Copy 1, Volume 1

Hand-book of chemistry / Translated by Henry Watts.

  • Gmelin, Leopold, 1788-1853
Date:
1848-1872
Catalogue details

Licence: Public Domain Mark

Credit: Hand-book of chemistry / Translated by Henry Watts. Source: Wellcome Collection.

    521/562 (page 497)
    &c, The liquid in a bundle of cotton fibres is more strongly heated than that contained in a glass tube of equal width and length; for the cells in which the liquid isjenclosed retard the transmission of the electricity. If the polar wires are inserted into the extremities of a cutting of a water- plant, the water near the wires rises to the boiling point. (De la Rive.) [Experiments of Prescott Joule (Phil. Mag. J. 19, 260). ] TECHNICAL APPLICATIONS OF GALVANISM. 1. Galvanic Precipitation of a thin Layer of one metal on the surface of another. Gilding. This process may be performed upon silver, brass, or copper, but not upon iron. The apparatus consists of a bladder containing dilute acid, in which zine is immersed, and a jar within which the bladder is placed. The jar contains the solution of gold, together with the metal to be gilt, which is connected by a wire with the zinc. (The gold solution and the metal to be gilt may also be placed within the bladder,—the dilute acid and a cylindrical zine plate surrounding the bladder being placed in the outer vessel.)—The more dilute the acid, the feebler is the current, and the better does the gilding go on;—e. g., six drops of acid to a glass of water. Sulphuric acid is used for silver, nitric acid with copper or brass. —The gold solution, which is made as neutral as possible, contains 5 mil- ligrammes of gold in a cubic centimetre, and therefore one gramme of gold in a litre (about 2 pounds). A weaker solution gives a darker, and a solution containing copper mixed with the gold a redder gilding —The metal to be gilt must be either polished or merely cleaned. In the for- mer case, the metal takes the gilding more readily, and the gilt surface has a much greater lustre, and merely requires rubbing with fine linen or with leather to give it a very high degree of polish; in the latter case, the gilding is taken slowly, has a duller surface, and requires to be rubbed with the burnishing steel. Ignited silver takes a finer gilding than that which has not been ignited. The zine is attached to a thick copper wire, and this to a silver or platinum wire, which touches at one point the metal to be gilt: this point must however be changed from time to time, otherwise no gold will be deposited upon it. Before the gilding process is commenced, the metal is dipped into dilute acid to free it from all impurities,—silver in sulphuric, copper and brass in nitric acid. If the zine, contained in a bladder filled with the same acid, be at the same time immersed in the liquid, the gas-bubbles evolved on the surface of the silver or copper will serve to cleanse it still more effectually. After this the gilding is commenced. The bladder with the zine being first placed in the gold solution, the circuit is closed by immersing the object previously metallically connected with the zinc. The metal to be gilt, especially if it be silver, must not be left for a moment in the old solution without galvanic connection,—otherwise it will either not be gilt at all or the gilding will be very bad. If therefore the inside of a vessel is to be gilt, the bladder with the acid and zinc being suspended within it, the gold solution must be poured into the vessel down the sides of the bladder, so that galvanic connection may be immediately formed. The galvanic current must be so weak that scarcely any gas VOL. I, 2K