MICROORGANISMS IN WATER. 2OI 



38 per c.c. Other figures are as follows: Loch Katrine, 74 per c.c., Lake 

 Lucerne, 8 to 51 per c.c., Lake Champlain, 82 per c.c. 



SEA WATER. There are few bacteria in sea water remote from the 

 coast; but near the shore and in the neighborhood of seaports there may 

 be large numbers. 



Examples: 350 meters from Naples, sea water contained 26,000 

 bacteria per c.c. At a distance of 3 kilometers, only 10. Samples taken 

 from depths of 75 to 800 meters at distances from 4 to 15 kilometers 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 are 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 remembered 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 generally 

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

 nation 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 organ- 

 isms, if the samples are kept cool. 



LIGHT (page 162). 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 current, 

 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 three meters (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 



