270 METALS. 



small quantity of muriatic acid has been added, is em- 

 ployed in this test. Gold alone is proof against its 

 action. In proportion to the permanence of the mark, 

 is the purity of the gold which has been submitted to 

 the assay. 



What is said 676. GOLD COIN. The gold employed 

 of gold coin ? f or CQ ^ plate and j e we i ry j s a ] wa y s alloyed 



with a certain portion of copper or silver, to give it 

 greater hardness. The standard gold of the United 

 States is nine-tenths pure gold, the remaining tenth 

 being an alloy of copper and silver. 



677. PURITY OF GOLD. The purity of 



How is the de- . 



gree of purity gold is expressed in carats , a carat signify- 

 mwMdr ing ' P racticall 7> one twenty-fourth. Thus, 

 when gold is said to be sixteen carats fine, 

 it is meant, that two-thirds of it is pure gold. Gold 

 eighteen carats fine is three-fourths pure gold, and one- 

 fourth alloy. 



678. GILDING. Gilding by the galvanic 



How is copper 



jewelry gild- battery has been already described. This 

 method is, in most cases, preferable to all 

 others. Copper jewelry is thinly gilded by boiling in 

 a solution of gold in carbonate of soda or potash. The 

 solution is prepared by first dissolving the gold in aqua 

 regia, and afterward precipitating and re-dissolving it, 

 by means of the carbonate above named. 



679. Gildine may also be effected by an 



Describe the J 



method of gild- amalgam of gold and mercury. The amal- 

 omof am/ am ^ em applied, the mercury is expelled 

 by heat and the gold remains. This me- 

 thod is very frequently employed. A coating of pure 



