_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. [Illustration: 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. [Illustration: 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_. =140. The Effects of Heat.=--There are five important changes produced by heat: (a) change of _size_, (b) change of _temperature_, (c) change of _state_, as the melting of ice or evaporating of water, (d) _chemical_ change, as the charring of sugar when it is overheated, and (e) _electrical_ change. This is illustrated by the production of an electric current, by the heating of the junction of two different metals. A thermo-electric generator (see Fig. 122) has been constructed upon this principle and works successfully. [Illustration: FIG. 122.--A thermo-electric generator.] Important Topics 1. Importance of a study of heat. 2. Four sources of heat. 3. Five effects of heat. 4. Examples of each. 5. Illustrations of transformation of energy which involve heat. Exercises 1. Write a list of the _sources_ of heat in the order of their importance to you. State why each is important to you.