Chemical Change and Energy 317 



but they would be combined in different compounds. 

 Instead of wood and coal and oxygen we should have 

 water and carbon dioxid ; instead of diamonds, we should 

 have just carbon dioxid; and so on with everything 

 that can burn. 



Why water puts out a fire. Water puts out a fire 

 because it will not let enough free oxygen get to the wood, 

 or whatever is burning, to combine with it. The oxygen 

 that is locked up in a compound, like water, you remem- 

 ber, has lost its ability to combine with other things. 

 Sand puts out a fire in the same way that water does. 

 Most fire extinguishers make a foam of carbon dioxid 

 (CO 2 ) which covers the burning material and keeps the 

 free oxygen in the air from coming near enough to com- 

 bine with it. 



Water will not put out burning oil, however, as the 

 oil floats up on top of the water and still combines with 

 the oxygen in the air. 



Why electric lamps are usually vacuums. Electric 

 lamps usually have vacuums inside because the fila- 

 ment gets so hot that it would burn up if there were any 

 oxygen to combine with it. But .in a globe containing 

 no oxygen the filament may be made ever so hot and it 

 cannot possibly burn. 



High-power electric lamps are not made with vacuums 

 but are " gas-filled." The gas that is oftenest put 

 into lamps is nitrogen, the same gas that is mixed 

 with the oxygen in air. By taking all the oxygen out 

 of a quantity of air, the lamp manufacturers can use 

 in perfect safety the nitrogen that is left. It will not 

 combine with the glowing filament. There is no oxygen 



