210 



COMMON SUBSTANCES 



SECTION III 

 MIXTURES 



246. Air. The air is a mixture of gases, the quan- 

 tities of which may vary somewhat. Pure air usually 

 contains about four fifths nitrogen and one fifth oxygen, 

 besides a very small amount of carbon dioxide. In the 

 atmosphere there is also more or less water vapor all 



the time, the quan- 

 tity varying greatly 

 at different times and 

 places. These four 

 substances, N, O, CO 2 , 

 and H 2 O, are each of 

 some important use 

 in the air; but there 

 are also other gases, 

 such as H, Cl, H 2 S, 

 NH 3 , etc., which mix 

 with the atmosphere 

 in very small amounts 

 now and then. The 

 quantity of such im- 

 purities in the air is generally greater in cities, near fac- 

 tories or chemical works, near marshes and swamps, in 

 mines, etc. Air in the country or over the sea is usually 

 more nearly pure, though no strict rule can be stated. 



Experiment 132 Float a cork on water in a large vessel. 



Place a bit of P on the cork and light it ; at once cover the cork 

 with a large jar, as in Fig. 137, holding the mouth of the jar 

 under water all the time. Allow the P to burn as long as it will, 



FIG. 137 



