BACTERIOLOGICAL EXAMINATION OF WATER 449 



known, since it depends on so many unknown factors and differs so 

 in different places. The typhoid bacilli probably rarely increase in 

 the soil and probably rarely survive a month in it The main danger 

 of soil bacteria is their being washed in water supplies by rains and 

 wind. 



Contamination and Purification of Drinking Waters. 



Brook-water and river-water are contaminated in two ways: through 

 chemicals, the waste products of manufacturing establishments, and 

 through harmful bacteria by the contents of drains, sewers, etc., the 

 latter method being by far the more dangerous. 



When water, which has been soiled by waste products of manufac- 

 tories only, becomes so diluted or purified that the contamination is 

 not noticeable to the senses and shows no dangerous products on chemi- 

 cal analysis, it is probably safe to drink. When sewage is the contami- 

 nation this rule no longer holds, and there may be no chemical impuri- 

 ties and no pathogenic bacteria found, and yet disease be produced. 

 That river-water which has been fouled by sewage will, in the course 

 of a few miles, through the dilution of additional supplies, through 

 sedimentation, and through oxidation, become greatly purified is an 

 indisputable fact. The increase in bacteria which occurs from con- 

 tamination 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 an absolutely safe water to 

 drink, although with the lapse of each day it becomes less and less 

 dangerous, nor will sand filter beds absolutely remove all danger. 

 These statements are founded upon the results of numerous investiga- 

 tions; 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 farther 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 purification 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 Law- 

 rence, nine miles below Lowell. It was estimated that the water took 

 ten days to pass from Lowell to Lawrence and through the reservoirs. 

 Typhoid bacilli usually die in river-water in from three to ten days, 

 but they may live for twenty-five days in other water; the Lawrence 

 epidemic is easily explained. Newark-on-Trent, England, averaged 

 seventy-five cases *a year from filtered water and only ten when it was 

 changed to deep-well supply. 



Purification of Water on a Large Scale. For detailed information 

 on this subject the reader is referred to works on hygiene. Surface 

 waters, if collected and held in sufficiently large lakes or reservoirs 

 usually beome so clarified by sedimentation, except shortly after heavy 

 rains, *as to require no further treatment so far as its appearance goes. 



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