72 BACTERIA 



tion of rivers, but we may take it that oxidation, dilution, 

 and sedimentation are three of the principal agencies. 



We may here digress to refer in passing to the facts ob- 

 tainable from Sir Edward Frankland's report on Metropoli- 

 tan water supply in 1894, as they will afford a connecting link 

 between self purification and artificial purification. Judged 

 by the relatively low proportion of carbon to nitrogen, the 

 organic matter present in the water was, as usual, found to 

 be chiefly, if not entirely, of vegetable origin. An immense 

 destruction of bacteria was found to be effected by storage 

 in subsidence reservoirs. The bacterial quality of the water 

 might differ widely from its chemical qualities. 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 refers to the physical conditions 

 affecting microbial life in river waters. The importance of 

 changes of temperature, the effect of sunlight, and rate of 

 fiow had been referred to in previous reports. Respecting 

 the relative proportion of these factors, he adds: 



** The number of microbes in Thames water is determined 

 mainly by the flow of the river, or, in other words, by the rain- 

 fall, 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 

 a comparatively thin stratum of water before they reach the living 

 organisms." 



From which it is clear that evidence favours the effect of 

 sedimentation and dilution. These two factors in conjunc- 

 tion with filtration are, practically speaking, the methods of 

 artificial water purification, with which we are now in a 

 position to deal. 



