MICROORGANISMS IN WATER. 203 



moving rivers the influence of this factor is also quite pronounced; and, 

 according to Jordan, "The influences summed up by the term sedi- 

 mentation are sufficiently powerful to obviate the necessity for summoning 

 another cause to explain the diminution in numbers of bacteria" in sewage 

 polluted rivers. The example already given (page 200) 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 comment, 

 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 peculiar fact, which has never been satisfactorily explained, is the 

 quick 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 belief 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 from wells in the vicinity of these rivers. 



INTERPRETATION or THE BACTERIOLOGICAL ANALYSIS OF WATER. 



In making any analysis of water, all 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 quality of water from the number of microorganisms 

 in it. It may, however, be safely assumed that high bacterial counts 

 indicate a large amount of organic matter. The number of organisms 

 growing in beef peptone gelatin at 20-22, 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. 



