256 ALBERT D. BROKAW 
gold sulphide, in the presence of hydrogen sulphide and sulphuric 
acid. Equations may be written as follows: 
8AuCl,+9H.S+4H.0 = 4Au.,S.+ 24HCI+H,SO, 
Au.S.+6Fe.(SO,),;+8H,0 = 2Au-+12FeSO,+8H.SO, 
These reactions may help explain the fact that, so far as we know, 
gold sulphide does not occur in nature. 
A number of natural tellurides, including all the commoner 
telluride ores, have been shown to react with gold chloride in a 
manner similar to that of free tellurium,’ for example: 
3Au.,Te+4AuCl,=10oAu+3TeCl, 
and as is the case with tellurium, this may help to account for the 
free gold in the oxidized portion of the ore body, such as the “rusty 
gold” or “flour gold”’ at Cripple Creek. 
b) Complex sulphides—From the fact that most of the simple 
sulphides precipitate gold from solution it is readily inferred that 
complex sulphides would’ be equally effective as precipitants. 
Polybasite was chosen for an experiment on this point. A small 
fragment of a good crystal of polybasite (Ag,SbS.) was placed 
in 5c.c. of gold chloride solution containing 0.5 per cent gold. 
After 10 days the gold had been completely precipitated, forming a 
dull coat over the crystal, resembling that formed on pyrrhotite. 
Doubtless many of the complex sulphides would react in a similar 
manner, though geological observations bearing on such precipi- 
tations are lacking. Perhaps this is because the complex sulphide, 
after precipitating gold, has been more or less completely removed 
by oxidation. 
III. FERROUS COMPOUNDS 
a) In solution.—The fact that ferrous salts can precipitate 
gold from solutions of its salts has long been known. The reaction 
is ordinarily written: 
3FeSO,+AuCl,=Au-+ Fe.(SO,),;+FeCl, , 
or ionically, 
3Fett+Auttt=3Fettt+Au 
This reaction was formerly used in metallurgical practice, but 
recently has been largely displaced by other methods. The 
tV. Lenher, op. cit. 
