i 7 4 SCIENCE PROGRESS. 



the year 1894. ^ n this diagram the black squares represent 

 the number of microbes in a given volume of the raw water 

 in each month, and the white centres the number remaining 

 in the same volume after filtration. 



Although deep well water has, from a bacterial point 

 of view, a decided advantage, the filtered river waters 

 are not very far behind, and there is every reason to believe 

 that with the improvements which are now being carried out 

 by the various river Water Companies, the Kent Company's 

 deep well water will, before long, be run very hard by the 

 other supplies. 



By the examination of the water as it issues from the 

 filters, the utmost freedom from microbes, or maximum 

 degree of sterility of each sample is determined. This 

 utmost freedom from bacterial life after all sources of con- 

 tamination have been passed is obviously the most 

 important moment in the history of the water ; for the 

 smaller the number of microbes found in a given volume at 

 that moment the less is the probability of pathogenic or 

 harmful organisms being present ; and although the non- 

 pathogenic may afterwards multiply indefinitely this is of 

 no consequence in the primary absence of the pathogenic ; 

 but it is only fair, in describing the character of the present 

 water supply of London, to say that not a single pathogenic 

 organism has ever been discovered even in the ^filtered 

 water as it enters the intakes of the various Companies, 

 although these organisms have been carefully sought for. 

 It is sometimes said that the non-pathogenic organisms 

 found in water may be beneficial to man ; but this idea is 

 not borne out by the fact of their entire absence from the 

 food which nature provides for young animals. Healthy 

 milk is absolutely sterile. 



As it is at present impracticable to obtain water, uni- 

 formly at least, free from microbes, it is desirable to adopt 

 some standard of bacterial purity ; and 100 microbes per 

 cubic centimetre has been fixed upon by Dr. Koch and 

 myself as the maximum number allowable in potable water. 

 This standard is very rarely infringed by the London 

 Water Companies, whilst I have every reason to hope that, 



