QUANTITATIVE BACTERIOLOGICAL EXAMINATION. 23 



certain species dying, and the sample, therefore, not being 

 a fair average of the water in the mains. 



If a sample is to be taken from a pump similar pre- 

 cautions are necessary. The pump should be in con- 

 tinuous operation for several minutes at least, and prefer- 

 ably for half an hour before the sample is taken, in order 

 to avoid excessively high numbers due to the growth of 

 bacteria within the well and pump, the bacterial condition 

 of the water as it passes through the ground being what 

 we wish to determine. Thus Heraeus (Heraeus, 1886) in 

 a well-water which had been but little used during the 

 preceding thirty-six hours found 5000 organisms per c.c.; 

 when the well was emptied by continuous pumping a 

 second sample, after an interval of half an hour, gave only 

 35. Maschek (Tiemann and Gartner, 1889) obtained 

 similar results shown in the following table: 



EFFECT OF PUMPING ON THE BACTERIAL CONTENT OF WELL-WATER. 



Well-water after continuous pumping for fifteen minutes 458 



" " " " many hours 140 



later 68 



after continuous pumping for fifteen minutes 578 



" " " " many hours 179 



later 73 



After a proper interval of pumping the sample of a well- 

 water may be collected from the pet- cock of the pump or 

 from a near-by tap. With a hand-pump, as in sampling 

 domestic shallow wells, the water is, of course, pumped 

 directly into the sample bottle. The difficulties in 

 securing an average sample from this latter source are 



