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 oftenis. 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-cleansing 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, 
