298 A MANUAL OF BACTERIOLOGY 



ones.' With all due respect to such an authority as 

 Yon Pettenkofer, 'we learn that ordinary sewage 

 forms a suitable medium not only for the indefinite 

 preservation of some pathogenic microbes, but also, 

 in some cases, for their rapid growth and multipli- 

 cation.' 



It is well known that surface waters (e.g. rivers, 

 ponds, etc.) are rich in microbial life ; but waters, 

 derived from deep wells and springs, which have 

 undergone natural filtration through porous strata, 

 contain only few microbes. Frankland has shown 

 that ' this removal of microbes from water also takes 

 place in a very marked manner when it is submitted 

 to some kinds of artificial filtration, such as that 

 through very finely-divided coke or charcoal, as well 

 as in the filtration of water on the large scale through 

 sand.' A glance at Frankland's table (p. 294) shows 

 the great reduction in the number of microbes 

 present in the water obtained from the Thames and 

 the Lea, after filtration through fine sand. But, 

 according to Frankland, the following factors are 

 calculated to influence the number of microbes 

 present in the distributed water : 



(a) Storage capacity for unfiltered water. 

 (6) Thickness of fine sand used in filtration. 



(c) Rate of filtration. 



(d) Renewal of filter-beds. 



(a) Through greater storage capacity, the neces- 

 sity of drawing the worst water from the river is 

 avoided, a matter which in the case of a stream like 

 the Thames, liable to frequent floods, is of great 

 importance. During the period of storage the water 



