118 MICRO-ORGANISMS IN WATER 



the case, is not objectionable on account of the absolute 

 quantity of organic matter, as revealed by chemical 

 analysis, which it contains, but only because of the 

 suspicious origin of a part of this organic matter, then 

 it is evident that in the purification of such water 

 the point to be taken primarily into consideration is 

 how the organic life it contains can be reduced to a 

 minimum. 



In estimating the value of such processes of purifica- 

 tion, it has hitherto been customary to assume that 

 those processes which effect the greatest chemical im- 

 provement in water may also safely be considered to be 

 biologically the most excellent ; and, conversely, that 

 those processes which effect little or no reduction in the 

 proportion of organic impurity are not calculated to be 

 of any service in removing organized matters. 



The following chemical analyses of river-water 

 before and after sand-filtration will sufficiently explain 

 how, for example, the process of sand-filtration found com- 

 paratively little favour as long as the chemical analysis 

 was the principal basis on which a judgment as to the 

 hygienic value of water-filtration could be formed. 



Chemical Analysis of Water of River Ouse before and after 



X< i nd- filtration (Percy Frankland) 

 Results of Analysis expressed in parts -per 100,000 



River Water 



Ditto after 

 Sand-filtration 



Total solid matters 28-40 26-20 



Organic carbon .... -123 -119 



nitrogen | -025 -022 



Ammonia 



Nitrogen as nitrates and nitrites . . -077 -089 



Total combined nitrogen . . . -102 -111 



Chlorine ....... 1-6 1-6 



(Temporary .... 11-5 10.9 



Permanent .... 7-1 7-1 



Total . 18-6 18-0 



Such slight chemical improvement as is here shown 



