176 Elements of Water Bacteriology. 



the litmus-lactose-agar plate serve for what Whipple has 

 well called presumptive tests. 



The results of the bacteriological examination have, in 

 several respects, a peculiar and unique significance. 

 First, this examination is the most direct method of sani- 

 tary 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 matter is present. By a determi- 

 nation 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 dan- 

 gerous character. 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 as these disease 

 germs do than will any chemical compounds. In the 

 second place, the bacteriological methods are superior 

 in delicacy to any others. Klein and Houston (1898) 

 showed by experiment with dilutions of sewage that 

 the colon test was from ten to one hundred times as 



