139. Principal Sources of Heat.—First and most important is the Sun, which is continually sending to us radiant energy in the form of light and heat waves. These warm the earth, make plants grow, evaporate water, besides producing many other important effects.
Second, chemical energy is often transformed into heat. One has but to think of the heat produced by burning coal, wood, oil, and gas, to recognize the importance of this source. Chemical energy is also the source of the heat produced within our bodies. The action of quicklime and water upon each other produces much heat. This action is sometimes employed during balloon trips as a means of warming things.
Third, Electrical Energy.—In many cities electric cars are heated by the electric current. We have all heard of electric toasters and other devices for heating by electricity. Electric light is produced by the heating of some material to incandescence by an electric current. The electric furnace has a wide application in the preparation and refining of metals.
Fig. 120.—Boy-scout method of making fire by friction.
Fourth, heat is also produced whenever mechanical energy of motion is overcome, whether it be by friction, concussion, or compression. Friction always results in the production of heat, as when we warm our hands by rubbing them together. When friction is excessive, such as in the case of a heavy bearing not properly oiled, the bearing may get very hot. This is the cause of the "hot box" on a railway car. Friction may produce heat enough to set wood on fire. Some fires in mills are believed to be due to this cause. Every boy scout must learn how to produce fire by friction. (See Fig. 120.) Concussion may be illustrated by the heating of a piece of metal by hammering it, while the compression of a gas always makes it warmer, as those who have used a bicycle pump have observed. The production of heat by compressing a gas is illustrated by the "fire syringe" (Fig. 121). This consists of a glass tube with a tightly fitted piston. A sudden compression of the air contained may ignite a trace of carbon bisulfid vapor.
Fig. 121.—A fire syringe.
The interior of the earth is hot, but its heat seldom gets to the surface except at hot springs and volcanoes.