82 BACTERIOLOGICAL EXAMINATION OF WATER. 



a medium free from organic matter and containing 0'05 gramme 

 potassium phosphate, 0*01 gramme magnesium sulphate, 0'05 

 gramme calcium chloride, and 1 gramme ammonium carbonate 

 per litre ; the same medium was also employed with the addition 

 of 1 gramme of glucose per litre. Growth and oxidation were 

 most marked in the medium free from sugar ; in strong contrast 

 to the reducing micro-organisms which worked most powerfully 

 in the medium containing sugar. 



Heraeus also examined certain well-known pathogenic and 

 non-pathogenic organisms, but as these did not grow well in 

 media containing only salts and sugar, sterilised urine, diluted 1 

 in 5, and broth, diluted 1 in 10, were used for the experiments. 

 He found that the M. prodigiosus, B. ramosus, B. anthracis, 

 B. typhosus, Staphylococcus citreus, and the spirilla of Finkler 

 and Miller produced nitrous acid in urine. 



Heraeus considered that in nature there existed nitrifying and 

 denitrifying organisms, and whether the oxidation or the 

 reduction process gained the ascendency depended on the supply 

 of nutriment present. If there was plenty of food material, 

 especially organic substances, the denitrification process was 

 active ; but if the food supply was poor the oxidising organisms 

 gained the upper hand. When no other source of nitrogen 

 except nitrates existed in the water it seemed possible that the 

 oxidising bacteria might reduce the nitrates in order to obtain 

 food necessary for their growth ; the same reduction also 

 appeared to take place when the entrance of air into the solutions 

 was prevented. 



Winogradsky considered that the amount of nitrites and 

 nitrates obtained by Heraeus was not sufficiently great to 

 establish beyond question the presence of a true nitrification 

 process. Nitrites and nitrates are constantly present in the air 

 of rooms and are readily absorbed by alkaline fluids ; conse- 

 quently, unless this source of error is avoided and a sufficiently 

 large quantity of an ammonium salt is completely oxidised by 

 the pure cultures under examination, it is impossible to ascribe 

 to them true oxidising powers. These conditions were not ful- 

 filled by the organisms investigated by Heraeus, so it is doubtful 

 whether he really succeeded in isolating true nitrifying 

 bacteria. 



