PRECIPITATION OF SULPHIDES. 1117 



precipitated in a different manner. For instance, in the case of lead 

 carbonate with pyrite and marcasite, the reaction is as follows: 



8FeS 2 +14PbC0 3 +Na,C0 3 =14PbS+4Fe 2 3 +^ T a 2 S 2 3 +15C0 2 



An interesting - point in connection with this reaction is the production 

 of hematite and sodium thiosulphate. Stokes shows also that zinc, copper, 

 and silver are precipitated as sulphides by iron as sulphide in an analogous 

 manner, the reactions being: 



8FeS 2 +15ZnC0 3 +Na 2 C0 3 =15ZnS+4Fe 2 3 +Na 2 SO i +16C0 2 

 and 



SFeS 2 fl5Ag 2 00 3 +Na 2 C0 3 =15Ag 2 S+4Fe 2 O 3 +Na 2 S0 4 +16CO 2 . 



While Stokes in the laboratory used temperatures of 100° C. and 180° 

 C, it does not follow that the above reactions do not take place at lower 

 temperatures but more slowly. 



The above reactions in the presence of an alkaline carbonate are very 

 suggestive, since they possibly explain in large part the association of lead, 

 zinc, copper, and silver ores with limestone; for if calcium carbonate be 

 substituted for sodium carbonate, similar reactions take place. It is not 

 necessary to rewrite the equations, but the character of the reactions may 

 be seen by substituting for Na 2 C0 3 on the left-hand side the equations 

 CaC0 3 , and for Na 2 S20 3 and Na 2 S0 4 on the right-hand side the equation 

 CaS 2 3 and CaS0 4 . Since these reactions are confirmed by experiment, 

 we probably have the clue to the world-wide preference of many ores of 

 lead, zinc, copper, and silver for the limestones. (See pp. 1086-1087.) 



i; EN ERA I. STATEMENT*. 



From the foregoing it appears that at the trunk channels of circulation 

 the sulphur may enter in sulphides or in oxidized compounds in the forms 

 of sulphites, sulphates, or other oxidized salts. If present as sulphides in 

 alkaline solutions, the sulphides may be precipitated in consequence of the 

 neutralization of the solution by an acid. They may be thrown down also 

 in the ascending solutions in consequence of diminishing temperature and 

 pressure. However, where the oxidized sulphur-bearing solutions reach 

 trunk channels, as they frequently do, in order to precipitate the metals 

 as sulphides they must become mingled with solutions bearing reducing 

 agents, hydrogen sulphide, or alkaline sulphides, or must come into contact 



