500 



GOLD. 



gravity is 19*3. In hardness it is above lead and (in, 

 but inferior to iron, copper, platina, and silver. Its 

 lustre does not equal that of steel, platina, or silver, but 

 it surpasses the other metals in this respect. It may be 

 exposed for any length of time to the atmosphere, 

 without suffering the least cliange. It is also equally 

 unalterable in the common fire ; but on being exposed 

 to powerful burning mirrors, or to the heat of the 

 oxy-hydrogen blowpipe, it melts, and even rises in 

 vapour. Gold is not oxidized or dissolved by any of 

 t lie pure acids. Its only solvents are chlorine and 

 nitro- muriatic acid ; and, according to Sir H. Davy, 

 tlte chlorine is the agent in both cases, since the 

 jii m-muriatic acid does not dissolve gold, except 

 when it gives rise to the formation of chlorine. It is 

 to be interred, therefore, that the chlorine unites 

 directly with the gold, and that the compound formed 

 is a chloride of gold. There is no inconvenience, 

 however, in regarding it as a muriate ; since reagents 

 act upon it as if it were such. The gold is precipi- 

 tated from its solvent by a great number of substances. 

 Lime and magnesia precipitate it in the form of a 

 yellowish powder. Alkalies exhibit the same ap- 

 pearance ; but an excess of alkali redissolves the pre- 

 cipitate. The precipitate of gold obtained by a fixed 

 alkali, appears to be a true oxide, and is soluble in 

 the sulphuric, nitric and muriatic acids ; from which, 

 however, it separates by standing. Gallic acid 

 precipitates gold of a reddish colour, and, very 

 soluble in nitric acid, to which it communicates a fine 

 blue colour. Ammonia precipitates the solution of 

 gold much more readily then fixed alkalies. This 

 precipitate, which is of a yellowish brown colour, 

 possesses the property of detonating with a very 

 considerable noise, when greatly heated. It is 

 known by the name of fulminating gold. Most 

 metallic substances precipitate gold from its solution 

 in nitre-muriatic acid. Lead, iron and, silver pre- 

 cipitate it of a deep and dull purple colour ; copper 

 and iron throw it down in its metallic state. A plate 

 of tin, immersed in a solution of gold, affords a purple 

 powder, called the purple powder of Cassius, which 

 is used to paint in enamel. Ether, naphtha, and 

 essential oils take gold from its solvent, and form 

 liquors, which have been called potable gold. The 

 gold which is precipitated on the evaporation of 

 these fluids, or by the addition of sulphate of iron to 

 the solution of gold, is of the utmost purity. 



The principal use of gold, as is well known, is in 

 coinage. It has been with mankind, from time 

 immemorial, the representative sign of every spe- 

 cies of property. Even before the art of coining 

 was invented, it passed for money in the condition in 

 which it was found in the earth ; and in this form it 

 still enjoys a currency in many parts of Africa. It is 

 rarely employed in a state of perfect purity, but is 

 almost universally alloyed with copper, or with 

 silver, in order to increase its hardness. The alloy 

 of gold and silver is found already formed in nature, 

 and is that most generally known. It is distinguish- 

 able from that of copper, by possessing a paler yel- 

 low than pure gold, while the copper alloy has a 

 colour bordering upon reddish yellow. A variety of 

 means are employed to judge of the quality of alloys, 

 supposed to consist in part, or principally, of gold, 

 without resorting to a regular analysis. The most 

 common of these consists in the use of the touchstone 

 (for the nature of this substance, see Quartz). A 

 mark is made upon the stone with the alloy, upon 

 which a drop of nitric acid is placed by means of a 

 feather ; if the metallic streak disappears, the alloy 

 is destitute of gold; if visible only in little points, at 

 distant intervals, it indicates a small proportion of 

 this metal ; whereas, if the continuity and density of 

 tlie mark remain unbroken, it evinces that the piece 



on trial is pure gold. This test is obviously founded 

 upon the property possessed by gold of being insolu- 

 ble in nitric acid, while silver, copper, and their 

 alloys, with zinc, are instantly taken up by this sol- 

 vent. It requires, however, much practice to deter- 

 mine, with any considerable degree of precision, the 

 amount of gold present in alloys by means of this 

 test. The trial of specific gravity is another mode of 

 ascertaining the proportion of gold in alloys ; and it 

 was in this manner that Archimedes detected the 

 amount of silver in a crown which was to have been 

 made of pure gold for Hiero, king of Syracuse. But 

 this method only gives approximations, since certain 

 alloys are more, and others less dense, than the mean 

 density of the metals which compose them. In the 

 coining of gold, where it is necessary to be assured 

 of the purity of the metal, the trials just mentioned 

 are never adopted. If the gold to be made use of 

 appears to contain copper (which is inferred from its 

 reddish tinge), it is made to undergo cupellation 

 with a given quantity of pure lead ; by which means 

 the copper quits its union with the gold, and unites 

 with the lead, leaving the former by itself, and, in 

 this way, the proportion of gold in the alloy is ascer- 

 tained. If silver is presumed to be the alloying 

 metal, the operation consists in melting the alloy 

 with three times its weight of silver, rolling the com- 

 pound into thin sheets, forming these into coils, and 

 plunging them into nitric acid, slightly diluted : the 

 silver is promptly dissolved, while the gold remains 

 unaffected. This operation is called quartation, and 

 the separation of the silver by nitric acid, parting. 



The art of gilding metals (see Gilding) depends 

 upon the double property which mercury possesses, 

 or amalgamating with gold, and of becoming volatile 

 by heat, and thus quitting the gold, which adheres 

 strongly to the metal upon which the mercurial 

 amalgam has been spread. The composition of the 

 amalgam generally nised, is eight parts of mercury to 

 one of gold. The malleability and extreme divisibi- 

 lity of gold are the foundation of the art of gold-beat- 

 ing; and these two properties are so remarkable in 

 this art, that natural philosophers are in the habit of 

 quoting the results it furnishes, as examples of the 

 divisibility of matter. Boyle has observed that a 

 grain of gold, reduced to leaves, will cover a surface 

 of fifty square inches ; that each one of these square 

 inches may be divided into 46,656 other little squares, 

 and that, of course, the entire amount of surface de- 

 rived from one grain of gold is capable of being 

 divided into 2,322,800 parts, each of which is visible 

 to the naked eye. In consequence of the wonderful 

 extension which the gold-beater is enabled to give to 

 this precious metal, it is employed for ornamental 

 purposes to an extent which, from its comparative 

 scarcity, would otherwise be impossible. Thus it is 

 estimated, that an equestrian statue, of the natural 

 size, may be gilded with a piece of gold not exceed- 

 ing in value ten shillings. The gilding of the dome 

 of the Hotel des Invalids at Paris, cost above 4,000. 



The following is a short account of the ingenious 

 art of gold-beating. The gold used is as pure as 

 possible, and the operation is commenced with 

 masses weighing about two ounces. These are 

 beaten into plates six or eight inches long, by three- 

 fourths of an inch wide. They are then passed be- 

 tween steel rollers, till they become long ribands as 

 thin as paper. Each one of these is now cut into 

 150 pieces, each of which is forged on an anvil, till 

 it is about an inch square, after which they are well 

 annealed. Each of the squares in this state weighs 

 6,* grs., and in thickness is equal to ,',, of an inch. 

 The 150 plates of gold, thus produced from one 

 mass, are interlaid with pieces of very fine vellum, 

 about four inches square, and about twenty vellum 



