CHAPTER XVII 

 THE AIR 



98. The Air a Mixture of Gases. The air is com- 

 posed of 79 per cent nitrogen, 20 per cent oxygen, .03 

 per cent carbon dioxide, some water vapor, and other 

 gases. These gases nitrogen, oxygen, and carbon 

 dioxide are mixed together, but are not combined 

 chemically. When we breathe, the oxygen is separated 

 from the nitrogen in our lungs. When natural gas burns, 

 oxygen from the air unites with the carbon that is in the 

 gas to form carbon dioxide. To prove that about four- 

 fifths of the air is nitrogen, try the following simple 

 experiment. 



Take a piece of yellow phosphorus about the size of a 

 pea, dry it with filter paper, 

 and place it on a prepared 

 float in a pneumatic trough. 

 Ignite the phosphorus with 

 a match and invert over it a 

 cylindrical glass vessel. The 

 phosphorous oxide formed 

 will be absorbed by the 

 water. The water will slowly 



rise in the vessel as the oxy- TAKING OXYGEN OUT OF THE AIR 

 gen is consumed. The gas 



remaining in the glass vessel above the water is nearly 

 pure nitrogen. Measure the gas and compare its volume 

 with that of the entire vessel. This experiment gives 

 evidence of what per cent of the air is oxygen. 



