80 Bacteria in Relation to Country Life 



the protection of the public health. The older views 

 on the subject are represented in the statement of 

 Pettenkoffer on the one hand, and in that of the sixth 

 report of the Rivers' Pollution Commissioners of Great 

 Britain, on the other. Pettenkoffer believed nature 

 could be trusted to purify polluted river-waters, provided 

 the amount of sewage added does not exceed i^ of the 

 volume of the stream, and that the velocity of the latter 

 is fully as great as that of the stream of sewage. The 

 Commissioners held, in their report, that nature can 

 not be rehed upon to do the work properly, even when 

 the sewage is greatly diluted by pure river-water, and 

 that "it is impossible to say how far such water must 

 flow before the sewage matter becomes thoroughly 

 oxidized." Their inference was that no river in England 

 was long enough to assure the complete destruction 

 of sewage by oxidation. 



Modern views steer the middle course between these 

 opposing opinions. We have strong proof that the pro- 

 cess of self-purification is quite variable, even in the 

 same stream, and that the destruction of the polluting 

 substances may be rapid or slow according to conditions. 

 In the water there is (1) decomposition of the organic 

 matter by bacteria, (2) a removal of a portion of it by 

 sedimentation, (3) and the destruction of disease germs 

 by other bacteria, by sunlight, and by animalcules and 

 other creatures feeding upon them. Gradually, but 

 surely, the putrescible matter largely disappears from 

 the water, and, with it, great hosts of injurious and 

 harmless bacteria. The organic matter carried down by 

 sedimentation is, in its turn, attacked by the classes 



