyanic acid, which combines with the potassium of the carbonate to form fresh cyanide, while the carbonic acid escapes in gaseous form. We have thus rehabilitated the cyanide with its dissolving properties, and, after proper dilution, the bath will be restored to its normal working condition.

The bath may also be regenerated by the addition of the cyanides of calcium or barium, the bases of which form insoluble carbonates, while the cyanogen combines with the potassium of the carbonate.

As has been elsewhere remarked, the cyanide plating baths may vary considerably from the standard before the fact becomes patent by the obvious inferiority of their work; a fact of some importance for the inexperienced plater to know. On this account it may be well to add to the foregoing hints on the management of such solutions, the statement that the rate at which the anode is being dissolved, will (in addition to its appearance) afford him an approximate knowledge of the condition of the bath.

The loss of weight of the anode should agree very closely with the gain in weight of the plated articles, if the bath is working properly. If the anode is losing more than the goods are receiving, the indication points to an excess of free cyanide in the bath; if the reverse be the case, a deficiency of free cyanide may safely be suspected.

In either case the remedies are obvious. It is a good rule to observe, not to correct the irregularities that show themselves in the working of the bath, by fresh additions of cyanide or silver salt, until the operator has assured himself by careful examination that the fault does not lie with an insufficient or excessive current, or a disproportion between the surface of the anode and of the recipient. Every fresh addition increases the quantity of the substances in the bath, and to maintain its density within proper limits its volume will soon become greatly increased by the necessary dilution with water.

When a bath is in good working order, the operator should, by careful attention to the hints and precautions. herein given, strive to keep it so. Such attention will go very far towards accomplishing this desirable result, but no amount of advice will take the place of experience.

A durable electro-silvering, that is to say, a thick, continuous, and adherent metallic deposit, is of such importance at the present day, that we repeat again in a synoptical form the several operations recommended by Roseleur for obtaining good results.

ROSELEUR'S METHOD FOR COPPER, BRASS, AND GERMAN-SILVER.

FIRST OPERATION.

Boil the articles a few minutes in a solution of—

Water

Caustic potassa .

and wash in fresh water.

10 parts,

1 part,

Plunge for a few seconds into the following mixture:

Nitric acid of 36° Baumé, yellow

Common salt

Calcined soot

and wash rapidly in abundance of water.

FOURTH OPERATION.

100 parts,

Immerse the articles for a few seconds, or until they

are entirely whitened, in a solution of

Water

Nitrate of mercury

100 parts,

1 part,

with the addition of sufficient sulphuric acid to render the solution clear on stirring with a glass rod.

Rinse in fresh water.

SIXTH OPERATION.

Immerse the articles in the bath, under the influence of a weak current, and at the expiration of a quarter of an hour examine carefully each supporting rod.

If the

* Some operators substitute for the third and fourth operations scouring with pumice-stone powder. For the preparation of Britanniametal, steel, etc., see our description, Chapter XXXIV., page 293.

articles are being coated uniformly and without spots the operation is allowed to proceed; on the contrary, should the silver-deposit be crystalline, the pieces are withdrawn, carefully scratch-brushed, passed through a hot solution of cyanide of potassium, rinsed in fresh water, plunged into the "quicking" solution, rinsed again, and then replaced in the silver bath, where they are permitted to remain until they have received the quantity of deposit determined upon.

SEVENTH OPERATION.

The current is arrested for a few minutes before the articles are withdrawn from the bath, in order that the solution may redissolve the subsalts of silver deposited along with the pure metal.

EIGHTH OPERATION.

Remove from the bath, wash in fresh water, and then in a very weak pickle of sulphuric acid.

The baths for electro-silvering very large pieces, or those of diverse forms, must obviously be contained in depositing vats of appropriate shapes, the soluble or insoluble anodes being placed, sometimes in the middle, sometimes about the sides.

The articles when removed from the bath have a dead lustre, which is crystalline and micaceous when the bath is fresh, and dull and earthy looking with old solutions. This dead lustre is easily scratched, and becomes yellow by contact with the air. In order to preserve its freshness, it should be covered with a thin coat of colorless alcohol varnish. We have already seen that an article, rendered

yellow by the alteration of the subcyanide silver, may be restored to its primitive whiteness by the combined action of heat, borax, and a feeble solution of either nitric or sulphuric acid; or, more simply, by an immersion for a few minutes in a tepid solution of cyanide of potassium, without the battery.

CHAPTER XXXVII.

METHODS BY WHICH THE WEIGHT OF DEPOSITED SILVER
IS ASCERTAINED.

To ascertain with exactness the quantity of silver deposited upon a given surface and in a given time, is an important problem for the silver electroplater to solve. With the different dispositions previously described, we cannot arrive at very correct results, and, although a long practice may facilitate the operation, there is always more or less guesswork and uncertainty.

We give herewith some of the methods employed by electro-platers, following the directions given by Roseleur: Let us suppose that twelve tablespoons are to be coated with two ounces of silver. The articles are cleansed by the processes already described, then dried in sawdust or otherwise, and weighed in a scale. However rapid this manipulation may be, the surface of the metal will be slightly oxidized and tarnished by contact with the fingers or by the sawdust; and, in order to restore their former brilliancy, the articles must be plunged into a strong pickle of sulphuric acid, and then into the “quicking" solution. After rinsing, and a more or less protracted immersion in the bath, the operator will judge, by his practical expe