CHAPTER III 

 AIR AND OXYGEN 



WE have seen that metals absorb a gas, called oxygen, from 

 the air, and turn into a rust or oxide. Let us now consider what 

 happens to the air during this process. 



The Nature of Air. Can a metal, like iron, when rusting, 

 absorb the whole of a sample of air, or does it select a part of the 

 air only? If we sprinkle some powdered iron in a test-tube, 

 having first moistened the interior to cause 

 the powder to adhere to the inside surface, 

 and then set the tube, mouth downwards, 

 in a dish of water (Fig. 15), we obtain be- 

 fore long an answer to this question. As 

 the iron slowly removes the oxygen, the pres- 

 sure of the atmosphere outside pushes the 

 water up the tube. But, after ascending 

 only about one-fifth of the total height of the 

 tube, the water comes to rest. Inspection shows reddened parti- 

 cles where rusting has taken place, but much of the iron is still 

 dark grey, and is as little able to rust in the remaining gas as in a 

 vacuum. Four-fifths of the air, then, is composed of gases which 

 do not combine with iron, and only one-fifth is oxygen. The 

 four-fifths is in fact almost all (99 per cent) nitrogen, a substance 

 which combines with very few materials, while the balance (1 per 

 cent) is made up of argon and other gases which do not enter 

 into combination with any known substances. Oxygen, on the 

 contrary, combines with almost all simple substances, although 

 in many cases, as in those of lead and tin, heating is required to 

 hasten the process. 



26 



FIG. 15 



