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They are then well washed in fresh water to free them from adhering acid, and placed at once in the depositing bath. It is particularly necessary to guard against substituting nitric acid in place of the hydrochloric or sulphuric acid in the above named acid solution.

Referring to the gilding and silvering of iron and steel, Roseleur remarks, that, while they may be well gilded without an intermediary coating of copper, in the hot gilding baths to be described further on, the direct silvering is always imperfect and without adherence, on which account he considers it necessary to coat such articles with a preparatory coating of copper. As regards steel, as we shall show in the special chapter on silverplating, Roseleur is evidently not familiar with what has been done in this country, and, if we mistake not, also in England. In all the large plating factories in the United States, not only is irreproachable silver-plating done upon table-knives directly on the steel, but such has been the practice for the past twenty-five years. The same may also be said in the case of Britannia metal so largely employed for articles of table service and artistic uses, of which Roseleur makes the assertion that "the direct deposit of the precious metals is difficult and does not adhere well." Not only is the direct silver-plating of such wares the universal practice in this country with the most admirable results, but it is questionable if coppered goods could command a market.

We shall, under Silvering, describe certain methods for preparing work, in use in some of the leading American establishments.

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GROVE'S BATTERY

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BICHROMATE BATTERY

-MARIE-DAVY BATTERY-SMEE'S BAT

TERY-THE SAME MODIFIED-LÉCLANCHÉ BATTERY.

Galvanic Batteries.

THIS name is applied to certain forms of apparatus devised for the purpose of generating an electrical current of more or less intensity and quantity and with more or less constancy and uniformity.

As in static electricity and magnetism, so likewise in the production of the electrical current in the battery, the action is accompanied by the phenomenon of polarity; the current presenting properties conveniently known as positive (corresponding to the vitreous of static electricity) and negative (or resinous); the former manifesting itself at one extremity of the apparatus, and the latter at the other. A galvanic couple or element is constituted by the union of a single electro-negative agent with a single electropositive agent. A number of such elements, coupled, as we shall see further on, constitutes a battery. The agents employed are commonly two dissimilar metals, which, when brought in contact by suitable means with a liquid capable of acting chemically on one of them, become electric. The degree of electric disturbance will depend on the nature of the dissimilar metals, and is termed electro-motive force. This electrical disturbance manifests

itself by the phenomenon of polarity in the two agents employed in the cell. The positive pole is designated by the sign, and the negative by

The electrical current generated in a cell may be conducted to any desired point by means of metallic bands or wires, which are called conductors.

When the two poles or the metallic conductors are united, the galvanic circuit is closed, and a continuous flow of the opposite electricities takes place in opposite directions, in virtue of the disposition to re-establish electrical equilibrium. As rapidly, however, as the opposite electricities passing through the conductors are neutralized, the electro-motive force in the cell originates a fresh electrical disturbance. The consequence of this continuous production of electrical tension and its equalization, is the formation of a continuous flow of electricity, which is called a galvanic current.

When the extremities of the two conductors (not in contact) are plunged into a liquid that is capable of transmitting the current, the liquid in question completes the circuit, and is termed an electrolyte.

It is nearly always to the wire which proceeds from the negative pole of the battery (generally zinc) that we attach the object which is to receive the galvanoplastic deposit, and the object takes the name of cathode. The positive conductor is generally terminated by a plate or wire of platinum, or by a plate of the metal which the bath contains in solution, or by any other good conductor of electricity, like the graphite (carbon) of gas-retorts, for instance. This wire, plate, or conducting body is called the anode.

There are two classes of batteries: One class depends for its action upon physical agents solely, the thermo

electric battery for example, and is comparatively feeble. On this account such batteries are rarely employed.

Batteries of the other class derive their action from chemical processes, decompositions, and recompositions, more or less energetic. It is with batteries of this class that we are specially interested.

There are many varieties of these batteries in use today. They have been modified extensively both in their shape and in the nature of their constituents. But we may summarize the subject by stating that the best battery is that which exhibits in the smallest volume the greatest energy, constancy, regularity, and economy.

We proceed, in the following, to describe the more important varieties of batteries which one may purchase, or, in case of necessity, construct for himself.

Daniell's Battery.

This battery develops quite a constant and lasting current; it is, however, wanting in intensity. It is especially adapted to those slow deposits which require to be thick and of uniform texture. A great advantage of this battery is that it will work without acids, and, therefore, without the production of gases or odors. It may be used in a private apartment without inconvenience.

Each cell of this battery is made of an exterior vessel of copper (Fig. 30), which is partially filled with a saturated solution of sulphate of copper (blue vitriol), and within which is placed a diaphragm of sail cloth, or hog's bladder, or, better still, a cup of porous porcelain or earthenware (Fig. 31), which causes the solution of sulphate of copper to rise to within about one inch of the top of the copper vessel. This diaphragm or cup is charged with a saturated solution of chloride of sodium

(common salt), into which is plunged a well-cleansed

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For the proper working of the apparatus, it is necessary that the levels of the two solutions should be practically the same. If there be any difference, the solution of chloride of sodium may be slightly above the other, as little inconvenience will result if a small quantity of salt becomes mixed with the sulphate of copper. On the contrary, if from a higher level the solution of sulphate of copper passes through the diaphragm, or porous cell, the zinc is immediately corroded and blackened, and the battery may cease to work.

When a single Daniell's element is employed, which seldom happens on account of the feeble intensity of the current, the conducting wire which supports the article to be electro-plated is connected with the zinc plate by a binding screw of brass, and the other wire supporting the anode is connected with the exterior vase of copper.

The solution of sulphate of copper must be kept constantly saturated with crystals of this salt, which may be laid upon a perforated shelf or box of copper in the upper portion of the solution, or suspended therein in a bag of linen or hair cloth. A similar process may be employed to keep the solution of common salt in a state of satura

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