6 MATTER AND ENERGY 



iron, cloth, paper, wax, ice, leather, and rock are ex- 

 amples of solid substances. 



Liquids keep the same size, but change their shape 

 according to the vessel in which they are. Water is the 

 most common liquid ; others are alcohol, benzine, kero- 

 sene, ether, and mercury. 



Crases do not keep either their size or shape, but expand 

 without limit. For this reason a gas cannot be kept 



FIG. 2 



pure unless it is in a tightly closed vessel. If a bottle 

 of some gas be uncorked and left so, in a few minutes 

 the gas will escape into the air and the bottle will con- 

 tain only a very little, mixed with a large amount of 

 air. This is because the tiny particles of any gas are 

 always in rapid motion, and so they become separated 

 from each other, mixing with particles of other gases. 

 This process is called diffusion. Gases are much lighter 

 than liquids or solids. Most of them are invisible (cannot 



