The subject of latent heat, described on page 12, has proved very puzzling to many. It is certainly a strange idea at first, that heat does anything more than make things warm. Still, a moment's consideration recalls to mind that heat can do many other things. Heat causes chemical change, for substances are often changed by strong heat. Heat causes most substances to expand. If a sealed can of any substance is strongly heated, it will probably explode. Heat causes liquids to evaporate, and solids to melt.

If a liquid is placed in an open dish on a source of heat, its temperature will rise until it begins to boil. After this, it gets no hotter, no matter how much heat is applied, unless the liquid is becoming more dense as it boils, as would be the case with a syrup, for example. The heat it receives is all expended in changing the liquid into vapor, or, as we say, changing the "state of matter." The particles (molecules) are driven farther apart by the heat. A cubic inch of water makes a cubic foot of steam. The amount of heat necessary to produce the change from liquid to gas varies with different substances. Water requires a very large amount. Four times as much heat is required to change an ounce of water into steam as to vaporize the same amount of alcohol. If heat is applied rapidly, the liquid will boil rapidly, but it does not affect the temperature. The heat used in this way is not lost, but is stored up in the vapor as latent heat. The steam is no hotter than the boiling water, and heat added keeps it from becoming liquid. When vapor condenses and changes back to liquid, the latent heat is given out, and warms surrounding things. In fact, the vapor cannot condense unless the latent heat it contains is removed, except under pressure. This latent heat makes steam an excellent medium for heating buildings, as it contains so much heat and passes through pipes rapidly. Not only is the steam itself hot, but it carries a vast amount of heat stored up, to be liberated in the cooler regions.

Latent heat is stored up in water, also, and is liberated when the water becomes ice. This is seldom apparent, for far less heat is thus stored in water than in steam, and, too, the temperature of freezing water is low. The heat given out when water freezes is at 320 F, while that given out when steam condenses is at 2120 F. Still, a cellar may be several degrees warmer if it contains a tank of water which freezes than if the water were not there. The temperature may keep about 320 F. where otherwise it might go to 26° or less.

A room is cooled in warm weather by sprinkling water upon the floor. The evaporation of the water takes much heat from the air, storing it in the vapor produced. Britannia and some other metals of which pitchers, teapots, etc., are made will melt if placed on a hot stove. If, however, they contain water, this is not likely to occur, for the water cannot be heated above its boiling point, and this is far below the melting point of the metal, and keeps the temperature of the metal low enough for safety. This reminds me of an experiment I once saw where candy was actually made in a pasteboard box. The syrup never became hot enough to scorch the paper, and thus the paper itself was kept fairly cool.