254 BACTERIOLOGY. 



greatly purified is an indisputable fact. The increase in 

 bacteria which occurs from contamination is also largely 

 or entirely lost after ten to twenty miles of river flow. 

 Nevertheless, the history of many epidemics seems to 

 show that a badly contaminated river is never a safe 

 water to drink, although with the lapse of time it be- 

 comes less and less dangerous, nor will sand filter-beds 

 absolutely remove all danger. These statements are 

 founded upon the results of numerous investigations; 

 thus the marked disappearance of bacteria is illustrated 

 by the following : Kummel found below the town of 

 Rosbock 48,000 bacteria to the cubic centimetre ; 

 twenty-five kilometres further down the stream only 

 200 were present about the same number as before 

 the sewage of Rosbock entered. On the other hand, 

 the doubtful security of depending on a river purifi- 

 cation is proved by such experiences as the following: 

 In the city of Lowell, Massachusetts, an alarming 

 epidemic followed the pollution of the Merrimac River 

 three miles above by typhoid feces, and six weeks later 

 an alarming epidemic attacked Lawrence, nine miles 

 below Lowell. It was estimated that the water took 

 ten days to pass from Lowell to Lawrence and through 

 the reservoirs. As typhoid bacilli may live for twenty- 

 five days in water, the Lawrence epidemic is easily 

 explained. Newark-on-Treut, England, averaged 

 seventy-five cases a year from filtered water and only 

 ten when it was changed to deep-well supply. 



The Purification of Water on a Large Scale. Surface 

 waters, if collected and held in sufficiently large lakes 

 or reservoirs, usually become so clarified by sedimen- 

 tation as to require no further treatment so far as its 

 appearance goes. The collection of water in large 



