366 METALS AND THEIR COMBINATIONS. 



and is a good conductor of heat and electricity. It is so malleable and 

 ductile that 1 grain can be hammered into a film covering 54 square 

 inches, and can be drawn into a wire, if protected by some more tena- 

 cious metal such as silver, so fine that 1 grain will measure 550 feet. 

 Tin, lead, antimony, arsenic, and bismuth destroy the ductility and malle- 

 ability of gold, making it very brittle. A small proportion of platinum con- 

 fers upon gold elasticity and increases its hardness. 



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

 various proportions of silver and copper. It is customary to express 

 the purity or fineness of gold in carats, an old term meaning a 

 twenty-fourth part. Pure gold is 24 carats, while an alloy contain- 

 ing 75 per cent, of gold is said to be of 18 carats fineness. Ameri- 

 can coin is an alloy of 90 parts of gold and 10 parts of copper ; 

 jeweller's gold contains generally 75 per cent, or more of gold, the 

 other metals 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 and 

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



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

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



Gold chloride, Au01 3 (Auric chloride). Obtained by dissolving 

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

 dryness. A mixture of equal parts by weight of gold chloride and 

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

 It is an orange-yellow, very soluble powder, containing about 30 per 

 cent, of metallic gold. 



Tests for gold. 

 (Solution of auric chloride, AuCl 3 , may be used.) 



1. Add hydrogen sulphide to the solution : brown auric sulphide, 

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



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

 hours metallic gold is precipitated as a dark powder : 



AuCl 3 + 3FeS0 4 = Fe,(S0 4 ) s -f FeCl 3 + AU. 



3. Many other reagents cause the separation of metallic gold from 

 its solution, as, for instance, oxalic acid, sulphurous and arsenous 

 acids, potassium nitrite, etc. 



