The Bacteria in Natural Waters. 21 



It is obvious that the efficiency of all the agencies which 

 tend to decrease the number of bacteria in surface waters 

 will increase with the prolongation of the period for which 

 they act. Time is the great measure of self-purification. 

 The longer the storage period, the greater the improve- 

 ment.* 



* The absolute time necessary to remove disease bacteria and render 

 a polluted water safe for drinking is impossible to fix with any cer- 

 tainty. Food supply, light, temperature and the activity of other living 

 forms vary widely, and in deposited material conditions are different 

 from those which obtain in the water itself. Jordan, Russell and Zeit 

 (1904), in an important series of experiments, added typhoid bacilli to 

 the unsterilized waters of Lake Michigan, the Chicago River and Drain- 

 age Canal and the Illinois River, in collodion sacs suspended in the 

 respective bodies of water. From the relatively pure Lake Michigan 

 water the specific organisms could be isolated for at least a week, but in 

 the polluted waters of the rivers and the Drainage Canal they were not 

 found after three days except in a single instance. Russell and Fuller, 

 (1906) confirmed these general results, finding that typhoid bacilli 

 would live for ten days in the unsterilized water of Lake Mendota while 

 they could isolate them only after five days when immersed in sewage. 

 Other observers record much greater viability for the typhoid bacillus. 

 Savage (1905) added a heavy dose of the organism to unsterilized tidal 

 mud and found it living after five weeks. Hoffmann (1905), after 

 inoculating a large aquarium with a rich typhoid culture, was able to 

 isolate the germ from the water after four weeks and from the mud at 

 the bottom after two months. Konrddi (1904) reports the persistence 

 of typhoid bacilli in unsterilized tap water for over a year. 



Under certain conditions it is even possible for intestinal bacteria to 

 multiply in water. At Harrisburg, Pa., a series of B. coli examina- 

 tions made in the midsummer of 1906 showed positive results in 7 per 

 cent of the -samples of water entering the storage reservoir and in 27 

 per cent of the samples leaving it. The storage period in this case was 

 about two days and the temperature of the water in the reservoir nearly 

 at blood heat (Harrisburg, 1907). It is improbable that true patho- 

 genic germs like the typhoid bacillus could increase even under such 

 exceptional conditions. 



