Mineral Sulphides of Iron. 191 



hydrogen sulphide. In the putrefaction of organic matter 

 hydrogen sulphide is one of the products, and Gautier* has 

 surmised that the pyrite which sometimes forms the substance 

 of fossil bones and shells is precipitated by hydrogen sulphide 

 which is given off slowly by the organic matter during its de- 

 composition. The formation of pyrite in this case would dif- 

 fer from that described in the previous pages only in the source 

 of the hydrogen sulphide, which is here a product of micro- 

 organisms. 



There is another way in which micro-organisms produce 

 hydrogen sulphide, and that is by the reduction of sulphates. 

 According to Beyerinck,f a considerable number of bacteria, 

 algae, flagellata and infusoria show this kind of activity. 

 Spirillum desulf uricans is one of the most important. As these 

 organisms are active only in neutral or alkaline solutions, fer- 

 rous sulphide is precipitated whenever ferrous salts as well as 

 sulphates are present. The black mud of many swamps, pools, 

 and even seas (e.g., the Black Sea), as well as sea coasts,;); 

 which are intermittently overflowed, contains ferrous sulphide. 

 Mr. C. A. Davis of the IT. S. Bureau of Mines, who has had 

 large experience on this subject, informs us that he has always 

 found hydrogen sulphide in peat-bogs into which tide-water 

 finds its way. Apparently, the formation of pyrite or mar- 

 casite through the agency of micro-organisms has not been 

 observed, but only an influx of air with excess of hydrogen 

 sulphide would be needed to change the ferrous sulphide into 

 disulphide. That micro-organisms are directly responsible for 

 any great quantity of the pyrite or marcasite of nature seems 

 unlikely because in the first place they are probably not active 

 far from the surface of the ground. They have been dis- 

 covered at depths of only four or five meters. § A fraction of 

 a per cent of free acid usually inhibits the growth of these 

 organisms ; therefore they could not live in the solutions from 

 which marcasite appears to have formed. Pyrite and marca- 

 site are not infrequently associated with minerals like chalco- 

 pyrite, which proves the presence of copper in the original 

 solutions, and copper is exceedingly poisonous to practically 

 all micro-organisms. It is possible, however, that the reduc- 

 tion of sulphates like gypsum and sodium sulphate by micro- 

 organisms may be an important source of hydrogen sulphide 

 in nature. 



Distinct conditions leading to pyrite or marcasite in nature. 

 — The geological relations of marcasite indicate that it is a 



*C.R, cxvi, 1494, 1893. 



f Centr. Bakter. u. Parasitenkunde, i, pp. 1, 49, 104, 1895. 



^:H 2 S was found in sea-water by B. Leroy, Ann. Ch. Ph., lviii, 332, 1846. 



§ Hygiene des Bodens, Josef von Fodor, Jena, vol. i, Pt. I, p. 137, 1893. 



