386 



APPLIED BACTERIOLOGY 



increase in numbers follows a correspondingly rapid decline, 

 so that the numbers again fall below those found in impure 

 surface-waters.' The above facts must be constantly before 

 one when interpreting the results yielded by the bacterio- 

 logical examination of a sample of water. 



A very large number of results showing the number of 

 bacteria contained in the various water-supplies of large 

 towns, both in England and abroad, have been pub- 

 lished by various investigators, but they have but little 

 practical importance. The following table is of interest. 

 It contains some results recently obtained by Frank- 

 land on the water of the rivers Thames and Lea, both 

 before and after filtration, during twelve months. The 

 results are of interest, as showing the monthly variations 

 during the year of the bacterial contents of the water 

 supplied by the London water companies. They also show 

 the great value a bacteriological examination of a water 

 has in showing if the filter-beds are working efiiciently, 

 as it has been shown that all the bacteria can be removed 

 from even a very impure water by proper filtration. The 

 following table shows the number of organisms per c.c. : 



