GOLD. 219 



reagents added to a stannic salt produce white stannic acid, H 2 SnO 3 . 

 Both precipitates are soluble in excess of the alkali. 



Gold, Au = 196.7 (Aurum). Gold occurs in nature chiefly in the 

 free state, often associated with silver, copper, and possibly with other 

 metals. This impure gold is separated from most of the adhering 

 sand and rock by a mechanical process of washing, in which advan- 

 tage is taken of the high specific gravity of the metallic masses. The 

 remaining mixture of heavy material is treated with mercury, which 

 dissolves gold and silver, leaving behind most other impurities. The 

 gold amalgam is placed in a retort and heated, when the mercury 

 distils over, while the gold is left behind. If this should contain 

 silver, the metals may be separated by treating the alloy with hot 

 sulphuric acid, which dissolves silver, leaving gold behind. 



Pure gold is too soft for general use, and therefore is alloyed with 

 various proportions of silver and copper. American coin is an alloy 

 of 90 parts of gold and 10 parts of copper; jeweller's gold contains 

 generally 75 per cent. (18 carat) of gold, the other 25 per cent, being 

 copper and silver ; the varying proportions are well indicated by the 

 color. 



Gold is not affected by either hydrochloric, nitric, or sulphuric 

 acid, but is dissolved by nitro-hydrochloric acid, by free chlorine or 

 bromine, and by mercury, with which it forms an amalgam. 



Gold is trivalent generally, as in auric chloride, AuCl 3 , but most 

 likely also univalent in some compounds, as in aurous chloride, AuCl. 



Auric chloride, AuCl 3 (Gold chloride). Obtained by dissolving 

 pure gold in nitro-hydrochloric acid and evaporating the solution to 

 dryness. A mixture of equal parts of auric chloride and sodium 

 chloride is official under the name of gold and sodium chloride. It is 

 an orange-yellow, very soluble powder. 



Analytical reactions. 

 (Auric chloride, AuCl 3 , may be used.) 



1. Add hydrogen sulphide to solution of gold : brown auric sul- 

 phide, Au 2 S 3 , is precipitated, which is soluble in yellow ammonium 

 sulphide. 



2. Add ferrous sulphate to solution of gold and set aside for a few 

 hours : metallic gold is precipitated as a dark powder, which by 

 fusion is converted into a metallic mass. 



