SEWAGE PURIFICATION 309 



very little organic impurity — so that, on the results of chemical 

 analysis alone, it might be passed as satisfactory, and yet 

 reveal the presence of B. coli vrhen examined bacteriologically 

 — yet in the author's experience, if a series of comparative 

 samples are taken, and the analysis carried out with special 

 care, the chemical and bacteriological indications are usually 

 of the same character, and the conditions which tend to improve 

 the chemical composition of the water, tend also to the removal 

 or diminution of dangerous organisms. 



Thus, e.g., Houston has shown that prolonged storage 

 tends gradually to decrease the number of organisms present 

 in a water supply, and especially the less resistant organisms 

 such as the typhoid bacillus. There is no doubt that the 

 number of bacteria decreases as the amount of pabulum 

 diminishes, and vice versa. Eecent experiments in France by 

 Miquel and Mouchet have shown that the impurities in water 

 can be oxidised by spraying over filters worked on similar 

 principles to the sewage trickhng filter, but of course with 

 material of smaller dimensions. With the chemical improve 

 ment of the water, there is again diminution in its bacterial 

 content, but an extraordinary increase in the number of 

 organisms takes place if the filters are dosed with a solution of 

 peptone. 



Besides the gradual destruction of their pabulum which 

 takes place on storage, the effect of sunlight is of great import- 

 ance in diminishing the number of bacteria present, especially 

 certain kinds, and those the more dangerous. This aspect of 

 the matter has been dealt with by Major W. W. Clemesha, in 

 his extensive study of ' the bacteriology of drinking water 

 supplies in tropical countries, undertaken particularly in 

 reference to the water supphes of Madras. He endeavoured 

 in his researches to differentiate the various organisms 

 alKed to Bacillus coli, by an extension of the method sug- 

 gested by MacConkey, who divided fsecal baciUi into four 

 groups : — 



