may have secured lodgment, and from which it is difficult to perfectly remove it. In such cases the work must be removed and given another thorough pumice brushing and rinsing, and again immersed in the depositing vat.

As has already been briefly noticed, the hardness of electro-deposited nickel renders it impossible to finish the plated articles by burnishing. It is therefore necessary to prepare the surfaces of the articles to receive the nickel deposited before they are plated in order to reduce the subsequent finishing operations as much as possible. On this account it is customary to polish the surfaces of articles to be plated on buffing wheels. In case the surface is very rough, as is sometimes the case with articles of iron or steel, it may be necessary to grind it smooth upon the emery wheel. The work when it is removed from the nickel bath is dipped for a few moments into boiling water, and then rapidly dried in sawdust. It is now ready to be polished on the buffing-wheels, when the work is finished.

The length of time required to produce a sufficiently heavy deposit of nickel will depend on the strength of the current, the condition of the bath, and the character of the articles. Brass and copper articles usually receive a sufficiently heavy coating in half an hour; for wares on which an extra heavy coating is desired the time of immersion is extended to an hour or even longer. Iron and steel, Britannia-metal, pewter, etc., require a longer time of immersion than brass or copper, even though given a preparatory coating of copper, because of their comparitively inferior conductibility. A good coating of nickel, properly laid on, possesses great durability, and with ordinary usage will last for many years.

Old nickel-plated work, which it is desired to replate, should first be "stripped," as is found necessary with the

precious metals. For this purpose a mixture of sulphuric and nitric acids is commonly employed. Watt (ElectroMetallurgy, 7th ed., 114 et seq.) recommends the following mixture as the most generally serviceable, viz: "4 pounds strong sulphuric acid, 1 pound nitric acid, and about 1 pint of water." By volume, these proportions would be, approximately: Strong sulphuric acid, 2 parts; nitric acid, 1 part; water, 1 part. The acids should be added to the water with constant stirring. This stripping liquid may be used either cold or slightly warm. It acts promptly, removing a light coating of nickel in less than a minute, and a heavy one in a few minutes. To avoid contaminating the solution as little as possible with the metal of the wares, the operation should be closely watched and the articles removed from the acid just as soon as the nickel has been dissolved. The preparation of the stripped articles for re-nickeling should be the same as for new work. Articles may be stripped in the nickel bath by the ordinary artifice of connecting them as anodes, but the practice is injudicious, as the purity of the bath will thereby become impaired by the solution of the metals composing the wares. Where the current is used for the purpose, therefore, a separate solution should be used, and for this purpose we approve Watt's recommendation to use as a stripping solution dilute sulphuric acid which will dissolve nickel readily without appreciably affecting brass. Under all circumstances, however, the articles should be looked at from time to time, and removed as soon as they are free from nickel. It is important, however, that the nickel be thoroughly cleaned off, to prevent the peeling of the subsequent nickel deposit.

Plating with Nickel by Immersion.-Stolba describes the following simple process for nickel-plating without the battery, which may be usefully applied in the case of

small objects. He dilutes a concentrated solution of chloride of zinc with twice its volume of water. This mixture he boils in a copper vessel, adding a few drops of muriatic acid should there appear a precipitate of basic chloride of zinc. He thereupon adds a small quantity of powdered zine. This addition causes a deposit of zinc upon the vessel. Thereupon sufficient chloride, or sulphate, of nickel is added to the bath to give it a distinctly green color, and the previously cleansed articles are then immersed in the liquid in contact with zinc, and allowed to remain there for about fifteen minutes, the temperature being maintained at boiling during the operation. If the coating is found to be insufficient the articles are again immersed until a deposit of sufficient thickness is obtained. In this way he claims to be able to coat satisfactorily articles of zinc, cast and wrought iron, steel, and copper. (Journal Chemical Society, xi. 465.)

By an analogous process described by C. Mène, it is af firmed that metallic articles may be plated with nickel by immersing them in contact with zinc, in a boiling neutral solution of chloride of zinc, in which is contained fragments, or a plate, of nickel. Should the solution be acid, the plating, it is asserted, will be dull. By this procedure the author claims to be able to coat articles of iron, steel, copper, brass, zinc, and lead. (Chemical News, xxv. 214.)

Where electrotypes of type or engravings are to be printed with colored inks, that are disposed to become chemically affected by contact with the usual copper surface (as for example, vermilion, which becomes brownish), it is customary to give the usual copper electrotype a thin coating of nickel in the usual manner. This nickel coating renders the electrotype proof against the abovenamed difficulty that printers experience with electrotypes not so protected.

By methods and solutions analogous to those described for nickel, electro-deposits of cobalt may be obtained. The electro-deposits of this metal, which we have seen, equal, if indeed they do not surpass, those of nickel in whiteness and brilliancy of lustre. The rarity and costliness of this metal, however, exclude it from any practical uses in plating.

CHAPTER XLVI.

DEPOSITION OF ZINC.

ELECTRO-ZINCING-ZINCING BY CONTACT WITH ANOTHER METAL.

ZINC is easily deposited by the wet way and with the aid of the battery. Its dead lustre is gray bluish-white. By precipitating a soluble zinc salt with ammonia, and then redissolving the precipitate in an excess of alkali, a bath is obtained which gives quite satisfactory results.

It is also possible to obtain satisfactory results with a bath made by dissolving any salt of zinc in cyanide of potassium, or in a soluble sulphite.

These zinc deposits obtained in the humid way, have nothing in common, either as regards the mode of operation or in point of durability, with such as are obtained by the method called galvanizing, by which cleansed iron, or other metallic objects, are plunged into a bath of molten zinc, and, receiving a substantial, adherent coating of this metal, become protected against oxidization for a long time. Electro-deposited zinc, in comparison with that deposited by "galvanizing," is much inferior as a protective coating. A few gilders employ deposits of this

metal, the dead lustre of which imitates chased aluminium; but it tarnishes entirely too soon to be of much service in the arts.

It may be useful, nevertheless, to give some of the most promising formula which have been proposed for obtaining electro-deposits of this metal :

Winckler (Handb. d. Metallüberzügen, 256) proposes the following for a cold electro-zincing bath:

The well-cleansed articles are immersed in this bath, which is worked with a large cast zinc plate as anode, and a weak current. Copper thus coated assumes the color of brass by subsequent heating. This bath gives goodresults with iron, for which it is specially recommended. [This solution is identical with that of MM. Person and Sire (Chem. News, ii. 275). The zinc, according to these authors, is deposited as readily as copper, and takes place on all metals, on platinum, as well as on copper iron.]

and

For the same purpose (i. e., electro-coating iron with zinc) this author also recommends an aqueous solution of sulphate of zinc, using 1 part of zinc salt to 4 of water (or an equivalent proportion of the chloride may be used instead of the sulphate). The bath should be used moderately warm, and with a zinc anode. The articles soon coat themselves with a bluish-white film of zinc, which may be burnished.

Dr. Elsner (Id., 257) recommends, for coating cast iron objects. a solution prepared by dissolving in the necessary quantity of rain water