60 BACTERIA IN WATER 



approving a water supply it is imjoortant to take all the findings of 

 chemistry, hacteriology, and topography into consideration. The whole 

 history of the sample must be considered, and too much reliance 

 must not be placed upon the mere presence or absence of B. coli, or 

 any single phenomenon or reaction. No ultimate reliance should, as 

 a rule, be placed upon any single test. 



Natural Purifleation of Water 



We have already noticed that rivers purify themselves as they 

 proceed. There are many excellent examples of such self- 

 purification. The Seine as it runs through Paris becomes highly 

 polluted with every sort of filthy contamination. It receives daily 

 about 250,000 cm. of sewage. But 20 or 30 miles below the city it 

 is found to be even purer than above the city before it received the 

 sewage. In small rivers it is the same, provided the pollution is less 

 in amount. The Thames and the Severn are excellent examples. 

 Whilst authorities differ with regard to the means of self-purification 

 which operate most effectually, all agree that in some way rivers 

 receiving crude sewage are able in a marvellous degree to become 

 pure again. 



The chief conditions influencing this phenomenon are as follow : — 



(a) The movement of the water. — It is probable that any beneficial 

 result accruing from this cause is due not to any mechanical factor in 

 the movement, but to the extra surface of water available for oxida- 

 tion processes. Delepine has shown that the effect of agitation is an 

 increase in the number of suspended bacteria which he attributes to 

 the dislodgment of deposit and side adhesions. The greatest amount 

 of purification in his experiments occurred when the rate of flow was 

 about 8 cm. per hour.* 



(&) The pressure of the water. — It is believed that the volume of 

 water pressing down upon any given area beneath it weakens the 

 vitality of "certain microbes. In support of this theory, it is urged 

 that the number of bacteria capable of developing is less the greater 

 the depth from the surface. Yet it must be remembered that mud at 

 the bottom of a river, or at the bottom of shallow sea, is teeming with 

 living organisms, and there is no evidence to show that pressure in 

 river water ever reaches a degree capable of affecting the life of 

 bacteria. Delepine found that in the Manchester mains increase of 

 pressure did not reduce the number of bacteria.f 



(c) Light. — We have seen how prejudicial is light to the growth 

 of organisms in culture media. This is so, though to a less extent, 

 in water (see p. 18). Arloing held that sunlight could not pierce 



* The Natural Purification of Running Water, .Tour, of titate Med., 1901, p. 517. 

 f Report to the Manchester Water Wm-ks Committee, 1894. 



