58 ELEMENTS OF GENERAL SCIENCE 



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 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. 



56. Expansion of gases. In experiments such as we have 

 mentioned we notice that a gas resists a compressing force. 

 It does not surprise us, therefore, that when the gas is 

 released it expands again. When a tennis ball filled with 

 air is compressed and then released, it illustrates this point. 



Suppose, furthermore, we attach an air pump to a bottle 

 and exhaust the air from it. When we have pumped half 

 of the air out of the bottle, is one half of the bottle filled 

 and the other half empty ? By no means. The air is just 

 as plentiful in one end of the bottle as in the other. Indeed, 

 no matter how much air we may remove, if there is any left 

 in the bottle it expands so as to fill the whole bottle. 



In the same way, if we inclose some air in a rubber bag 

 and pull the sides of the bag outward so as to make more 

 space inside of it, the bag will still be filled with air. No 

 matter how much the bag is enlarged, the air will continue 

 to expand and fill it. 



Air and other gases seem to possess the power of indefi- 

 nite expansion. A gas always expands so as to occupy the 

 whole space that is open to it. 



