220 ELEMENTS OF WATER BACTERIOLOGY 



The full bacteriological analysis should then consist 

 of three parts, the gelatin-plate count, as an estimate 

 of the amount of organic decomposition in process; 

 the total count, and the count of red colonies, on 

 litmus-lactose-agar, as a measure of the organisms 

 which form acids and thrive at the body temperature; 

 and the study of a series of lactose bile tubes for the 

 isolation of colon bacilli. 



Special Advantage of the Bacteriological Examination. 

 The results of the bacteriological examination have, in 

 several respects, a peculiar and unique significance. 

 First, this examination is the most direct method of 

 sanitary water analysis. The occurrence of nitrites 

 or free ammonia in a small fraction of one part per 

 million, or of chlorine in several parts per million, do 

 not in themselves render a water objectionable or 

 dangerous. They merely serve as indicators to show 

 that germ-containing and germ-sustaining organic mat- 

 ter is present. By a determination of the chlorine 

 and study of the relations of carbon and nitrogen, 

 it is possible to determine with some degree of accuracy 

 whether this organic matter is of plant or animal origin, 

 and hence to rate its objectionable or dangerous char- 

 acter. By the bacteriological examination, on the 

 other hand, we are able to determine directly whether 

 particular kinds of organisms characteristic of sewage 

 are, or are not, actually present in the water. What 

 we dread in drinking-water is the presence of pathogenic 

 bacteria, mainly from the intestinal tract of man, 

 and it is quite certain that the related non-pathogenic 

 bacteria from the same source will behave more nearly 



