76 



AIR 



does the air in other places. Why? If you leave a 

 glass of cold water in a warm room, what collects on the 

 outside? There is another part of the atmosphere called 

 carbon dioxide, which also varies according to the local- 

 ity. In the open country there is very little of this gas, 

 but in cities, and in rooms where there are a large num- 

 ber of persons, it may increase so as to harm us, for it 

 is not a good gas to breathe. 



Although the amount of water and carbon dioxide 

 in the atmosphere varies greatly, the largest portion of 

 the air is composed of two gases which do not vary 

 About one-fifth of the air is oxygen, and about four- 

 fifths is nitrogen. These gases are so important that we 

 will study them one at a time. Thus you see that the air 

 which you thought was just one thing is really composed 

 chiefly of four substances: nitrogen, oxygen, carbon 

 dioxide, and water. Besides these, there are argon, dust, 

 and bacteria. 



Experiment 38. The Amount of Oxygen in the Air. 



Apparatus: Saucer, glass, candle, flat cork stopper, 

 a. Cut the can- 

 dle so that it is 

 not more than one 

 inch long and 

 make a "life pre- 

 server" for it from 

 the stopper. To do 

 this, cut a round 

 hole in the stop- 

 per, in which the 

 candle will fit 

 tightly. The cork 



