SILVER. —Estimation.

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goes on charging in the same way four or six leavesat a time, till he has applied, one over another,thirty, forty, fifty, or sixty leaves, according to thedesired solidity of the silvering. He then burnishesdown with great pressure and address, till he hasgiven the surface a uniform silvery aspect.

Silvering by the aid of heat may be performed invarious ways, of which the following are the mostimportant:—The surface, previously cleaned withcare, and dipped in a solution of nitrate of mercury,is covered with a uniform layer of silver amalgam,and subsequently heated, first gently, and whenthe layer is found to be uniform, the heat is raisedto expel the mercury, and the surface is subsequentlyburnished.

A mixture consisting of 1 part of silver in a finestate of subdivision, 4 of sal-ammoniac, 4 of commonsalt, and one-fourth part of corrosive sublimate, ismade and formed into a paste, with a sufficientquantity of water; and when the surface of thearticle has been coated with this mixture, it isheated to the fusing point of the mixture of salt.

Another method consists in rubbing the surfacewith a mixture of equal parts of chloride of silver,silver in a fine state of subdivision, and calcinedborax, heating to a dull redness, and then introducingthe article into a solution of common salt and creamof tartar; then covering the surface with a paste ofequal parts of sal-ammoniac, common salt, andsulphate of zinc, again heating and dipping into thesame liquid. This process is repeated until a surfaceof the requisite thickness is obtained.

Silvering without the aid of heat, and by Immersion .—The following are a few of the preparations andsolutions which have been recommended by variousauthors as suitable for depositing silver without thenecessity of raising the temperature of the body—apractice which would in many cases destroy thearticle by fusing the soldered joints :—

The old method of obtaining a thin coat of silverconsisted in employing a paste formed by mixingwater with equal parts of chalk, cream of tartar, andchloride of silver; or with 1 part of freshly precipi-tated chloride of silver, 5 of common salt, and 5 ofcream of tartar.

A second process, recommended by Roseleur andLanaux, is to dissolve 6 parts of sulphite of sodaand 1 part of a salt of silver in 6 parts of water, theobject being warmed after rubbing with this mixture.

Another method—frequently adopted for platingsmall brass or copper articles, such as the scales ofthermometers—consists in rubbing them, after beingproperly cleaned, with a mixture of 100 parts ofcream of tartar, 10 of chloride of silver, and 1 ofcorrosive sublimate: the surface should then bepolished.

If a wet rag be dipped in a powder consisting of2 parts of cyanide of potassium, 1 of nitrate of silver,and 5 of chalk, and rubbed on the surface of anobject, it is, according to Boudier, covered with afirmly adhering layer of silver.

Silvering of Glass, &c.— Drayton proposed in 1843a method of coating glass surfaces, such as mirrors,

&c., by the reduction of an ammoniacal solution ofnitrate of silver by one of the essential oils.

The process, as modified by Petitjean, is carriedout as follows:—The glass to be silvered havingbeen laid on a horizontal cast-iron table, is wellcleaned, and solutions of the silver salt and tartaricacid are poured over it. In twenty minutes thedeposition commences, and the whole process iscompleted in an hour and a quarter. The liquid isnow poured off, and the glass washed, dried, andcovered with a coat of varnish. The deposition bythis means has the advantage of being a perfectlyhealthy occupation, and costing less than whenmercury is employed; but the method, nevertheless,has certain objections, the principal of which is ayellowish tint on the glass; but all of them areeffectually avoided by the modification introducedby Lenoir, which consists in sprinkling the surfaceafter washing with a dilute solution of the doublecyanide of mercury and potassium. The silver ispartially dissolved, while an amalgam is instantan-eously produced, both more adhesive and muchwhiter than pure silver. The solar rays, moreover,have less effect upon it than before such a treatment,neither are sulphur vapours so liable to attack it.

Estimation of Silver. — The behaviour of variousreagents with solutions of silver is given in a tabularform in the article on Gold, to which the reader isreferred. In general, the soluble salts of silver are re-cognized by furnishing a white precipitate with hydro-chloric acid and the soluble chlorides, which blackensby exposure to light, and which is readily soluble inammonia; and by affording metallic silver uponthe immersion of a plate of copper. The salts in-soluble in water are mostly soluble in ammonia; andwhen heated with carbonate of soda on charcoalbefore the blow-pipe they afford a globule of silver.A yellow precipitate on the addition of phosphateof soda to a neutral solution, a red-brown witharseniates, a crimson with the chromates, and whitewith ferrocyanide of potassium, are further character-istics of the soluble salts of silver.

Tin and lead are the most rapid precipitants ofmetallic silver from the nitrate; cadmium, zinc,copper, bismuth, and antimony are more slow intheir operation, and mercury still more tardy.Chloride of silver is rapidly reduced by most of themetals which form soluble chlorides, such as zinc,iron, cadmium, cobalt, and arsenic. Zinc, copper,and arsenic rapidly reduce the ammoniacal solutionof oxide of silver. Of all the metallic precipitantszinc and cadmium are the most effective ; when zincor antimony is used, the separated silver containsthe precipitating metals.

The solubility of silver in nitric acid, and thecomplete insolubility of its chloride in acid solutions,facilitate the separation of silver from other metals.When its amount is to be estimated quantita-tively, the alloy or pulverized ore is dissolvedin nitric acid, and the silver is precipitated as achloride by adding a slight excess of hydrochloricacid. In order that a dense precipitate may beobtained, the solution should be heated nearly to