1092 A TREATISE ON METAMORPHISM. 



heating, or several days at room temperatures. Tire only conditions to be 

 observed, in order to obtain quantitative precipitation of the gold by the 

 tellurium, are sufficient time and the direct contact of all the tellurium with 

 the gold solution."" Hall and Lenher have also shown that selenium 

 precipitates gold from its solutions in a similar manner according to the 

 reaction: 



4AuCl 3 +3Se=4Au+3SeCl 4 



They say: "From six to eight hours of continued boiling are necessary to 

 insure complete precipitation, or better, from two to three days at a tem- 

 perature of 70° to 80°." l 



Not only is gold precipitated from its solutions by various metals, but 

 it is precipitated by maii}^ ous salts and oxides ; for instance, it is well known 

 that ferrous sulphate readily reduces gold to the metallic condition. Stokes 

 has recently shown also that ferrous salts in silicates are sufficient to pre- 

 cipitate copper from its solutions, and if this be so, it is certain that gold may 

 be precipitated in like manner. There is little doubt, also, that ferrous 

 oxide in magnetite is adequate to precipitate gold from its solutions. It is 

 to be noted that Stokes writes the equations 



AuCl 3 + 3FeCl 2 2 Au +3FeCl 3 

 AuCl 3 +3CuCl^Au+3CuCl 2 



as reversible reactions. He states that "with rising temperature the equi- 

 librium moves from left to right, with falling temperature from right to 

 left." c 



It follows that where there is lessening temperature and pressure, 

 ferrous chloride and cuprous chloride reduce gold solutions, throwing down 

 the metallic gold. Normally, the conditions for falling temperature are 

 those for ascending solutions, and therefore, so far as gold is precipitated 

 by these reactions, it is more likely to take place where the waters are 

 ascending. 



So far as I know, experiments have not been made in the precipitation 

 of gold by cuprous oxide and cuprous salts other than cuprous chloride; 

 but in general it may be stated with a considerable degree of certainty that 

 any of the ferrous and cuprous compounds which occur in nature are ade- 

 quate precipitating agents for gold solutions. From the descriptions of 



"Hall and Lenher, cit., p. 920. &Hall and Lenher, cit., p. 921. '■ Stokes, manuscript, p. 7. 



