HOT-AIR ENGINE FIREPROOFING 



185 



A.r 



72. Hot-Air Engines. 



If air is heated it expands, as was learned in Section 

 18. This principle has been used to obtain 

 power. The illustration shows a very sim- j 

 pie hot-air engine. If a tube is closed at 

 one end and has the other end sliding, which 

 is called a piston, when the air inside is 

 strongly heated by a Bunsen burner it. will 

 expand and force the piston out. After re- 

 moving the burner the air will cool and con- 

 tract and the piston will return to its first 

 position. This is the principle of the hot- 

 air engine but the usual engine has many 

 more parts, and the hot air is exchanged for cold air 

 instead of waiting for the hot air to become cold. You 

 know now why the engine works. To learn just how it 

 works will require further study after you have had more 

 about machines. 



73. Fireproofing. 



Some materials burn much more easily than others 

 and the same kind of material burns more easily if il is 

 opened up to the effect of the air than if it is tightly rolled 

 or compressed. The reason for this is, that if a body is 

 porous the oxygen of the air can get at more of the mate- 

 rial at one time and more burning or combustion can take 

 place. Since this is so, if we wish to make some mate- 

 rial less easy to burn we must put something upon it 

 which will keep away the air from its surface. A solu- 

 tion which contains tin, or a solution of water glass, will 

 accomplish this purpose quite well without harming the 

 material. 



