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slowly, for the reason that the layer of water in contact with them soon becomes saturated, and, being denser than the liquid above, does not mix with it, but forms a saturated solution surrounding the crystals. In this case it is, therefore, necessary to agitate the liquid frequently with a glass or wooden rod, in order to mix the dense and saturated layers with the lighter ones which have dissolved little or no sulphate. But it is preferable to operate in a different manner; thus, the crystals of sulphate of copper may be placed in a perforated ladle of copper or stoneware, or in a bag of cloth, suspended near the surface of the liquid. With this arrangement, as soon as a portion of the liquid has become saturated with sulphate of copper, it becomes denser and flows downward, while a lighter layer takes its place, and becomes saturated in its turn. When the liquid refuses to dissolve any more of the crystals, it is said to be saturated, and marks about 25° of Baumé's hydrometer for liquids heavier than water.

Water, without the addition of sulphuric acid, will dissolve the sulphate of copper equally well, and may be used to form a galvanoplastic bath. But, if the water is not distilled, and contains, as is nearly always the case where ordinary water is used, bicarbonate of calcium, the solution is rendered turbid by the precipitation of carbonate of copper.

Baths of water and sulphate of copper only are rarely employed, because they are poor conductors of electricity, and because the deposited copper is irregular, granular, and brittle. It is possible to replace the sulphuric acid by other acids or salts which increase the specific gravity of the liquid, and increase its electric conductivity; such are acetic, tartaric, citric, etc., acids, or the bisulphates of potassium and sodium, the alums, and sul

phate of aluminium. But, in the arts, sulphuric acid is nearly always employed.

To sum up, a galvanoplastic bath consists of water, more or less acidulated with sulphuric acid, and saturated with sulphate of copper at the ordinary temperature.

Fig. 132.

Baths of sulphate of copper, while working, must always be kept saturated; that is to say, new supplies of sulphate of copper must be furnished to the bath to replace that decomposed to form the metallic deposit. This result is simply accomplished by suspending to the top of the vessel, so as to dip in the upper portion of the liquid, bags of cloth or hair, or perforated baskets of gutta peralways provided with crystals of

cha (Fig. 132), kept sulphate of copper.

One cannot be too particular about the choice of the sulphate of copper, of which three very distinct varieties are found in the market. The first, which is the best, is produced from the treatment of copper, or its oxide, with sulphuric acid, more or less concentrated. This sulphate comes in the form of rhomboidal crystals, semitransparent, and of a fine blue color. Its solution is also a pure blue.

The second variety is produced from the spontaneous oxidization of natural copper sulphides, or pyrites. As these ores contain a large percentage of arsenic and other metals, the resulting sulphates are very impure, and often exhibit veins of white and green.

The third is the product resulting from the treatment by sulphuric acid of the dipping liquors employed for cleansing copper and its alloys. These sulphates are mixed with those of zinc and of other metals entering into

the composition of bronzes, brasses, or other alloys. Besides they still retain a certain proportion of nitric acid, which is injurious to the operation.

We shall give in the third part of this work (CHEMICAL PRODUCTS), the best and readiest methods for ascertaining the purity of the sulphate of

copper.

The galvanoplastic baths are always employed in the cold, and are kept in vessels of various shapes, according to the wants of the operator. Stoneware, porcelain, and glass are the best materials for the purpose; but as it is quite difficult to find in the trade such vessels sufficiently large, it is also customary to employ wooden troughs, vats, covered inside with a layer of gutta percha, marine glue, various resinous substances, or with sheet lead, which may be coated with a layer of varnish.

Zinc, iron, and tin must, manifestly, be excluded from the bath on account of the chemical reactions taking place between these metals and the sulphate of copper.

The working of galvanoplastic baths remains satisfactory, when the specific gravity is retained between 20° and 25° Baumé.

CHAPTER LI.

MODES OF OPERATING-DEPOSITS BY SEPARATE CURRENTSIMPLE APPARATUS-THE SAME FOR THE AMATEUR".

LARGE PLANT.

Modes of Operating.

66

IT has been stated that copper can be reduced by two distinct methods, viz: by the separate current process, employing either the voltaic battery or the dynamo

machine, or by a modified procedure in which the object to be covered constitutes the positive pole. We shall describe these two processes.

Separate Current Process.

The solution of sulphate of copper having been prepared and disposed as above described, the object which is to receive the deposit is connected (Fig. 133) with

Fig. 133.

the conducting wire attached to the negative pole of the battery or dynamo, and immersed in the solution. On the other hand, to the conducting wire leading from the positive pole (carbon or copper) is attached a sheet or plate of copper, and this anode is placed in the liquid parallel to the object connected with the other pole. This plate should have a surface at least equal to that of the article to be covered. The deposit begins immediately, and its progress may be observed by removing the object now and then from the solution.

If we operate upon a clean metallic surface, the deposit of copper will be formed instantaneously on every part of it; on the contrary, if the surface is only a mediocre conductor of electricity (as plumbago or graphite), the deposit begins at the points nearest by the conducting wire, and from which it spreads over the surface from one

[merged small][graphic]

molecule to the other.

Thus, for instance, a medal of

wax covered with plumbago (Fig. 134), and joined at the rim alone with the conducting wire, will receive a

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