be found at the bottom of
the vessel. Because of this fact, it is possible to purify water in a
very simple way. Place over a fire a large kettle closed except for a
spout which is long enough to reach across the stove and dip into a
bottle. As the liquid boils, steam escapes through the spout, and on
reaching the cold bottle condenses and drops into the bottle as pure
water. The impurities remain behind in the kettle. Water freed from
impurities in this way is called _distilled water_, and the process is
called _distillation_ (Fig. 19). By this method, the salt water of the
ocean may be separated into pure drinking water and salt, and many of
the large ocean liners distill from the briny deep all the drinking
water used on their ocean voyages.

[Illustration: FIG. 19.--In order that the steam which passes through
the coiled tube may be quickly cooled and condensed, cold water is
made to circulate around the coil. The condensed steam escapes at
_w_.]

Commercially, distillation is a very important process. Turpentine,
for example, is made by distilling the sap of pine trees. Incisions
are cut in the bark of the long-leaf pine trees, and these serve as
channels for the escape of crude resin. This crude liquid is collected
in barrels and taken to a distillery, where it is distilled into
turpentine and rosin. The turpentine is the product which passes off
as vapor, and the rosin is the mass left in the boiler after the
distillation of the turpentine.

26. Evaporation. If a stopper is left off a cologne bottle, the
contents of the bottle will slowly evaporate; if a dish of water is
placed out of doors on a hot day, evaporation occurs very rapidly. The
liquids which have disappeared from the bottle and the dish have
passed into the surrounding air in the form of vapor. In Section 20,
we saw that water could not pass into vapor without the addition of
heat; now the heat necessary for the evaporation of the cologne and
water was taken from the air, leaving it slightly cooler. If wet hands
are not dried with a towel, but are left to dry by evaporation, heat
is taken from the hand in the process, leaving a sensation of
coolness. Damp clothing should never be worn, because the moisture in
it tends to evaporate at the expense of the bodily heat, and this
undue loss of heat from the body produces chills. After a bath the
body should be well rubbed, otherwise evaporation occurs at the
expense of heat which the body cannot ordin