ELEMENTS AND COMPOUNDS 
11 
plants and animals, energy is needed. Energy is defined 
as the power to do work. It is produced in animals chiefly 
by the action of oxygen on carbon compounds. Oxidation 
is the source of the larger amount of animal energy. The 
amount of energy is in proportion to the amount of oxidation. 
Heat is a form of energy. 
Conservation of Energy. — The source of all energy on the 
earth is the sun. The heat and light of the sun make it 
possible for plants to grow. Plants store up energy in the 
form of starch, wood, and sugar. Animals eat these stores 
of energy and convert them into the energy of heat and 
motion. 
Coal is stored energy from the plants of ages ago. The coal 
is placed in furnaces and burned. This furnishes energy in 
the form of heat. The heat produces steam, which is con¬ 
verted into energy in the form of motion. This motion may 
be converted into energy in the form of electricity. Elec¬ 
tricity may be converted into light or motion. 
We may sum up in this way. The light of the sun made 
plants grow which were transformed into coal. The coal 
was burned under the boiler, making the steam which ran 
the engine. The engine turned the dynamo and produced 
electricity. This electricity was passed through a small 
wire in an electric bulb and furnished light. The light of 
ages ago is again made into light by this long process. It 
has traveled in a circle, so to speak. Energy may be con¬ 
verted into various forms. Electricity may be changed to 
heat, as in the electric flat-iron; into light, as in the electric 
bulb; or into motion, as in the trolley car. One form may 
be converted or changed into other forms of energy. If 
energy is used in one form, it will appear in some other 
form. Energy cannot be created or destroyed. 
Elements and Compounds. — The bodies of plants and 
animals as well as the earth and air are composed of a great 
variety of compounds and elements. The starch found in 
