126 MICRO-OEGANISMS IN WATER 



' The factors which, in my opinion,^ are more espe- 

 cially calculated to influence the number of micro- 

 organisms present in the distributed water are the 

 following : — 



' 1. Storage capacity for unfiltered water. 



' 2. Thickness of fine sand through which filtration 



is carried on. 

 '3. Eate of filtration. 

 ' 4. Renewal of filter-beds. 



' 1. Influence of storage capacity for unfiltered water. — 

 The influence which this factor may exercise upon the 

 organised matter in water is manifold. In the first 

 place, through greater storage capacity, the necessity 

 of drawing the worst water from the river is avoided, 

 a matter which in the case of a river like the Thames, 

 which is liable to frequent floods, is of great import- 

 ance. During the period of storage subsidence takes 

 place, the water becoming poorer in suspended parti- 

 cles of all kinds. Again, in these storage reservoirs a 

 process of starvation may go on, for the organisms 

 present in the impounded water find themselves im- 

 prisoned with a limited amount of sustenance, which 

 they rapidly exhaust and then perish in large numbers, 

 falling to the bottom. This phenomenon is sufficiently 

 familiar to all who have made the cultivation of micro- 

 organisms a subject of study, 



' 2. Influence of thickness of fine sand. — That the 

 thickness of the filtering stratum should exercise an im- 

 portant influence on the number of micro-organisms 

 passing through the filter will be sufficiently obvious 

 to everyone. In referring to my laboratory experi- 



^ ' Water-purification, its Biological and Chemical Basis,' Percy 

 Frankland. Trmisactio7is of Institution of Civil Engineers^ 1886. 



