WHAT MAKES THE AIR, USEFUL TO MAN? 49 



2. Plunge the glowing end of a splint into oxygen. Remove 

 instantly and cover the jar. Result? Twist a wire around a 

 small bundle of steel wool. Heat the 

 steel wool in a flame and immediately 

 plunge it into the oxygen. Result ? 



The results are so striking that 

 there is no doubt what it is that 

 helps things to burn. The two 

 gases, nitrogen and oxygen, appear 

 to have opposite properties. Nitro- 

 gen quenches a fire just as water 

 would, but oxygen causes it to burn 

 with greater force. Air supports a 

 flame because of the oxygen in it. 

 But things do not burn as fiercely in air as in pure oxygen 

 because of the large amount of nitrogen in the air. 



What Is Oxidation ? When a substance combines with 

 oxygen, the process is called oxidation. When this com- 

 bination of a substance with oxygen results in a flame, 

 the process is called combustion. Iron combines with 

 oxygen when it rusts, but there is no flame ; therefore 

 this process is oxidation but not combustion. 



What Is a Flame ? Flame is defined as a burning gas. 

 It is easy to understand this in the case of burning manu- 

 factured or natural gas. When oil burns, it must first be 

 changed to a gas by heat before there can be any flame. 

 Have you ever noticed when a candle burns that the wax 

 melts and is taken by the wick up to the flame ? There 

 it is changed to a gas which burns, and in doing so, pro- 

 duces the candle flame. If you hold one end of a glass 

 tube in the center of a candle flame so as to conduct gas 

 through it, the gas will burn with a flame at the other end. 



Demonstration 3. What Substances Result When a Candle Burns ? 

 1. Bring a dry pint jar down on a burning candle. When the 

 candle goes out, remove the jar and quickly close with a glass plate 

 or cardboard. What appears to be on the inside surface? 



H. & W. SCI. I 5 



