There is, however, another reason for the rise in temperature.  If a large pail of hot soup is placed in a larger pail of cold water, the 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 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.