242 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



This change is hastened by the presence of certain metallic 

 oxides, particularly those of iron and manganese ; thus in 

 presence of oxide of iron the following changes may take 

 place : — 



SHjS + Fe^Os = 2FeS + SH^O + S 



In presence of oxygen and moisture FeS may readily 

 oxidise to ferrous sulphate, FeS04, thus : — 



FeS + 20j = FeSO^ 



And this may further oxidise with formation of fenic 

 sulphate, thus : — 



(FeSOJs+O + H^O = Fe2(SOj3 + Fe(OH), 



It is quite possible that pyrites, especially when found in 

 coal, may owe its origin to the interaction of oxide of iron and 

 the sulphides produced by the decay of vegetable matter. 

 When such ' coal brasses," as this form of pyrites is termed, 

 is exposed to the air, it oxidises with formation of ferrous 

 sulphate, or eventually, it may be, of ferric sulphate. 



When black sewage mud is exposed to the air it turns 

 brown and becomes acid, owing to the formation of hydrated 

 oxide of iron and sulphuric acid. 



How far hydrogen sulphide and sulphides are capable of 

 being directly oxidised by solutions of nitrates does not 

 appear to have been sufficiently studied. There is no doubt 

 that nitrates are rapidly reduced in presence of sulphide 

 mud ; how far this is a purely chemical change and, if so, 

 what is the exact cause of the change, has not been fully 

 determined. 



Beyerinck claims to have isolated an organism, B. thioparus, 

 which brings about the following decomposition : — 



5S + 6KNO3 + 2H2O = K2SO4 + 4KHS0^ + 3N3 



The most frequently occurring and obvious case of oxida- 



