OXIDATION 37 



bygone ages was conserved in the coal, and now can be 

 made to appear again as force in a steam engine. This 

 force may run an electric dynamo, and the electricity can 

 be transported silently over miles of wire, to appear as light 

 rivaling its original source, the sun. Through all its 

 changes the original energy is preserved. 



Observation of the three facts, (i) the heat of the sun 

 acting through plants to tear the oxygen from the carbon 

 and hydrogen, (2) the reunion of the substances in oxi- 

 dation with the development of the original heat of the 

 sun, and (3) the various forms of power into which the 

 energy can be changed, has given rise to the principle that 

 any form of energy can be changed into another form without 

 loss. This principle is called the conservation of energy. 



This principle is exemplified in the human body. The 

 energy for the work done by the body is the heat derived 

 from the oxidation of its food. 



45. Relation of plants to animals. The oxygen of the 

 air would all be used up in a few years if it were not continually torn 

 away by plants from its combinations in carbonic acid gas and water. 

 The carbon and hydrogen would also disappear; but the sun and 

 chlorophyll continually renew the supply both of food and of oxygen. 

 Thus there is a stream of material flowing from lifeless soil and air. It 

 becomes alive in the plant and again in the animal, and then is suddenly 

 oxidized to a lifeless form, and given back to the soil and air, only to 

 repeat the round of life. Plants build up living material which animals 

 use as food and then oxidize back to the form in which it existed before 

 the plant touched it. Plants give off oxygen which supports animal 

 life. Each lives upon what the other discards. 



46. Organic substances. Substances which are built 

 up by living beings are called organic. Thus the plant 

 takes carbon from the carbonic acid gas in the air, and 

 builds it up into an organic substance, which forms part of 

 the plant. 



