filtration beds are prepared from sand and gravel, and
the water is allowed to filter through these. Some of the
impurities are strained out by the filter, while others are
decomposed by the action of certain kinds of bacteria present
in the sand. Fig. 25 shows a cross section of a portion of the
filter used in purifying the water supply of Philadelphia. The
water filters through the sand and gravel and passes into the
porous pipe A, from which it is pumped into the city mains.
The filters are covered to prevent the water from freezing in
cold weather.

[Illustration: Fig. 25]

3. _Boiling._ A simpler and equally efficient method for purifying water
for drinking purposes consists in boiling the water. It is the germs in
water that render it dangerous to health. These germs are living forms
of matter. If the water is boiled, the germs are killed and the water
rendered safe. While these germs are destroyed by heat, cold has little
effect upon them. Thus Dewar, in working with liquid hydrogen, exposed
some of these minute forms of life to the temperature of boiling
hydrogen (-252 deg.) without killing them.

~Self-purification of water.~ It has long been known that water
contaminated with organic matter tends to purify itself when exposed to
the air. This is due to the fact that the water takes up a small amount
of oxygen from the air, which gradually oxidizes the organic matter
present in the water. While water is undoubtedly purified in this way,
the method cannot be relied upon to purify a contaminated water so as to
render it safe for drinking purposes.

~Physical properties.~ Pure water is an odorless and tasteless liquid,
colorless in thin layers, but having a bluish tinge when observed
through a considerable thickness. It solidifies at 0 deg. and boils at 100 deg.
under the normal pressure of one atmosphere. If the pressure is
increased, the boiling point is raised. When water is cooled it steadily
contracts until the temperature of 4 deg. is reached: it then expands. Water
is remarkable for its ability to dissolve other substances, and is the
best solvent known. Solutions of solids in water are more frequently
employed in chemical work than are the solid substances, for chemical
action between substances goes on more readily when they are in solution
than it does when they are in the solid state.

~Chemical properties.~ Water is a very stable substance, or, in othe