GOLD AND SILVER. 



44. GOLD AND SILVER. 



I. From an alloy containing less than about 15 per 

 cent, of silver, aqua regia dissolves all the gold, while 

 the whole of the silver is left as chloride ; for this pur- 

 pose, however, the metal must be employed in a very 

 thinly laminated state. The solution is evaporated, to 

 expel as much of the nitric acid as possible; and di- 

 luted with water, to effect the complete separation of 

 the chloride of silver. From the solution the gold is 

 precipitated by oxalic acid, or by protosulphate of 

 iron. 



II. If the alloy contain more than 80 per cent, of 

 silver, pure nitric acid dissolves the whole of the silver, 

 and leaves the gold. Here also the alloy must be thinly 

 laminated. The silver is precipitated by hydrochloric 

 acid. The gold is well washed, and dissolved in aqua- 

 regia, to ascertain if any trace of silver be left in it. 



III. When the quantity of silver present in the alloy 

 is between 15 and 80 per cent., it cannot be entirely 

 extracted by nitric acid, neither can all the gold be 

 dissolved out by aqua regia, since the metal becomes 

 covered with a thick layer of chloride of silver. Such 

 an alloy should be fused in a porcelain crucible with 

 3 times its weight of pure lead. From this alloy, nitric 

 acid then dissolves all the lead and silver, leaving pure 

 gold. 



From the solution filtered from the gold, the silver 

 is precipitated by hydrocyanic acid ; or, after diluting 

 largely, and heating nearly to boiling, by hydrochloric 

 acid. 



IV. Silver and gold in alloys of these metals may 

 also be separated by concentrated sulphuric acid, 

 whatever may be the relative proportion of the two 

 metals. The thinly laminated alloy is heated with the 

 acid in a capacious dish, until all evolution of gas 



