GOLD. 365 



per, if present) by treating the granulated alloy with nitric acid of a specific 

 gravity of 1.32. This dissolves silver and copper, but does not act on the gold. 

 The process, however, is not applicable to an alloy containing more than 33 

 per cent, of gold, and it was believed that it should not exceed 25 per cent. 

 In order to subject to this process an alloy which is richer in gold, the alloy is 

 first fused with silver, and as it was customary to use 3 parts of silver for 1 part 

 of impure gold the process became known as quartation or inquartation. In 

 place of nitric acid, sulphuric acid of a specific gravity of 1.84 may be used. Gold 

 which has been freed from base metals and silver by any of the above-described 

 methods is known as refined gold, but it is rarely absolutely pure. It retains 

 traces of base metals or silver and all of the platinum if originally present. 



Chemically pure gold, or as it is termed by the mints gold 1000 fine, may be 

 obtained by the following process. Nitro-hydrochloric acid, consisting of one 

 part of nitric acid and two parts of hydrochloric acid, is added in small por- 

 tions to the granulated gold, refined by one of the ordinary processes, until its 

 solution, with the aid of heat, has been effected. This solution of gold chloride 

 is evaporated nearly to dryness at a moderate heat. If platinum be suspected 

 the remaining mass is dissolved in very little water, and to the solution is added 

 an equal volume of alcohol and some ammonium chloride. If platinum be 

 present it is precipitated as ammonium platinic chloride and separated by fil- 

 tration. The filtrate is diluted with four parts of water and permitted to stand 

 for several days in order to cause complete precipitation of any silver chloride 

 present. To the filtrate a clear solution of ferrous sulphate is added which 

 causes the precipitation of gold. After decanting the supernatant liquid and 

 thoroughly washing the precipitate with distilled water it is treated with hot 

 concentrated acid, to eliminate traces of iron or copper. The purified gold is 

 again washed, dried, fused in a borax-lined crucible and poured into an ingot 

 mold. The chemical reaction which occurs in the precipitation of gold with 

 ferrous sulphate is this : 



AuCl 3 + 3FeS0 4 = FeCl 3 + Fe 2 (SO 4 ) 3 + Au. 



Many other reducing agents may be used in place of ferrous sulphate for 

 the precipitation of gold. Thus it is precipitated in a spongy or crystalline 

 condition by gently heating the gold solution with oxalic acid: 

 2AuCl 3 + 3H 2 C 2 O 4 = 6HC1 + 6CO 2 + 2Au. 



Sulphurous acid precipitates gold in scales: 



2AuCl 3 -f- 3H,SO 3 + 3H 2 = 6HC1 + 3H 2 SO 4 + 2Au. 

 Zinc and many other base metals precipitate gold as a brown powder: 

 2AuCl 3 -f 3Zn = 3ZnCl 2 + 2Au. 



Elementary phosphorus, and many other organic and inorganic reducing 

 agents, may be used similarly for the precipitation of gold from its solutions, 

 or this precipitation may be effected electrolytically. 



Cohesive gold, used in dentistry, may be obtained by heating gold foil to 

 redness, by which is restored its cohesiveness, which is greatly diminished 

 during the conversion of pieces of gold into foil by beating. 



Gold is orange-yellow by reflected light, and green by transmitted 

 light ; it fuses at 1200 C. (2192 F.), has a specific gravity of 19.36, 



