THE STORY OF THE BACTERIA. 12$ 



longer observed and more definitely recog- 

 nized than in our own, legal enactments have 

 been long in force to prevent the pollution of 

 streams which might be sources of water sup- 

 ply of towns. But still large cities like Lon- 

 don and Berlin have found it necessary to 

 further protect themselves against disease-pro- 

 ducing organisms and against filth, by the 

 maintenance of filtering systems on the large 

 scale, by which the dangerous elements of a 

 contaminated water may be largely or en- 

 tirely removed. 



We should not forget that contaminated 

 water always tends to purify itself in certain 

 ways when exposed to the air in large volumes, 

 as in lakes or running streams. Nor should 

 we lose sight of the fact that a moderate 

 amount of sewage, when poured into a large 

 volume of water, becomes so considerably 

 diluted, that its dangerous elements are much 

 less numerous in any given glass or volume 

 of water than in sewage itself. But such con- 

 siderations can afford but little real consola- 

 tion to those who find themselves forced to 

 drink sewage, even though it be very largely 



