20 BACTERIOLOGICAL EXAMINATION OF WATER. 



sedimentation of different species. Very few observations have 

 been made on the sedimentation of bacteria in large masses of 

 water. Large lakes, away from the shore, appear to contain 

 very few micro-organisms ; yet during rain large numbers of 

 bacteria are brought down by rivers, and the mud at the bottom 

 of lakes is found to be very rich in micro-organisms. 



Frankland's experiments with Thames water at the works of 

 the West Middlesex Company show the effects produced by 

 storage extremely well. Samples were collected (a) from the 

 Thames water at Hampton; (b) from the same water after 

 passing through one storage reservoir; and (c) after passing 

 through two storage reservoirs. The following results were 

 obtained : 



No. of organisms 

 per n.c. 



(#) Thames water at Hampton 1.437 



(&) after passing through one reservoir . 318 



(c) ,. two reservoirs . 177 



It thus appears probable that a water after receiving a large 

 number of micro-organisms will, in a very short time, show a 

 great diminution in its bacterial contents. But sedimentation 

 is not the only factor in the change ; currents, oxygen require- 

 ments, inorganic and organic matters sinking to the bottom, all 

 exercise great influence. Moreover many of the bacteria are 

 not natural inhabitants of water but are derived from earth, 

 house-washings, &c., which are rich in nutritive material. 

 Consequently, with the sinking of suspended material and the 

 loss of food material, these foreign bacteria may quickly die out. 



The self-purification of water may be due to the following 

 causes : 



(1) To the death of those bacteria which either do not belong 

 to the so-called water-bacteria or are sensitive to the influence 

 of light. 



(2) To a sedimentation of non-motile microbes and the spores 

 of motile bacteria. 



(3) To a spontaneous sinking of bacteria with solid materials 

 which form nutritive centres. 



(4) To a mechanical sinking of bacteria with the subsidence of 

 heavy materials. 



