CHAPTER V 

 STRUCTURE OF AIR: MOLECULAR THEORY 



44. Compressibility of air. We have already discussed the 

 general nature and movements of the air. In the experience 

 of everyone many questions and problems arise which can be 

 explained only through a further discussion of the structure 

 and composition of the air. In this chapter we shall consider 

 some of the problems and their explanations. 



If a pneumatic tire is filled with air so that it is fully ex- 

 panded and also feels quite firm, we say that it is "filled" with 

 air, and so it is. If it were filled with water in like manner, 

 we should not expect to be able to get any more water into 

 it, but we know very well that if we apply the pump, we can 

 force in more air. As we continue to pump, the tire gets 

 firmer and the pump works harder, showing that there is 

 greater pressure in the tire. There seems scarcely any limit 

 to the amount of air that can be forced in, so long as our 

 strength endures and the tire does not burst. How can this 

 additional amount of air get into the tire ? Plainly the air 

 that was in there before must have been forced to occupy less 

 space than before, or else the old and the new have mingled 

 within the space formerly occupied by the old. In either case 

 they both together occupy less space than they would occupy 

 if they had not been forced into the tire. In other words, the 

 air has been compressed. 



Instead of pumping air into the tire, we might have squeezed 

 the tire together. We should then have forced the tire and 

 the air that it contains to occupy less space than they did 

 before, again showing that it is possible to compress air into 

 smaller space. 



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