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 ordinarily afford to lose.

Evaporation is a slow process occurring at all times; it is hastened during the summer, because of the large amount of heat present in the atmosphere.  Many large cities make use of the cooling effect of evaporation to lower the temperature of the air in summer; streets are sprinkled not only to lay the dust, but in order that the surrounding air may be cooled by the evaporation of the water.

Some thrifty housewives economize by utilizing the cooling effects of evaporation.  Butter, cheese, and other foods sensitive to heat are placed in porous vessels wrapped in wet cloths.  Rapid evaporation of the water from the wet cloths keeps the contents of the jars cool, and that without expense other than the muscular energy needed for wetting the cloths frequently.

27.  Rain, Snow, Frost, Dew.  The heat of the sun causes constant evaporation of the waters of oceans, rivers, streams, and marshes, and the water vapor set free by evaporation passes into the air, which becomes charged with vapor or is said to be humid.  Constant, unceasing evaporation of our lakes, streams, and pools would mean a steady decrease in the supply of water available for daily use, if the escaped water were all retained by the atmosphere and lost to the earth.  But although the escaped vapor mingles with the atmosphere, hovering near the earth’s surface, or rising far above the level of the mountains, it does not remain there permanently.  When this vapor meets a cold wind or is chilled in any way, condensation takes place, and a mass of tiny drops of water or of small particles of snow is formed.  When these drops or particles become large enough, they fall to the earth as rain or snow, and in this way the earth is compensated for the great loss of moisture due to evaporation.  Fog is formed when vapor condenses near the surface of the earth, and when the drops are so small that they do not fall but hover in the air, the fog is said “not to lift” or “not to clear.”