Report on the Bacteriology of Water, 



185 



more caution and reserve were exercised in this respect, the results 

 being tentatively chronicled without comment or deduction as to 

 purity. 



The prudence of this caution was soon apparent, when both in 

 England and in Germany it was discovered in 1885 that many forms 

 of micro-organisms could multiply to an astonishing extent in waters 

 of great organic purity, including distilled water itself. It is obvious 

 that this discovery at once subverted the artificial standards which 

 had been proposed, for, although a small number of micro-organisms 

 might frequently point to little or no contamination having taken 

 place, a large number could only under special circumstances be 

 viewed as conclusive evidence of proportionate contamination. 



Another question was, however, being simultaneously attacked, 

 both in this country and on the Continent, viz., the effect of various 

 processes of purification on the bacterial life present in water. This 

 question, which is of evident importance, could be investigated 

 with considerable precision by means of the gelatine-plate method, in 

 spite of its imperfections, for by applying the same mode of culture to 

 a particular water before and after purification, it is obvious that the 

 defects neutralise each other, and that a true differential result can 

 be obtained. In this manner, the value of a number of filtering- 

 materials has been ascertained, and the remarkable efficiency of 

 water- works sand- filtration for the first time established. Again, the 

 fact that practically all surface waters exhibit a large number of 

 microbes by the gelatine test, whilst deep well and spring waters 

 yield very few, or in some cases none, shows how perfect, as regards 

 removal of micro-organisms, must be the natural filtration which 

 water undergoes in traversing great depths of porous strata in the 

 earth. 



It has also been shown that in the subsidence of solid particles 

 often a surprisingly large proportion of the micro-organisms present 

 in water are carried to the bottom, a matter which is of par- 

 ticular interest and importance in connexion with the natural 

 purification of surface waters in rivers, lakes, and storage reservoirs, 

 as well as in the artificial treatment of water-supplies by Clark's 

 softening process, and other methods of precipitation. In applying 

 the gelatine test to these various purification processes it is, however, 

 essential, if an accurate result is to be obtained, that the water should 

 be examined immediately (within a few hours) after the purification 

 is completed, and before the number of microbes in the purified 

 water can have multiplied naturally. 



Another use to which the bacteriological m ethods have been applied 

 is to the actual discovery of pathogenic microbes in water, and, in 

 the opinion of some, this should indeed be the primary object of 

 bacteriology in connexion with water-supply. This view appears 



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