74 BACTERIA 



alone is rarely sufficient to purify water. It is true that the 

 collection of water in large reservoirs permits subsidence of 

 suspended matters, and affords time for the action of light 

 and the competitive suicidal behaviour of the common water 

 bacteria. Yet, after all, filtration is the most important 

 and most reliable method. 



Sand Filtration, as a means of purifying water, has been 

 practised since the early part of the present century. But 

 it was not till 1885 that Percy Frankland first demonstrated 

 the great difference in bacterial content between a water un- 

 filtered and a water which had passed through a sand filter. 

 Previous to this time the criterion of efficiency in water puri- 

 fication had been a chemical one only, and the presence or 

 absence of bacteria in any appreciable quantity was described, 

 not in mathematical terms, but in indefinite descriptive 

 words, like " turbid," "cloudy," etc. It is needless to say 

 that this difference in estimation was due to the introduction 

 by Koch of the gelatine-plate method of examination. As 

 a result of Percy Frankland's work, he formulated the fol- 

 lowing conclusions as regards the chief factors influencing 

 the number of microbes passing through the filter. 



It depends upon: 



(1) The Storage Capacity for Unfiltered Water. This, of 

 course, has reference to the advantages, which we have 

 noticed above, of securing a large collection of water pre- 

 vious to filtration for subsidence, etc. 



(2) The Thickness of Fine Sand through which Filtration is 

 Carried on. An argument needing no further support, for 

 it is clear, other things being equal, the more sand water 

 passes through the greater the opportunity of leaving its 

 impurities behind. 



(3) Rate of Filtration. The slower filtration will be gen- 

 erally the more complete in its results. 



(4) Renewal of Filter-Beds. After a certain time the filter- 

 bed becomes worn out and inefficient ; at such times renewal 



