MICROORGANISMS IN WATER 265 



of the self-purification of the Chicago drainage canal illustrates Jordan's 

 contention. 



Other Causes. — ^There is a number of other causes, not well 

 known nor of sufficient practical importance for more detailed com- 

 ment, which may increase or decrease the number of bacteria in water, 

 such as the inhibiting action of microorganisms and their products 

 on one another, the effects of pressure, etc. 



A pecuhar fact, which has never been satisfactorily explained, is the 

 qmck death (in three to five hours) of the cholera vibrio in the waters 

 of the Ganges and Jumna. When one remembers that these rivers 

 are grossly contaminated by sewage, by numerous corpses of natives 

 (often dead of cholera), and by the bathing of thousands of natives, 

 it seems remarkable that the behef of the Hindoos, the water of these 

 rivers is pure and cannot be defiled, and they can safely drink it and 

 bathe in it, should be confirmed by means of modern bacteriological 

 research. It is also a curious fact that the bactericidal power of 

 Jumna water is lost when it is boiled; and that the cholera vibrio 

 propagates at once, if placed in water taken froip wells in the vicinity 

 of the rivers. 



Interpretation of the Bacteriological Analysis oe Water 



In making any analysis of water, aU data, such as the kind of 

 water and the particulars regarding collection, transmission, sampling, 

 rainfall, etc., should be given, as these are a great help in interpreting 

 the results. One analysis is rarely sufficient; examinations should 

 be regularly and systematically made. 



Quantitative Standards. — No absolute guide can be given to 

 determine the potable quahty of water from the number of micro- 

 organisms in it. It may, however, be safely assumed that high bacte- 

 rial counts indicate a large amount of organic matter. The number of 

 organisms growing in beef peptone gelatin at 20° to 22°, and termed 

 the "gelatin count," should be given. For deep wells and springs, 

 this should not exceed 50 per c.c; and for shallow wells and rivers, 

 not over 500 per c.c. After rains or floods, these figures might be 

 exceeded, and would not necessarily indicate dangerous pollution. 



The number of organisms which develop on beef peptone agar 

 incubated at blood heat, commonly termed the "agar" or "blood- 



