PURIFICATION OF WATER. 255 



reservoirs allows not only the living and dead matter 

 to subside, but allows time also for the pathogenic germs 

 to perish through light and antagonistic bacteria and 

 other deleterious influences. Filtration of water ex- 

 erts a very marked purification, taking out 99 per cent, 

 of the organisms in those best constructed and at least 

 90 per cent, in those commonly used in cities. The 

 construction of filters is too large a subject to enter on 

 minutely here; they consist, as a rule, of several layers, 

 beginning with fine sand, and then smaller and larger 

 gravel, and finally rough stones. A certain time elapses 

 before the best results are obtained; this seems to wait 

 for the formation of a film of organic material on the 

 sand, which is full of nitrifying bacteria. Even the 

 best filters only greatly diminish the dangers of pol- 

 luted water. Spring and well waters are, in fact, 

 filtered waters. 



Domestic Purification. Water which requires private 

 filtering should not be supplied for drinking purposes. 

 Unhappily, however, it often is. Filters may be divided, 

 roughly, into those for low and high pressure. The 

 former are directly connected with the water main, while 

 the others simply have the slight pressure of the column 

 of water standing in the filter. Many high-pressure 

 filters contain animal charcoal, silicated carbon, etc., 

 either in a pressed condition or in one porous mass. 

 These filters remove much of the deleterious matter 

 from the suspected waters, but the majority cannot be 

 depended upon to remove all bacteria. Even those 

 which are equipped for self -clean sing become in a little 

 while foul, and, if not cleaned, unfit for use. The best 

 of the class are the Berkefeld and Pasteur filters. 

 These yield a water, if too great pressure is not used, 



