CHAPTER VI 

 COMPOSITION OF AIR: ATOMIC THEORY 



54. The air not a simple substance. When you consider 

 what we have learned about the air in connection with pre- 

 ceding chapters, you may recall several facts which have 

 shown that the air is not composed of one substance only. 

 For instance, we found that water vapor is often a very con- 

 siderable part of the air. The air is in fact a mixture of sev- 

 eral gases, of which water vapor is one. We should get some 

 acquaintance with at least the more important of these, and 

 learn how they act. As a means of securing such acquaintance 

 we shall study the flame of a candle. 



55. The flame and the air. If a candle is thrust into a 

 lamp chimney which is held in an upright position, the 

 flame will burn more brightly and steadily than in the open 

 air; but if the lower end of the chimney is closed by the 

 hand, the flame becomes smoky, flickers, and possibly is 

 extinguished. In the first case the heat of the flame warmed 

 and expanded the air in the chimney, thus causing an up- 

 ward current of air (sect. 11), so that the flame was better 

 supplied with air than when it was in the open. When the 

 hand was placed across the bottom of the chimney, however, 

 the upward current of air was interrupted and the supply 

 to the flame was so much reduced that burning was no longer 

 possible. 



It is clear that air has much to do with burning and with 

 flames. Just what part air takes in the burning, or com- 

 bustion, and what are the effects upon b.oth the air and the 

 candle, we shall study at some length ; but first we must 

 become familiar with the flame. 



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