1018 BACTERIA IN AIR, SOIL, WATER, AND MILK 



leads to a diminution of the number of bacteria in the upper layers 

 of the water. In rivers where sedimentation is to a certain extent 

 prevented by rapidity of Current, the effectiveness of such sedimen- 

 tation is, of course, entirely dependent upon the speed of the current. 



The influence of light in purifying surface waters is important 

 chiefly in ponds, lakes, and sheets of water which expose a large 

 surface to the sunlight, and where the surroundings are such that 

 the sun has free access throughout the day. According to the 

 researches of Buchner, 2 the bactericidal effects of light penetrate 

 through water to a depth of three feet. 



The effects of temperature in purifying surface waters under 

 natural conditions are probably not great. There is, however, a 

 general tendency toward diminution of the bacterial flora as the 

 temperature of such waters becomes lower. 



The presence of protozoa in natural waters as purifying agents 

 has recently been emphasized by Huntemiiller, 3 who claims that 

 these organisms by phagocytosis greatly diminish the number of 

 bacteria in any given body of water. It is self-evident that the 

 number of bacteria in any of these waters is never constant, since 

 all factors which tend to a diminution or increase in volume, such 

 as drying up of tributary streams or the occurrence of heavy rains, 

 would lead to differences of dilution which would materially change 

 numerical bacterial estimations. The influence of rains, furthermore, 

 may be a twofold one. On the one hand, heavy rain-falls, by wash- 

 ing a large amount of dirt into the rivers and lakes from the sur- 

 rounding land, have a tendency to increase the bacterial flora. This 

 influence would be especially marked in all bodies of water which 

 are surrounded by cultivated land where manured fields and graz- 

 ing-meadows supply a plentiful source of bacteria. On the other 

 hand, in regions where arid, uninhabited lands surround any given 

 river or lake, the rain would carry with it very little living con- 

 tamination and would act chiefly as a diluent and diminish the 

 actual proportion of bacteria in the water. 



Another and extremely important source of water supply is that 

 spoken of technically as "ground water/' The "ground waters" 

 include the shallow wells employed in the country districts, springs, 

 and deep or artesian wells. The shallow wells that form the water 



2 Buchner, Arch. f. Hyg., xvii, 1895. 

 8 HuntemiiUer, Arch. f. Hyg., liv, 1905. 



