Rain water is relied upon in some localities for drinking purposes. That
collected in cities and in the vicinity of barns and dwellings contains
appreciable amounts of organic impurities. The brown color is due to the
impurities, ammonium carbonate being one of these. There are also traces
of nitrates and nitrites obtained from the air. When used for drinking,
rain water should be boiled.

270. Improvement of Waters.--Waters are improved by: (1) boiling,
which destroys the disease-producing organisms; (2) filtration, which
removes the materials mechanically suspended in the water; and (3)
distillation, which eliminates the impurities in suspension and
solution, as well as destroys all germ life.

271. Boiling Water.--In order to destroy the bacteria that may be in
drinking water, it is not sufficient to heat the water or merely let it
come to a boil. It has been found that if water is only partially
sterilized and then cooled in the open air, the bacteria develop more
rapidly than if the water had not been heated at all. It should boil
vigorously five to ten minutes; cholera and typhoid bacteria succumb in
five minutes or less. Care should be taken in cooling that the water is
not exposed to dust particles from the air nor placed in open vessels in
a dirty refrigerator. It should be kept in perfectly clean,
tight-stoppered bottles. These bottles should be frequently scalded.
Great reliance may be placed upon this method of water purification when
properly carried out.

272. Filtration.--Among the most efficient forms of water filters are
the Berkefeld and Pasteur. The Pasteur filter is made of unglazed
porcelain, and the Berkefeld of fine infusorial earth (finely divided
SiO_{2}). Both are porous and allow a moderately rapid flow of water.
The flow from the Berkefeld filter is more rapid than from the Pasteur.
The mechanical impurities of the water are deposited upon the filtering
surface, due to the attraction which the material has for particles in
suspension. These particles usually are the sources of contamination and
carry bacteria. When first used, filters are satisfactory, but unless
carefully looked after they soon lose their ability to remove germs from
the water and may increase the impurity by accumulation. Small faucet
filters are made of porous stone, asbestos, charcoal, etc. Many of them
are of no value whatever or are even worse than valueless. Filters
should be frequently cleansed in boiling water or