78 THE PRODUCTION AND USE OF HEAT 



may be converted into the energy of the moving parts of the 

 machine. The working steam-engine at the electric station 

 drives the dynamo which generates electricity. Physicists 

 tell us that there is no gain nor loss of energy when it is con- 

 verted from one kind into another. This principle is known as 

 the CONSERVATION" OF ENERGY. 



86. Chemical Energy. It is evident that, since the coal 

 produces heat which causes the steam-engine to do work, and 

 since the food eaten keeps the body warm and enables it to do 

 work, and since kerosene, gasoline, and acetylene produce light 

 and heat, each of these substances possesses energy of a dif- 

 ferent kind from that of heat, light, electricity, kinetic energy, 

 or potential energy. It is also to be remembered that the coal, 

 the kerosene, and the other fuels must be burned in order to 

 liberate their energy as light and heat. The food must be 

 eaten, digested, and assimilated in order to liberate its energy 

 to the body. The kind of energy possessed by such materials 

 is called CHEMICAL ENERGY. This kind of energy, of course, 

 may be transformed into other kinds. The principle of 

 conservation of energy holds for this kind of energy as it does 

 for other kinds. In fact, chemical energy is one of the funda- 

 mental kinds of energy. From it most of our other kinds are 

 produced. 



87. Chemical Change. When coal burns and liberates its 

 chemical energy, it undergoes what is called a CHEMICAL 

 CHANGE. The coal and the oxygen of the air with which it 

 unites produce new substances. Gasoline, kerosene, and 

 acetylene undergo chemical changes as they liberate light and 

 heat. Food undergoes many complex chemical changes as it 

 liberates its chemical energy to the body. All of the sub- 

 stances just mentioned give out energy as they undergo 

 chemical change. But some substances must take in energy as 

 they undergo chemical change. Thus the compound, calcium 

 carbide, which was used in making acetylene, took in a large 

 amount of energy in the electric furnace when it was made 

 from lime and coke (Chap. I, Sec. X). Many other com- 



