NATURAL PURIFICATION OF WATER 61 



a layer of water an inch in thickness and still act inirnically on 

 micro-organisms. But Buchuer found that the sun's rays could 

 pass through 15 or 20 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 in some degree 

 bactericidal and causes a diminution in the quantities of organisms 

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

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

 stationary, or moving but slowly.* But taken as a whole it may 

 be said that light does not exert a marked influence in water puri- 

 fication. There is, on the other hand, evidence to prove that water 

 in its passage through dark mains of various sizes gradually becomes 

 deprived of a great part of its bacterial contents. 



(d) 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. Boyce has 

 pointed out that in the river Severn, in addition to the temperature 

 and movement being unfavourable to B. coli and presumably patho- 

 genic bacteria, that (a) lack of pabulum, and (&) antagonism due to 

 the fauna and flora of the river exert an unfavourable influence upon 

 these bacteria. The organic matter so abundant when the river 

 becomes polluted at Shrewsbury is diluted and destroyed lower down 

 stream, and therefore the water becomes purified of bacteria living 

 on the organic matter. Fish, birds, rats, protozoa, and forms of river 

 life generally contribute their share to the consumption of organic 

 pabulum. The water Ranunculus, Spkcerotilus, Lcptomitus lactcus, 

 sewage fungi, chlorophyll containing protophytes, and river plants 

 generally assist in the destruction of organic matter and bacteria.f 



(e) Dilution. The pollutions passing into a flowing river are very 

 soon diluted with the large quantities of comparatively pure water 

 always forthcoming. And this, whilst it lowers the percentage of 

 impurity, also raises the percentage of oxygenated water. Delepine 

 has pointed out as a result of artificial experiments that dilution 

 exerts a double effect on the bacterial content of water. In the first 

 place it has the mechanical effect of increasing the space occupied by 

 a definite number of bacteria, and in the second place it causes a 

 diminution in the amount of pabulum present in a given bulk of the 

 impure fluid. Dilution and deposition acting together exert a power- 

 ful influence as purifiers. Clark and Gage of the Lawrence station 

 pointed out in 1903 that the number of B. coli in a polluted river 

 varies in inverse ratio with the dilution of the entering sewage by 

 the river water, and is affected by the temperature, the number of 

 B. coli being larger during the warm weather than in the cold. In 



* See also Spitta's work on the Spree at Berlin, Arcliw fiir Hyg., vol. xxxviii. 

 t Roy. Com. on Sewage Disposal, Second Report, 1902, pp. 104-109. 



