THE WATER SUPPLY OF LONDON. 



171 



such as cholera and typhoid fever through the agency of 

 drinking water has no connection whatever with the chemical 

 or organic purity of the water. These diseases are propa- 

 gated by living organisms of extreme minuteness, to which 

 the names bacilli, bacteria, and microbes have been given, 

 and here comes the important question how, if at all, does 

 filtration secure immunity from these water-borne diseases ? 

 To Dr. Koch of Berlin, we are indebted for the answer 

 to this question. By his discovery of a means of isolating 

 and counting the number of bacteria, or bacilli, or microbes 

 and their spores in a given volume of water, we were, for 

 the first time, put into possession of a method by which the 

 condition of water as regards these living organisms, before 



PROPORTIONAL AMOUNT OF ORGANIC ELEMENTS 

 IN NEW RIVER AND DEEP-WELL WATERS. 



1835 



so 



No. 5. 



and after filtration, can be determined with quantitative 

 exactness. The enormous importance of this invention 

 (which was first made known and practised in England in 

 1882 by the late Dr. Angus Smith) is evident, when it is 

 borne in mind that the living organisms, harmful or harm- 

 less, contained in water are of such extreme minuteness as 

 practically to defy detection by ordinary microscopical 

 examination. But although the microscope cannot detect 

 with certainty single bacteria or their spores, even the 

 naked eye can easily discern towns or colonies consisting of 

 thousands or even millions of such inhabitants. 



Dr. Koch's method accomplishes at once two things : 

 it isolates, in the first place, each individual microbe or 



