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296.]

ELECTRO-GILDING AND PLATINIZING.

605 or larger surface to the action of the acid. The superficial area, and number of the plates used, are made to vary according to the size and nature of the objects to be operated upon. The workman judges from experience as to the number of pairs to be employed; it seldom happens that more than two or three pairs of plates are needed. In Paris, Bunsen's carbon and zinc batteries are also employed with success in these operations. Magneto-electric machines (315) are now frequently used instead of batteries.

(296) Electro-Gilding and Platinizing. It is possible to gild most of the ordinary metals by voltaic action. Articles which consist of brass, bronze, copper, or German silver are first annealed, then pickled, as the operation of immersing them into the mixture of diluted nitric and sulphuric acids is termed, after which they are scrubbed and dipped' in strong nitric acid, and then rinsed in water, as is practised in preparing them for plating. Silver articles are cleansed in a similar manner, but they do not require to be 'dipped.' Iron and steel may be gilt by cleansing them from grease, first with potash, and then by dipping in nitric acid, and scouring the surface with burnt clay finely sifted, in order to remove the black stains produced by the liberation of carbon. A more powerful current is required for gilding upon iron than upon the metals previously mentioned.

The gilding bath most usually employed consists of aurous cyanide dissolved in potassic cyanide. It may be prepared by dissolving gold in aqua regia, and adding potassic cyanide to the diluted liquid so long as it produces a precipitate; a brisk effervescence accompanies the action, and a yellow deposit of aurous cyanide (AuCy) is formed: the clear liquid is decanted, and the precipitate is redissolved in a solution containing between 7 and 8 parts of potassic cyanide to 1 part of gold: the solution is then diluted until 100 parts of the liquid contain 1 part of gold.

M. Ruolz has shown that various other gilding baths may be used instead of the auro-potassic cyanide: for example, he finds that aurous cyanide may be employed when brought into solution by the ferrocyanide or by the ferricyanide of potassium; he has also used with success the aurous sodic sulphite, the solution of the sodic-auric chloride (NaCl, AuCl) or iodide with an excess of soda, and even the sulphide of gold dissolved in a solution of protosulphide of potassium.

As yet the deposition of platinum by voltaic action has not been practised to any considerable extent, but it is said that a solution of the potassic-platinic chloride (2 KCl, PtCl) in caustic potash may be applied to this purpose with tolerable success.

(297) Resemblances between the Electricity of the Machine and that of the Voltaic Battery.-Notwithstanding the extremely brief duration of the discharge from the electrical machine, it produces, whilst it lasts, phenomena similar to those of the voltaic current, which, indeed, may be regarded as a succession of discharges renewed so frequently as to become continuous. By repeating the discharge from the electrical machine many times through the same liquid conductor, Faraday was enabled to obtain true electrolytic decomposition. The following simple experiment may be adduced as an illustration of this fact:-Upon a plate of glass place a small piece of turmeric paper, moistened with a solution of potassic iodide which has been mixed with a little starch; upon one end of this piece of paper allow the point of a fine platinum wire to rest, the other end of the wire being in communication with the prime conductor of the machine; on the other extremity of the paper place a similar wire in communication with the earth: it will be found on setting the machine in action that, after the lapse of one or two minutes, a small blue spot will appear round the point of the wire connected with the prime conductor, owing to the liberation of iodine; while round the wire which communicates with the earth a brown spot will be formed, from the action of the alkali which is set free. If the wires, instead of being connected through the medium of a solution of potassic iodide, be made to dip into a drop of a solution of cupric sulphate, metallic copper will be deposited on the wire connected with the earth, and oxygen and sulphuric acid will appear on the other wire. If a piece of litmus or turmeric paper, moistened with a solution of sodic sulphate, be supported on a thread of glass between two wires, one of which proceeds from the prime conductor, whilst the other is in communication with the earth, the saline solution in the paper will be decomposed by the electricity, even if the paper does not touch either of the wires the litmus paper on the side towards the prime conductor will gradually be reddened, whilst the turmeric paper will be turned brown at the extremity which is furthest from the prime conductor.

The quantity of electricity which is required to produce chemical decomposition is very great. This fact is strikingly illustrated by a comparison which was made by Faraday between the amount of electricity developed from the machine by friction and that which is furnished by the chemical action of the battery. The experiment was performed in the following manner :-A wire of platinum and another wire of zinc, each of an inch in

298.]

DELUC'S DRY PILE.

607

diameter, were immersed of an inch apart, to a depth of of an inch in an extremely dilute acid liquid, prepared by adding a single drop of oil of vitriol to four ounces of water. The current obtained from this combination, at a temperature of 15° C., was passed through the coil of a galvanometer consisting of 18 feet of copper wire of an inch thick. It produced in about three seconds as great a deviation of the needle as was obtained by the electricity furnished by thirty turns of a powerful platemachine in excellent action. This quantity, if concentrated within a space of time constituting only a minute fraction of a second, by discharging it in a single flash from a Leyden battery, exposing 3500 square inches of coated surface, would have been sufficient to kill a small animal, such as a cat or a rat; but the chemical action upon the zinc by which it was produced was so trifling as to be quite inappreciable; and it is estimated by Faraday that not less than 800,coo discharges, each equal in quantity to this, would be required for the decomposition of a single grain of water! Extraordinary as this estimate appears, it has been amply confirmed by later experiments of Becquerel upon this subject: and from the experiments of Weber, it may be calculated that, if the whole of the positive electricity required to decompose a grain of water were accumulated upon a cloud 1000 metres (3281 feet) above the surface of the earth, the attractive force exerted between the cloud and the portion of the earth beneath it would be equal to 1497 tons!

A recent experiment made by Messrs. Warren De La Rue and Hugo W. Müller, of which they kindly permit a description to appear here in anticipation of a paper to be communicated to the Royal Society, illustrates very strikingly the powerful heating effects obtainable from a quantity of electricity sufficient to decompose only a very small quantity of water. A condenser of 448 microfarad capacity was charged by 3000 cells of their battery, the current flowing through a voltameter in which only 50% of a grain of water was decomposed by the electricity which accumulated in the condenser; this charge was, however, capable of deflagrating a piece of platinum wire 8 inches long and of an inch in thickness.

(298) Deluc's Dry Pile.-The relation between the electricity of the voltaic battery and that of the ordinary electrical machine admits of being traced in an interesting manner by intermediate steps. Deluc, soon after the discovery of the voltaic pile, contrived what he termed the dry pile. It may be constructed in the following manner :-Take a number of sheets of paper, one

surface of which has been coated with gold or silver leaf, and paste upon the uncoated surface a sheet of zinc foil; when sufficiently dry, place several of these sheets of paper one over another, the zinc faces all being arranged in one direction; then cut out, with a punch, a number of circular disks, and arrange them, to the number of 2000 or upwards, in a glass tube, the diameter of which is rather greater than that of the circular disks of paper, taking care that all the zine surfaces are in one direction, and all the silvered or gilt surfaces in the opposite direction. A pile analogous to Volta's will thus be obtained; and if these disks be pressed together and connected at each end with a metallic wire, such a pile will cause divergence of the leaves of the gold-leaf electroscope when one extremity of it is made to touch the cap of the instrument, whilst the other end is connected with the earth, either through the human body or by means of any other conductor. If the pile be reversed, and then presented to the still diverging electroscope, the leaves will first collapse, and will then immediately open with the opposite kind of electricity. Indeed, if the wires attached to the two extremities of the pile be bent round and made each to terminate in a small metallic disk, the two disks being placed at a distance of about an inch and a half (3 or 4 centimetres) from each other, care being taken to maintain their insulation, an insulated slip of gold-leaf, suspended midway between the two disks will oscillate backwards and forwards between them, if an impulse be first given to it towards either side:-suppose it to approach the positive plate, it acquires a positive charge; it is then repelled from the positive plate, but is attracted by the negative plate, when it gives up its positive charge and becomes negatively electrified, in which state it is again atractted by the positive plate; this alternate movement of the gold leaf will continue uninterruptedly for months or even years. With a dry pile, which contained 20,000 pairs, or disks, of zinc and silver paper, sparks have been obtained, and a Leyden battery has been charged sufficiently to produce shocks. It is worthy of remark that these actions are produced in Deluc's column only when the paper contains that amount of moisture

*

*Bohnenberger has contrived an extremely sensitive electroscope, which depends upon a modification of this experiment. Midway between the two insulated terminating disks of Deluc's pile, he suspends a single strip of gold-leaf by a metallic wire from an insulated plate of metal; this gold-leaf, however, is not near enough to either disk to touch it. If a body with the feeblest electrical charge be made to touch the insulated plate, the gold-leaf becomes electric, and is attracted towards the oppositely electrified pole of the pile.

299.]

WATER BATTERY.

609

which is found in it under ordinary circumstances, and which is considerable, although it usually passes unnoticed. If the paper be artificially dried, the pile loses its activity, but again recovers its energy as the paper reabsorbs moisture from the air. Provided that the two extremities of the pile be insulated from each other, it will retain its activity unimpaired for years; but if the ends be permanently connected by means of a good conductor, the zinc becomes gradually oxidized, and the electrical effects disappear.

leaf.

Zamboni obtains a more effective instrument by substituting finely-powdered black oxide of manganese for the gold or silver One surface of the paper is coated with zinc or tin-foil, and the coating of peroxide may be given to the other surface either by rubbing it on in a dry state, or by applying it in admixture with water to which a little honey has been added. The paper disks are arranged in a column, and are terminated at either extremity by a metallic plate. These metallic plates are made to compress the paper disks by means of ligatures of silk which pass from end to end of the pile and bind the disks firmly together whilst effectual insulation is provided for by giving the pile a non-conducting coat of sulphur, which is easily applied by a momentary immersion of the whole instrument in a bath of melted sulphur.

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(299) Water Battery.—It has been already stated (259) that even with a single pair of zinc and copper plates excited by dilute acid, polarization and electric tension may be proved to precede the voltaic current, although the experiment is one of considerable delicacy. These effects of tension are strikingly exhibited in the case of Deluc's pile; but they may be shown in a manner still more decided by employing a numerous series of alternations of zinc and copper, each of which need expose only a very small surface, and may be excited simply with distilled water. Such an arrangement or water battery, consisting of a thousand couples, produces, if insulated, and connected at each of its extremities with a gold-leaf electroscope, considerable divergence of the leaves of each instrument. Such a combination will communicate a charge to a Leyden battery: this charge, although it rises only to a small extent, may be renewed and discharged for an indefinite number of times in very rapid succession. The wire which is connected with the last zinc plate of this battery is negative, whilst that which is attached to the copper is positive.

Gassiot (Phil. Trans. 1844, 39) has given an account of a very powerful and carefully constructed water battery, from which

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