1100 A TREATISE ON METAMOKPHISM. 



extended to equilibrium it will continue farther where the temperature is 

 rising, and thus descending solutions with increasing temperature are 

 favorable to solution of metallic silver. Doubtless silver is soluble to some 

 extent in all the strong acids which occur underground and also in their 

 salts. When, however, the silver becomes a salt, the form in which it would 

 subsequently travel depends on the amount of acids in the solutions and 

 their strength. It is well known that silver sulphide is soluble in alkaline 

 carbonates and in hydrosulphuric acid, but probably silver thus transported 

 is not an important source of metallic silver of the first concentration by 

 ascending waters. From such compounds the silver is likely to be thrown 

 down as a sulphide. 



precipitation. — The precipitation of silver in the metallic form from its 

 solutions follows to a certain extent the same lines as that of gold; but silver 

 is not nearly so readily reducible as gold, and therefore is not thrown down 

 in the metallic form by so many compounds Silver is precipitated by 

 metallic iron, by metallic copper, by cuprous compounds, readily by ferrous 

 sulphate, and probably slowly by all other ferrous compounds. Of the ous 

 salts, ferrous sulphate is doubtless the most important, The precipitation 

 of the silver by ferrous sulphate is commonly written — 



Ag 2 S0 4 + 2FeS0 4 = 2Ag + Fe 2 (S0 4 ) s 



The reduction is a function of the abundance of the reducing agent. Where 

 the ferrous sulphate is abundant, and the temperature moderate it is believed 

 that the more probable reaction for the precipitation of silver from its 

 solutions is represented by the following equation: 



Ag,S0 4 + 3FeS0 4 + 4H. 2 =2Ag + Fe 3 4 + 4H 2 S0 4 



Since ferric sulphate will dissolve silver, and ferrous sulphate will 

 reduce silver, the relations of these two compounds are, as Stokes notes, 

 reversible according to the following expression: 



2Ag + Fe 2 ( S0 4 ) 3 ^ Ag,S0 4 + 2FeS0 4 



Stokes says that the reaction moves in the direction of solution when the 

 temperature is rising, and in the direction of precipitation when the tempera- 

 ture is falling. Thus, in ascending solutions, where the temperature is 

 falling, silver is especially likely to be thrown down by the ferrous salts. 



Silver may be also precipitated by ferrous compounds in a solid form, 

 as in mao-netite and the silicates. That this is certain follows from the fact 



