rmed
by the condensation of the steam collects in the receiver E and is
known as _distilled_ water. Such water is practically pure, since the
impurities are nonvolatile and remain in the flask A.

~Commercial distillation.~ In preparing distilled water on a
large scale, the steam is generated in a boiler or other metal
container and condensed by passing it through a pipe made of
metal, generally tin. This pipe is wound into a spiral and is
surrounded by a current of cold water. Distilled water is used
by the chemist in almost all of his work. It is also used in
the manufacture of artificial ice and for drinking water.

~Fractional distillation.~ In preparing distilled water, it is
evident that if the natural water contains some substance which
is volatile its vapor will pass over and be condensed with the
steam, so that the distillate will not be pure water. Even such
mixtures, however, may generally be separated by repeated
distillation. Thus, if a mixture of water (boiling point 100 deg.)
and alcohol (boiling point 78 deg.) is distilled, the alcohol,
having the lower boiling point, tends to distill first,
followed by the water. The separation of the two is not
perfect, however, but may be made nearly so by repeated
distillations. The process of separating a mixture of volatile
substances by distillation is known as _fractional
distillation_.

2. _Filtration._ The process of distillation practically removes all
nonvolatile foreign matter, mineral as well as organic. In purifying
water for drinking purposes, however, it is only necessary to eliminate
the latter or to render it harmless. This is ordinarily done either by
filtration or boiling. In filtration the water is passed through some
medium which will retain the organic matter. Ordinary charcoal is a
porous substance and will condense within its pores the organic matter
in water if brought in contact with it. It is therefore well adapted to
the construction of filters. Such filters to be effective must be kept
clean, since it is evident that the charcoal is useless after its pores
are filled. A more effective type of filter is the Chamberlain-Pasteur
filter. In this the water is forced through a porous cylindrical cup,
the pores being so minute as to strain out the organic matter.

~City filtration beds.~ For purifying the water supply of cities,
large