32 WILLIAM II . EMMONS 



That a weighable quantity of gold is dissolved when only a trace 

 of chlorine is present is shown by experiment 13 (b), in which 

 chlorine was introduced without intention. 



The precipitation of gold. — In igneous rocks ferrous sulphate is 

 the chief precipitating agent. Ferrous sulphate is formed by the 

 oxidation of pyrite, but in the presence of oxygen and H 2 S0 4 it 

 becomes ferric sulphate, which does not precipitate gold. Below 

 the water-table, where pyrite is more abundant and free oxygen 

 less abundant, ferrous sulphate may persist in the mine waters. 

 Ferrous sulphate is so effective as a precipitant of gold that it is 

 used for that purpose in metallurgical processes. Experiment 19 

 shows that a minute amount of ferrous sulphate greatly decreases 

 the solubility of gold, although it does not precipitate it com- 

 pletely. With excess of ferrous salt practically all of the gold is 

 precipitated. 



Ferrous sulphate is formed in the upper part of a lode above 

 the water-table; but owing to the open condition of that part of 

 the lode, air is freely admitted and ferric sulphate forms, at the 

 expense of ferrous sulphate and sulphuric acid. This reaction takes 

 place almost instantaneously if Mn0 2 is present (experiment 20), 

 for ferrous sulphate and manganese dioxide are under these condi- 

 tions incompatible. Manganese dioxide then not only releases 

 the solvent for gold, but eliminates the salt which precipitates it. 

 It is doubtful whether appreciable amounts of gold are ever carried 

 far below the water-table in mines where the waters carry ferrous 

 sulphate, but, in the presence of Mn0 2 , ferrous sulphate may be 

 eliminated below the water-table. 



When manganese dioxide takes part in the reactions by which, 

 under the conditions named, gold is dissolved, transported, and 

 precipitated, the manganese salt is itself changed. At the surface 

 pyrolusite, Mn0 2 , forms, for there the excess of oxygen prevails; 

 and this mineral is commonly found in the gossan of manganiferous 

 lodes. When solutions containing H 2 S0 4 and NaCl react on Mn0 2 

 there is a tendency to form MnS0 4 , and some manganese goes into 

 solution as sulphate, but salts of manganese with higher valence 

 may also form. In this connection Dr. R. C. Wells has offered the 

 following statement: 



