PURIFICATION OF WATER FOR DRINKING PURPOSES 127 



ments on filtration, I have already pointed out that 

 comparatively thin strata of various materials are capa- 

 ble of largely, and sometimes wholly, removing the 

 micro-organisms in the water passing through them, but 

 that this power is gradually lost ; it is only reasonable 

 to suppose that a thicker stratum will lose this power 

 less rapidly than a thinner one. In estimating the 

 thickness of the filtering stratum, the fine sand only 

 should be taken into consideration, as it is only this 

 portion of the filter which can have any effect in the 

 removal of micro-organisms. 



4 3. Influence of rate of filtration. That the removal 

 of micro-organisms is less perfect when the rate of 

 filtration is increased, and vice versa, has been illus- 

 trated by the results obtained in my laboratory experi- 

 ments, already referred to. 



'4. Influence of renewal of filter-beds. As already 

 pointed out, even the most perfect filtering media 

 sooner or later lose their power of retaining micro- 

 organisms, and hence the importance of frequent 

 renewal is sufficiently apparent. 



' In considering how the differences in these various 

 factors, which the statistics of the water companies ex- 

 hibit, may be expected to influence the results obtained 

 in the removal of the micro-organisms, attention must be 

 restricted to the five companies drawing water from the 

 Thames, as it is only these which have approximately 

 the same raw material to deal with ; for from the tables 

 it is seen that the amount of organic life found in the 

 Eiver Lea at the intake of the East London Company 

 is often very different from that in the Thames at 

 Hampton, and the difference in the case of the New 

 Eiver Company is doubtless even still greater, besides 

 the problem being there complicated by the admixture 

 of a very considerable proportion of deep-well water. 



