228 SCIENCE OF COMMON THINGS. 



Unprofitable to burn green wood. Chemical changes produced by combustion. 



1530 Js there any truth in the remark often made " that it is economy 

 to burn green wood because it is more durable, and therefore in the end more 

 cheap ?" 



No j it is true that the consumption of green wood 

 is less rapid than dry, but to produce a given amount 

 of heat, a far greater amount of fuel must be con- 

 sumed. 



1531 J n ordinary fuel, what three elements enter into the process of 

 combustion f 



Hydrogen gas, carbon., and oxygen gas; the two 

 former in the fuel, and the latter principally in the air 

 which surrounds the fuel. 



1533 What chemical changes in air and fuel are produced by com- 

 bustion ? 



1. Some of the oxygen of the air, combining with the 

 hydrogen of the fuel, forms vapor of water ; and 



2. Some of the oxygen of the air, combining with the 

 carbon of the fuel, forms carbonic acid gas. 



1523 Why is there more smoke when fresh fuel is added than when the 

 fuel is red hot ? 



Because more carbon and volatile matters are sepa- 

 rated from the fuel than can be reduced by combustion / 

 and the surplus flies off in smoke. 



1534 Why is there so little smoke with a red-hot fir ef 



Because the entire surface of the coals is in a state of 

 combustion ; and, as very little of the escaping carbon 

 remains unconsumed, there is but little smoke. 



1535 When a coal fire is lighted, why are paper and wood laid at the 

 bottom, against the grate f 



That the flame may ascend through the fuel to heat 

 it. If the fire were kindled from the top, the flame 

 would not come in contact with the fuel placed below. 



1536 Why do we cover up afire with ashes or cinders to preserve it f 



The covering of ashes or cinders protects the fire 

 from the action of the air, and when fuel is deprived of 

 air it ceases to burn. 



1537 Why does afire burn so fiercely in windy weather ? 



Because the air is rapidly changed, and affords 

 plentiful nourishment to the fire. 



