244 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



sulphide (sulphuretted hydrogen). This may be re-oxidised 

 to sulphates, either directly by chemical means, e.g., oxides of 

 iron, etc., or by the intervention of bacteria. Certain of 

 these oxidise it directly to sulphate, while others make use 

 of the presence of nitrates. 



Sulphates are capable of being directly reduced to hydrogen 

 sulphide by certain bacteria, in presence of small quantities 

 of organic matter, but such changes only take place in absence 

 of air. These various changes clearly indicate the importance 

 of abundant supphes of oxygen, if the evolution of hydrogen 

 sulphide, and the other less well-defined objectionable gases 

 which accompany it, are to be avoided. The bearing of this 

 principle on the purification and disposal of sewage and 

 other waste organic matter, wiU be further referred to in 

 Chapter XVII. 



