BACTERIA IN WATER 691 



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,* the bac- 

 tericidal 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 HuntemiiUer,^ who claims that these organ- 

 isms 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 bacteria] estimations. The 

 influence of rains, furthermore, may be a twofold one. On the one hand, 

 heavy rain-falls, by washing a large amount of dirt into the rivers and 

 lakes from the surrounding 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 grazing-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 supply for a 

 large proportion of farms in the eastern United States are usually very 

 rich in bacteria and are by no means to be regarded as safe sources, ex- 

 cept in cases where great care is observed as to cleanliness of the sur- 

 roundings. In such wells the filtration of the water entering the well 

 may be subject to great variation according to the geological con- 

 ditions of the surrounding ground. The proximity of barns and sinks 

 may lead to dangerous contamination of such waters. 



' Buchner, Arch. f. Hyg., xvii, 1895. ' Huntemuller, Arch. f. Hyg., liv, 1905. 



