MICROORGANISMS IN WATER 319 



Examples: 350 M. from Naples, sea water contained 26,000 bac- 

 teria per c.c. At a distance of 3 KM., only 10. Samples taken from 

 depths of 75 to 800 M. at distances from 4 to 15 KM. from shore were 

 found to contain from 6 to 78 bacteria per c.c. in surface water, and 

 from 3 'to 260 at various depths below. 



CAUSES AFFECTING THE INCREASE AND DECREASE OF THE 

 NUMBER OF BACTERIA IN WATER 



There is a number of causes which influence the multiplication 

 or diminution of microorganisms in natural waters; and while it is 

 necessary to discuss each of these causes in detail, it must be remem- 

 bered that a number of them may be simultaneously influencing the 

 increase or decrease. 



TEMPERATURE. In natural waters, a low temperature probably 

 acts injuriously on parasitic bacteria, reducing their numbers; but 

 the bacterial content of water during the hot summer months is gener- 

 ally not so large as during the cooler seasons. Water collected for 

 examination should be analyzed at once; otherwise, contradictory 

 results as to numbers will be found. Usually, in most waters, there is 

 a reduction in numbers for a few hours, followed by a large increase. 

 Very much polluted waters, however, show a marked decrease of 

 intestinal organisms, if the samples are kept cool. 



LIGHT. Although the germicidal effect of sunlight is well known, 

 yet it has not such powerful effects on the bacteria in water. 

 Much depends, no doubt, on the turbidity and speed of the cur- 

 rent, the maximum killing effect being produced in shallow, clear 

 and slow-moving water. It has been found by experiment that the 

 germ-killing power of light extends to a depth of 3 M (about 9.84 feet). 

 As a means of purifying water, direct light produces very little effect. 



FOOD SUPPLY. The amount of organic matter in water directly 

 influences the growth of bacteria. Where a large amount of this is 

 present, the number of microorganisms is also large. Rivers containing 

 considerable organic matter derived from vegetable debris, etc., contain, 

 as a rule, more organisms than rivers in which there is but little of 

 such material. Thus the Ottawa River, which drains a large area of 

 forest lands and is characterized as an upland peaty water carrying a 

 rather high percentage of organic and volatile matter, contains through- 



