BACTERIA IN WATER 7 1 



(/) Sedimentation. Whilst Pettenkofer attributes self- 

 purification to oxygenation and vegetation, most authorities 

 are now agreed that it is largely brought about by the sub- 

 sidence of impure matters, and by their subsequent dis- 

 integration at the bottom of the river. Sedimentation 

 obviously is greatest in still waters. Hence lake water 

 contains as a rule very few bacteria. ** The improvement 

 in water during subsidence is the more rapid and pronounced 

 the greater the amount of suspended matter initially pre- 

 sent " (Frankland). Tils has pointed out that the number 

 of micro-organisms was invariably smaller in the water col- 

 lected from the reservoir than in that taken from the source 

 supplying the latter. Percy Frankland has demonstrated 

 the same effect of sedimentation by storage as follows : 



No. of Colonies in 

 I cc. of water. 



1. Intake from Thames, June 25, 1892 i)99i 



2. First small storage reservoir i,703 



3. Second " " 1,156 



4. Large storage reservoir 464 



The large reservoir would of course necessitate a prolonged 

 subsidence, and hence a greater diminution than in the small 

 reservoirs. Many like examples might be cited, but a typi- 

 cal one such as the above will suffice. 



(^g) Oxidation, Many experiments and observations have 

 been made to prove that large quantities of oxygen are used 

 up daily in oxygenising the Thames water. Oxygenated 

 water will come up with the tide and down with the fresh 

 water from above London. There will also be oxygen ab- 

 sorption going on upon the surface of the water, and from 

 these three sources enough oxygen is obtained to oxidise 

 impurities and produce what is really an effluent. In many 

 smaller streams the opportunity for oxidation is afTorded by 

 weirs and falls. 



Probably all these factors play a part in the self-purifica- 



