soup will gradually cool and the cold water will gradually become
warmer. A red-hot iron placed on a stand gradually cools, but warms
the stand. A hot body loses heat so long as a cooler body is near it;
the cold object is heated at the expense of the warmer object, and one
loses heat and the other gains heat until the temperature of both is
the same. Now the hot water in the tub gradually loses heat and the
cold air of the room gradually gains heat by convection, but the
amount given the room by convection is relatively small compared with
the large amount set free by the condensing steam.

25. Distillation. If impure, muddy water is boiled, drops of water
will collect on a cold plate held in the path of the steam, but the
drops will be clear and pure. When impure water is boiled, the steam
from it does not contain any of the impurities because these are left
behind in the vessel. If all the water were allowed to boil away, a
layer of mud or of other impurities would 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