BACTERIA IN WATER 77 



To maintain this nitrifying action of a filter in efficiency, 

 Koch suggested, in the second place, that the rate of filtra- 

 tion must not exceed four inches per hour. At the Altona 

 water-works this rate of filtration was maintained, and the 

 number of organisms always remained below 100 per cc., 

 which, as we have seen, is the standard. Thirdly, it is im- 

 portant that periodic bacteriological examinations should be 

 made. Koch's emphasis upon this point is well known, and 

 the cumulative experience of bacteriologists only further 

 supports such a course being taken. If it be true that 

 efficient sand filtration is a safeguard against pathogenic 

 germs like typhoid and cholera, then there can be but one 

 criterion of efficiency, viz., their absence in the filtered 

 water, which can only be ascertained by regular examina- 

 tion. But it is not alone for pathogenic germs that filtration 

 is proposed. Filtered water containing more than 100 

 micro-organisms of any kind per cc. is below the standard 

 in purity, and should on no account be distributed for drink- 

 ing purposes. In this country chemical analysis, with a 

 more or less cursory microscopic examination, has been 

 almost invariably accepted as reliable indication of the con- 

 dition of the water. But such an examination is not really 

 any more a fair test of the working of the filter than it is of 

 the actual condition of the water. It is true, the quantity 

 of organic matter can be estimated and the condition in 

 which it exists in combination obtained ; but it cannot tell 

 us what a bacteriological examination can tell us, viz., the 

 quantity and quality of living micro-organisms present in 

 the water. Upon this fact, after all, an accurate conclusion 

 depends. There is abundant evidence to show that no 

 valuable opinion can be passed upon a water except by both 

 a chemical and a bacteriological examination, and further 

 by a personal investigation, outside the laboratory, of the 

 origin of the water and its liabilities to pollution.' 



So convinced was Koch of the efficiency of sand filtration 



