COPPER, .SILVER, AtfD GOLD 447 



for the quartation. The amount of pure gold (assay) is determined by 

 weighing the gold which remains after this treatment. English gold 

 (= 22 carats) coinage is composed of an alloy containing 91-66 p.c. of 

 gold, but for many articles gold is frequently used containing a larger 

 lamount of foreign metals. 



Pure gold may be obtained from gold alloys by dissolving in aqua 

 regia, and then adding ferrous sulphate to the solution or heating it 

 with a solution of oxalic acid. These deoxidising agents reduce the 

 gold, but not the other metals. The chlorine combined with the gold 

 then acts like free chlorine. The gold, thus reduced, is precipitated as 

 ftn exceedingly fine brown powder. 31 bis It is then washed with water, 

 and fused with nitre or borax. Pure gold reflects a yellow light, and 

 in the form of very thin sheets (gold leaf), into which it can be 

 hammered and rolled, 31 trl it transmits a bluish-green light. The 

 specific gravity of gold is about 19'5, the sp. gr. of gold coin is about 

 17'1. It fuses at 1090 at a higher temperature than silver and can 

 be drawn into exceedingly fine wires or hammered into thin sheets. 

 With its softness and ductility, gold is distinguished for its tenacity, 

 and a gold wire two millimetres thick breaks only under a load of 68 

 kilograms. Gold vaporises even at a furnace heat, and imparts a 

 greenish colour to a flame passing over it in a furnace. Gold , alloys 

 with copper almost without changing its volume. 32 In its chemical 



si t>is Schottlander (1893) obtained gold in a soluble colloid form (the solution is violet) 

 by the action of a mixture of solutions of cerium acetate .and NaHO upon a solution of' 

 AuClj. The gold separates out from such a solution in exactly~the same manner as Ag- 

 does from the solution of colloid, silver mentioned above. There always remains 

 certain amount of a higher oxide of cerium, CeO 2 , in the. solution i.e. the gold is 

 reduced by converting -the cerium into a higher grade 'of oxidation. Besides ., which 

 Kriiss and Hofmann show'ed that sulphide of gold precipitated by the -action of H 2 S upon 

 a solution of AuKCyg mixed with HC1 easily passea into a colloid solution after being 

 properly washed (like As 2 S 3 , CuS, &c., Chapter I., Note 57). 



31 trl Gold-leaf is used for gilding wood (leather, cardboard, and suchlike, upon which 

 it is glued by means of varnish,, &c.), and is about O'OOS millimetre thick. It is obtained 

 from thin sheets (weighing at first about grm. to a square inch), rolled between gold 

 rollers, by gradually hammering them (in packets of a number at once) between sheets 

 of moist (but not wet) parchment, and then, after cutting them into four pieces, between 

 a specially prepared membrane, which, when at the right degree of moisture, does not 

 tear or stick together under the blows of the hammer. 



32 The. formation of the alloys Cu + Zn, Cu + Sn, Cu + Bi, Cu + Sb, Pb-f Sb, Ag + Pb, 

 Ag+.S3, "A'u'4-Zn, Au + Sn, &c., is accompanied by a contraction (and evolution of heat). 

 The formation of the alloys Fe + Sb, Fe + Pb, Cu + Pb, Pb + Sn, Pb+Sn, Pb + Sb, 

 Zn + Sb, Ag + Cu, Au + Cu, 'Au + Pb, takes place with a certain increase in volume. 

 With regard to the alloys of gold,' it may be mentioned that gold is only slightly 

 dissolved by mercury (about 0'06 p.c., Dudley, 1890) j the remaining portion forms a 

 granular alloy, whose composition has not been definitely determined. Aluminium (and 

 silicon) also have the capacity of forming alloys with gold. The presence of a small 

 amount of aluminium lowers the melting point of gold considerably (Roberts- Austen, 

 1692) ; thus the addition of 4 p.c. of aluminium lowers it by U 0> 28, the addition of 10 P.O. 



