64 BACTERIA IN WATER 



number and character of the bacteria at different stages of the 

 river (e.g., see Table of Bacteria in Severn, p. 38). Jordan has 

 pointed out the peculiar value of the reduction of B. coli.* 



We may here refer in passing to the facts obtainable from the 

 late Sir Edward Frankland's report on Metropolitan water supply in 

 1894, as they will afford a connecting link between natural purifica- 

 tion and artificial purification. First, judged by the relatively low 

 proportion of carbon to nitrogen, the organic matter present in the 

 water was, as usual, found to be chiefly of vegetable origin. Secondly, 

 an immense destruction of bacteria was effected by storage in 

 subsidence reservoirs. Thirdly, the bacterial quality of the water 

 might differ widely from its chemical qualities. It is, of course, a 

 much finer index of pollution. These three facts are of primary 

 importance in the interpretation of water reports, and it will be well 

 to bear them in mind. Sir E. Frankland also referred to the physical 

 conditions affecting microbial life in river waters, and, as in previous 

 reports, to the importance of changes of temperature, the effect of 

 sunlight, and rate of flow. Eespecting the relative proportion of 

 these factors, he wrote : " The number of microbes in Thames water is 

 determined mainly by the flow of the river, or, in other words, by 

 the rainfall, and but slightly, if at all, by either the presence or 

 absence of sunshine, or a high or low temperature. With regard to 

 the effect of sunshine, the interesting researches of Dr Marshall 

 Ward leave no doubt that this agent is a powerful germicide, but it 

 is probable that the germicidal effect is greatly diminished, if not 

 entirely prevented, when the solar rays have to pass through even 

 a comparatively thin stratum of water before they reach the living 

 organisms." Subsequent investigations have confirmed the im- 

 portance of these broad principles, and from which it is clear that 

 evidence favours the effect of sedimentation and dilution. These two 

 factors in conjunction with filtration are, practically speaking, the 

 methods of artificial water purification, to which reference will now 

 be made. 



Artificial Purification of Water 



Sedimentation and Precipitation. In nature we see this 

 factor in operation in lakes and reservoirs. For example, the water 

 supply of Glasgow is the untreated overflow from Loch Katrine. 

 Purification has been brought about by means of subsidence of 

 impurities. Nothing further is needed. Much of the purification 

 obtained in reservoirs supplying large towns is due to the same 

 factor. Artificially we find it is this factor which is the mechanical 

 purifier of biological impurity in such methods as Clark's process. 

 By this mode "temporary hardness," or that due to soluble 



* Jour. ofHyg., 1901, p. 293. 



