240 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



(4) Nitrogen compounds are only required in very small 

 quantities ; sufficient indeed is contained in ordinary tap water. 



(5) The most favourable temperature for sulphate re- 

 duction is about 25 C. Beyerinck succeeded in isolating an 

 organism which he termed Spirillum desulphuricans ; it is 

 a strictly anaerobic organism, and this circumstance, in con- 

 junction with its small need for nitrogenous nutriment, 

 enables it best to grow in solutions which have been worked 

 over by other organisms. These facts are of not a little 

 practical interest. Those who have had to deal with samples 

 of sewage and effluents will have noticed that such samples, 

 if kept in stoppered bottles, may become in time practically 

 clear, having only a small black sediment at the bottom ; 

 but if they have been tightly stoppered, they may also retain 

 considerable quantities of hydrogen sulphide. If this is 

 removed by boiling, very little residual organic matter will be 

 found to be present. 



Stagnant polluted waters, e.g., the Manchester Ship Canal, 

 show the same phenomenon. It is evident, in both these 

 cases, that the nitrogenous organic matter is broken down by 

 ordinary putrefactive organisms, and that final sulphate reduc- 

 tion takes place. In such cases sulphides, or hydrogen sulphide, 

 will be found to constitute almost all the oxidisable matter left. 



To demonstrate the reduction of sulphates, the following 

 solution was made use of by van Delden : 



Tap water 1000 grams 



Common salt . . . . . . 30 



Sodium lactate . . . . . . 10 



Crystallised magnesium sulphate . . 8 



Potassium phosphate . . . . 0*5 gram 



Asparagin . . . . . . . . 0*5 



This solution may be inoculated with a little sewage 

 sludge, from which sulphate-reducing organisms are seldom, 

 if ever, absent. 



