6*4 GOLD. 



standard silver of England is an alloy of 222 pennyweights of 

 silver with 18 pennyweights of copper. When the proportion 

 of copper is considerable, the alloy becomes red by wear, 

 shewing that the silver of the alloy yields more readily to 

 attrition than the copper. This effect is very visible in the 

 smaller silver pieces of some continental states. 



SECTION III. 



GOLD. 

 Eg. 1243 or 99.6; Au (aurum). 



Gold is found in small quantity in most countries, sometimes 

 in iron pyrites, but generally native, massive and disseminated in 

 threads through a rock, or in grains among the sand of rivers. 

 It occurs crystallized in the cube and its allied forms. At 

 present the principal supply of this metal is from the mines of 

 South America, Hungary and of the Uralian mountains in 

 Siberia. It is generally separated from earthy and all other 

 metallic substances, except silver, by amalgamation. It may be 

 separated from silver by nitric acid, which dissolves the latter 

 metal, but only when it forms a large proportion of the alloy. 

 When nitric acid does not dissolve the silver, the alloy is sub- 

 mitted to an operation termed quart ation ; it is fused with four 

 times its weight of silver, after which the whole silver may be 

 dissolved out by nitric acid. 



To obtain gold in a state of purity, the alloy containing it is 

 dissolved in a mixture of two measures of hydrochloric, and 

 one measure of nitric acid a mixture which, from its application 

 to dissolve gold has acquired the name of aqua regia. The 

 solution of gold is evaporated by a water-bath, till acid vapours 

 cease to be exhaled ; it is then dissolved in water and hydro- 

 chloric acid is mixed with it. On adding protosulphate of iron 

 to this solution, the gold is wholly precipitated as a brown or 

 brownish yellow powder, quite destitute of the metallic lustre, 

 which, however, appears when the powder is rubbed. The 

 protosulphate of iron is, at the same time, converted into per- 

 sulphate and perchloride : 



6(FeO, So 3 ) and Au 2 C1 3 = 2(Fe 2 O 3 , 3SO 3 ) and Fe 2 C1 3 and 2Au. 



