250 



STEPHEN DEM. GAGE 



nearly all the remaining samples had ratios above 40. The normal 

 ratio for this latter group of sea-water samples is probably between 

 50 and 60. The well-water samples are characterized by the fact 

 that 68 per cent of the samples have ratios less than i, the ratios 

 for the remaining samples being distributed in a fairly uniform 

 manner. 



TABLE 15. 

 DISTRIBUTION OF THE RATIOS BETWEEN THE TOTAL BACTERIA AND THE ACID-PRODUCING BACTERIA 



AT 20, 30, AND 40 AMONG DIFFERENT SAMPLES FOR VARIOUS CLASSES OF WATERS. 



PLATES INCUBATED AT 2O C. 



PLATES INCUBATED AT 40 C. 



Causes o) variation in bacterial contents of Merrimack River water. 

 As fluctuating factors which are most liable to influence the numbers 

 of bacteria and B. coli, and the ratio between bacteria and B. coli, 

 in the water from such a source as the Merrimack River, we have 

 the volume of water flowing in the river, the temperature of the water, 

 and the amount of dissolved oxygen in the water. For a clear under- 

 standing of the influence of these three factors it is first necessary 

 to consider the influence of the various factors upon one another. 

 It is quite generally understood that the amount of oxygen dissolved 

 in water varies inversely as the temperature, a greater amount of 



