a BACTERIOLOGICAL EXAMINATION OF WATER. 



distinguishing bacteria were unknown or imperfectly practised, 

 eager investigators of the bacterial flora of waters isolated a 

 large number of water-organisms. Many micro-organisms were 

 described as distinct species which were really only varieties, the 

 variation being caused by differences in food, habitat, tempera- 

 ture, c. The confusion thus originated gradually became so 

 great that many hygienists lost faith in the bacteriological 

 examination of water and were inclined to believe that a reliable 

 opinion as to the purity of a supply could be given only from 

 the results of a chemical examination. But the indications given 

 by chemical means are merely relative, the results so obtained 

 cannot be correctly interpreted except by reference to local 

 standards. There is no general standard which can be fixed for 

 all waters; the source and immediate surroundings of the supply 

 must be known before a reliable opinion as to the quality of the 

 water can be given. Also it is a matter of common knowledge 

 how quickly some waters change after collection, and this is 

 especially the case when the sample is obtained from a polluted 

 source. It is now well known that in most waters there are two 

 groups of micro-organisms which may largely influence the 

 chemical composition of the water. In the first group are 

 included the micro-organisms which break up organic matter 

 into its chemical elements and reduce nitrates and nitrites to 

 ammonia ; to the second group belong the oxidising micro- 

 organisms which convert ammonia into nitrites and nitrates. 

 The exact conditions under which these two groups of micro- 

 organisms operate are not worked out at the present time. The 

 presence of oxygen is supposed to have an important influence, 

 but there must be other conditions, as yet unknown, which 

 influence the final result. It appears from the researches of 

 Heraeus that one species of bacteria in the absence of nitrates 

 oxidises a part of the ammonium salts to nitrites; but if nitrates 

 are present they are reduced to nitrites, and ammonia notwith- 

 standing an abundant supply of air. Another species also 

 appears to oxidise ammonia to nitric acid in the presence of air, 

 but in the absence of oxygen reduces nitric acid to ammonia. 

 Winogradsky doubts the possibility of the same species possess- 

 ing both oxidising and reducing characteristics. It seems 

 probable that Heraeus 1 cultures were impure, and that the 



