chamois leather contain, at the ordinary temperature, 126 per cent of gold; at 0° C. the percentage is 110; at 100° C. 650 per cent. These amalgams therefore behave like aqueous solutions. The residues left after the action of nitric acid on solid or liquid gold amalgams are not homogeneous, which proves that there probably exist definite compounds of gold and mercury dissolved in excess of mercury. A mixture of gold and mercury heated to a temperature a little above the boiling point of mercury, till its weight became constant, left an amalgam containing 10·02 to 10.5 per cent of mercury which corresponds to the formula Au,Hg.

The pasty amalgam of gold and mercury is the base of the various processes for gilding base metals by the old method, known as fire-gilding. For gilding copper, brass, etc., 8 to 9 parts of mercury to 1 part gold are used. The amalgam is prepared by heating the gold, cut up small, in a plumbago crucible to a red heat, then adding one-eighth or one-ninth of its weight of the above-mentioned amalgam, previously heated to boiling. The contents of the crucible are then well stirred with an iron rod, and poured into a large vessel cooled on the outside by water. If the amalgam is allowed to cool in the crucible, or if kept for some time after being poured into the cold vessel, it crystallises. The crystalline amalgam may, however, be restored to working condition by heating in a crucible with excess of mercury.

The articles to receive the deposit of gold are first cleansed by heating and dipping in dilute sulphuric acid, and sometimes in nitric acid; they are then dipped in a solution of nitrate of mercury, by which means they become coated with mercury, they are afterwards pressed upon the pasty amalgam, a portion of which adheres to it. The mercury is then expelled by heat and the gold surface polished. A mixture of bees-wax, red-ochre, copper acetate, and alum is then applied for the removal of the last traces of mercury, and to impart a redder shade to the surface. The articles are then burnished, washed with dilute nitric acid, then with water, and finally dried.

A native amalgam of gold in small yellowish crystals is found in California having the formula Au2Hg, (gold 39′02 to 41.63 per cent, mercury 60.98 to 58.37 per cent). An amalgam of gold and silver is found in small white grains in New Granada containing 38-59 gold, 5 silver, 56.4 mercury per cent.

§ 121. Silver - Amalgam.—Silver and mercury unite slowly at ordinary temperatures, but much more quickly when at a red heat. The affinity of these metals for each other is nearly the same as that of mercury for gold, but with a greater tendency towards crystallisation. The more finely divided the silver, the more rapidly does amalgamation take place. Silver precipitated from its solutions by metallic zinc, then washed and dried, is in a very favourable condition for amalgamation. When such silver is thrown into a hot crucible containing mercury, combination rapidly takes place. By pressing the product in a chamois leather bag, the free mercury runs through and a soft white amalgam is left.

Silver-amalgam can be prepared by adding mercury to a solution of silver nitrate, the amalgam is precipitated in a crystalline form called a silver tree, or Arbor Diana. The silver solution should be somewhat acid, and not too concentrated. Joule gives the formula Ag2Hg as the average composition of this amalgam. When the action is assisted by the aid of a battery, amalgams richer in silver are obtained.

Silver-amalgam varies in character according to its composition and mode of formation, being soft, granular, or crystalline. The soft amalgam obtained by uniting the two metals was formerly used for coating articles with silver in a manner similar to that described for gilding. A native compound, termed amalgam, and having the composition Ag2Hg, or Ag2Hg, is found in a crystallised state, and sometimes massive. Another amalgam, termed arquerite, is found in Chili, having the formula Ag,Hg, or 13.5 silver and 86.5 mercury per cent.

§ 122. Magnesium-Amalgam. This amalgam is slowly formed by contact of mercury with pure magnesium in the cold, but quickly at the boiling point of mercury. In this amalgam the affinities of magnesium are exalted. An amalgam containing 5 per cent magnesium swells up instantly in contact with air, and loses its lustre; it decomposes water readily. Magnesium-amalgam may also be prepared by covering sodium-amalgam with a solution of magnesium sulphate.

§ 123. Sodium-Amalgam.-Sodium combines rapidly with mercury at ordinary temperatures, the combination being attended with a hissing noise and vivid combustion. A piece of sodium forcibly thrown upon mercury is thrown out of the vessel with explosion, in consequence of the great heat produced. It can be prepared by melting sodium under petroleum, and introducing the mercury through a narrow glass tube. A silver-white solid amalgam is formed which must be kept under petroleum to prevent the oxidation of the sodium. Sodium-amalgam is also prepared by triturating the two metals together in a dry mortar, fitted with a cover, until the combustion ceases. With 30 parts mercury to 1 of sodium, it is tolerably hard under the file, and exhibits a crystalline laminar fracture. With 40 parts mercury to 1

of sodium the amalgam is still solid, but softer than the former. With 60 parts mercury to 1 of sodium it forms a stiff paste at 21° C.; 100 parts mercury to 1 of sodium produces a viscous mass; and 128 parts mercury to 1 of sodium is liquid.

Sodium-amalgam is used in the preparation of other amalgams. Metallic chlorides, such as those of silver and gold, for example, are decomposed by sodium-amalgam, and the reduced metal then unites with the mercury. Metals which do not readily unite directly with mercury may be amalgamated by the action of sodium-amalgam on certain solutions of their salts; thus: iron-amalgam is obtained by

1 Chem. Soc. J. (2) vol. iv. p. 141.

immersing sodium-amalgam, containing 1 per cent of sodium in a clear saturated solution of ferrous sulphate.

Sodium-amalgam is used for the extraction of gold and silver from their ores instead of mercury. It is said to facilitate the amalgamation, and to prevent flouring of the mercury; i.e. it prevents the formation of oxide, sulphide, arsenide, etc., which would form a coat on the mercury and prevent contact with the gold or silver.

§ 124. Potassium-Amalgam.-Potassium unites with mercury with great violence at ordinary temperatures, and forms amalgams similar in properties to those of sodium.

CHAPTER IX

GOLD ALLOYS

§ 125. The properties which make gold so valuable in the Arts have been already mentioned in the introductory part of this work, and it has been shown that this metal is improved for manufacturing purposes by alloying with certain other metals, such as silver and copper, in small quantity. The great value of gold is the reason, in most cases, why the proportion of added metal should be small. Many metals not only reduce the value of gold, but impart qualities which destroy its high malleability and ductility, while others render the gold unfit for any useful purpose. With a few exceptions, it may be stated that the higher the value of a metal, the greater should be the proportion of this metal in the alloy.

Taking into consideration its high intrinsic value, gold may be considered the most perfect of all the metals, but it is necessary to impart to it a greater degree of hardness than it alone possesses, for the manufacture of coins, medals, jewellery, etc. Silver and copper are the metals invariably used for this purpose, and, when added in small quantity, do not materially alter the malleability and other working properties of gold, but increase its fusibility. The above remarks are only true when the silver and copper employed for alloying are practically pure, as a small quantity of lead, arsenic, antimony, etc., in the copper would considerably reduce the malleability and ductility of the alloy.

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