The Air We Breathe 



35 



monly estimated that 78% of the air is nitrogen, 21 % oxygen, 

 and the remaining \% the other gases which are of little impor- 

 tance to us here. Most air also contains microbes, or germs, 

 and particles of dust ; these, however, are not found in pure air. 



Properties and Uses of the Parts of Air. Certain of these 

 gases which make up the air have properties and uses so important 

 to our welfare as to deserve special study at this time. 



Oxygen. The most important property of oxygen is that it 

 will support combustion or burning. This is a chemical property. 

 Other properties are that it is colorless, tasteless and odorless. 

 These are physical properties. 



The most important value of oxygen is in respiration. During 

 both day and night we breathe about sixteen times each minute. 

 The air we breathe in provides oxygen for all parts of the body. 

 But respiration means more than breathing. It includes the work 

 oxygen does in burning up the foods we eat. This burhing 

 releases heat, part of which 

 keeps the body warm and part 

 of which provides the energy 

 necessary to carry on life. 

 Oxygen performs a similar 

 service for all animals and all 

 plants. 



Experiments to Show Oxy- 

 gen in Air. It is an interest- 

 ing experiment to show the 

 presence and the proportion 

 of oxygen in the air. You 

 can test for its presence by 

 lighting a candle. If it con- 

 tinues to burn there is oxygen 

 in the air. 



To find the proportion of 

 oxygen in air burn a small piece 



REMOVING OXYGEN FROM THE AIR BY 

 BURNING PHOSPHORUS IN A WATER- 

 SEALED BELL JAR 



of phosphorus under a bell jar over water. The burning of the 

 phosphorus will use up the oxygen and leave the nitrogen. Water 

 will rise in the jar and occupy the space of the oxygen. By 



