FIRE 13 



Which of these three gases is it that is used up in a fire ? 

 Or are two of them, or are all, necessary for fire ? 



Each of these gases can easily be prepared in the laboratory, 

 and we may try each in turn, to see how it behaves in relation 

 to fire. From these tests we discover the importance of oxygen 

 in relation to burning. 



21. Oxidation. The chemist describes the facts of burning 

 by the statement that some substance combines with oxygen, 

 producing new compounds. The heat and light energy that 

 are set free are the equivalent of the combining force, or 

 chemical energy, that the fuel has in relation to oxygen the 

 attraction between the two substances. The new compound that 

 is formed when a simple substance (an element) burns is called 

 an oxid. Thus, when magnesium is burned, magnesium oxid 

 is produced ; when sulfur is burned, sulfur oxid is produced ; 

 when phosphorus is burned, phosphorus oxid is produced ; and 

 so on. The carbon dioxid which is found in the air is a com- 

 pound of oxygen and carbon. Water is a compound of oxygen 

 and hydrogen. 



When a compound, or mixture (like sugar or wood), is 

 burned, several oxids may be formed at the same time. When 

 sugar burns, the carbon of the sugar goes to form carbon 

 dioxid and the hydrogen of the sugar goes to form water. 

 The carbon and the hydrogen of the wood behave in a 

 similar manner when wood burns. 



22. Oxidation in living things. In living bodies the energy 

 transformed in the various activities is derived from food. 

 This food is not burned directly, like the gasoline in an engine ; 

 it first undergoes many changes and becomes part of the living 

 body. And the burning, or oxidation, takes place in all the 

 several parts of the body instead of in one central furnace. 

 Another difference between the oxidation in the living body 

 and in our ordinary engines is this : In the living plant or 

 animal there is no flame. Indeed, the oxidation always takes 

 place in the presence of water, whereas the fires with which 



