230 ORIGIN OF METALLIFEROUS DEPOSITS. [XII. 



iron, is even to-day forming in certain waters and in beds of 

 mud and silt, where it sometimes takes a beautifully crystalline 

 shape. What are the conditions in which the sulphide of 

 iron is formed and deposited, instead of the oxide or carbonate 

 of iron? Its production depends, like these, on decaying 

 organic matters. The sulphates of lime and magnesia, which 

 abound in sea-water, and in many other natural waters, when 

 exposed to the action of decaying plants or animals, out of 

 contact of air, are, like peroxide of iron, deoxidized, and are 

 thereby converted into soluble sulphides ; from which, if car- 

 bonic acid be present, sulphuretted hydrogen gas is set free. 

 Such soluble sulphides, or sulphuretted hydrogen, are the 

 reagents constantly employed in our laboratories to convert the 

 soluble compounds of many of the common metals, such as 

 iron, zinc, lead, copper, and silver, into sulphides, which are 

 insoluble in water and in many acids, and are thus conven- 

 iently separated from a great many other bodies. Now, when 

 in a water holding iron-oxide, sulphates are also present, the 

 action of organic matter, deoxidizing the latter, furnishes the 

 reagent necessary to convert the iron into a sulphide ; which 

 in some conditions, not well understood, contains two equiva- 

 lents of sulphur for one of iron, and constitutes iron-pyrites. 

 I may here say that I have found that the unstable protosul- 

 phide, which would naturally be first formed, may, under the 

 influence of a persalt of iron, lose one half of its combined 

 iron ; and that from this reaction a stable bisulphide results. 

 This subject of the origin of iron-pyrites is still under investi- 

 gation. 



The reducing action of organic matters upon soluble sul- 

 phates is well seen in the sulphuretted hydrogen which is 

 evolved from the stagnant sea-water in the hold of a ship, and 

 which coats silver exposed to it with a black film of sulphide 

 of silver, and for the same reason discolors white-lead paint. 

 The presence of sulphur in the exhalations from some other 

 decaying matters is well known, and in all these cases a solu- 

 ble compound of iron will act as a disinfectant, partly by fixing 

 the sulphur as an insoluble sulphide. Silver coins brought from 



