136 



MICRO-ORaANISMS IN WATER 



previously ; the second sample was taken from the 

 filtrate of a bed which had been at work for twenty-one 

 days ; and the third sample was taken from the main 

 at the works, and is thus representative of the united 

 waters after filtration. 



Samples of unfiltered water were also taken from 

 the surface of the same filter beds ; the number of micro- 

 organisms found was very small for unfiltered Thames 

 water, and thus again points to the removal of micro- 

 organisms by subsidence, to which we have already 

 referred. 



Chelsea Waterworks, Thames Ditton, August 20, 1892 

 Filtered Water 



Thus the water from seven filter beds was examined, 

 and the series is particularly interesting in consequence 

 of the great length of time (76, 68, 54, and 40 days) 

 during which some of these beds had been at work. 

 There was no indication of their efficiency having become 

 impaired through this prolonged service. The number 

 of micro-organisms in the filtered water varied from 

 6 to 27, and averaged 17 in the cubic centimetre. The 



