WHAT MAKES THE AIR USEFUL TO MAN? 47 



atmosphere is always pressing upon us and upon every- 

 thing it touches. 



Air Is a Mixture. In the latter part of the eighteenth 

 century several men of science, working in their labora- 

 tories, proved that air consists of several gases, the most 

 important of which are oxygen, nitrogen, carbon dioxide, 

 and water vapor. Among other substances in the air are 

 the gases, argon, neon, and helium. Besides this, there 

 is a variable quantity of dust, consisting of pollen, soot, 

 soil, and many other tiny particles of matter. Oxygen 

 forms about one fifth and nitrogen nearly four fifths of 

 the air near the earth. 



What Causes Rust. You have all seen examples of 

 rusting : the brown flakes and yellow dust on unpainted 

 iron fences, unused rails, your knife on a fishing trip, on a 

 tin can left for a time in a damp place. 



Since unprotected iron surfaces are so easily acted upon 

 by the oxygen in moist air, exposed surfaces of iron are 

 covered with material which keeps the air from them. A 

 "tin" can is iron with a thin wash of tin on the surface. 

 But you know that a tin can will rust. This is because 

 there are microscopic openings in the tin covering through 

 which air reaches the iron. A "tin" roof again is tin- 

 coated iron. To protect it, various paints may be applied. 

 Steel bridges costing millions of dollars are preserved 

 for many years by 

 painting at proper in- 

 tervals. When iron 

 is dipped into molten 

 zinc and withdrawn, 

 a coat of zinc clings 

 to the iron, making 



What is Called gal- 

 iron. This 



What is rust and what causes rusting? How 

 could the screen have been protected from rust? 



protects the iron even better than the coat of tin. 



