70 BACTERIA 



The conditions influencing this phenomenon are as follows : 



{a) The Movement of the Water. It is probable, however, 

 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 oxidation processes. 



(U) The Pressure of the Water. It is believed that the 

 volume of water pressing down upon any given area be- 

 neath it weakens the vitality of certain microbes. In sup- 

 port 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 bot- 

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

 living organisms. 



{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. Arloing held that sunlight could 

 not pierce a layer of water an inch in thickness and still act 

 inimically on micro-organisms. But Buchner found that the 

 sun's rays could pass through fifteen or twenty inches and 

 yet be bactericidal. This evidence appears contradictory. 

 On the whole, however, authorities agree that the influence 

 of the sun*s rays upon water is distinctly bactericidal and 

 causes a marked diminution in the quantity of organisms 

 after acting for some hours. Especially will this be so when 

 the water is spread over a wide area and is therefore shallow 

 and stationary, or moving but slowly. 



((f) Vegetation in Water. Pettenkofer, in his observations 

 upon the Iser below Munich, has shown how algse bring 

 about a marked reduction in the organic matters present in 

 water. 



{e) Dilution, There can be no doubt in anyone's mind 

 that the pollutions passing into a flowing river are very soon 

 diluted with the large quantities of comparatively pure 

 water always forthcoming. But this, whilst it would lower 

 the percentage of impurity, cannot remove impurities. 



