60 ELEMENTS OF GENERAL SCIENCE 



From either of these experiments and from a great many 

 of the experiences of life we get the notion that air, and other 

 gases as well, may be compressed by applying force to them 

 in the proper way, and that they resist the compressing force. 



45. 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 to just the volume it had before. 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 one half 

 of the air out of the bottle, it is then only half filled. 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 we may re- 

 move, if there is any air 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 in the inside of it, the bag will still be filled with ah*. 

 No matter how much the bag is enlarged, the air will continue 

 to expand and fill it. 



Such experiments might be carried much farther, and we 

 should find that the air seems to possess the power of indefi- 

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

 whole space that is open to it. 



46. Diffusion of gases. If there is a leaking gas pipe in a 

 closed room, the odor of gas can soon be noticed in any part 

 of the room. The fact lhat the odor can be found in all parts 

 of the room indicates that the gas has spread to the most 

 remote corners. How did it get there ? If there were cur- 

 rents of air, it might be carried by the currents, but the same 

 result is observed even if the air is quiet. It seems, there- 

 fore, that the small amount of gas that has been released 



