62 ELEMENTS OF GENERAL SCIENCE 



63. Liquids and solids. It may be more difficult to believe 

 that liquids and solids are composed of molecules, but this 

 is thought to be the case. It is not very difficult if we 

 recall the fact that gases can be condensed into liquids for 

 instance, when steam becomes liquid water, or in the forma- 

 tion of liquid air. Likewise, liquids may be frozen into solid 

 form. It follows, then, that solids and liquids, like gases, 

 are made of molecules. 



There are some differences, however. The molecules of 

 a liquid seem to move about readily almost as readily as 

 they do in a gas, but they do not have the same tendency 

 to fly apart. Thus the liquid changes form, but it remains 

 the same in volume. It does not readily expand and is not 

 easily compressed. The solid also is not easily compressible, 

 since its molecules do not readily shift their positions. It 

 has a permanent shape. The spaces between the molecules 

 in a liquid or a solid are much smaller than in a gas, but 

 there must be some space, else we could not dissolve sub- 

 stances in water. Even so compact a substance as gold will 

 absorb certain other substances, and we believe that the 

 molecules of the dissolved substance find their way into 

 the spaces between the molecules of gold. 



If alcohol, which is lighter than water, is carefully poured 

 into a test tube half filled with water, at first most of the 

 alcohol will float upon the water, but after some time it will 

 be found that the two liquids have become evenly mixed. 

 In a similar manner a lump of salt or sugar dropped into 

 a vessel of water will dissolve and diffuse throughout the 

 water. Diffusion occurs in liquids as it does in gases. 



