20 



THE FOUNDATION 



Part I 



actually fill the space. Air is a gas and its density varies with the compression, 

 with the pressure and temperature of the atmosphere. In high places where 

 pressure is lessened, its molecules are relatively far apart and it may be too 

 "thin" in oxygen to be adequate for respiration. In liquids, the molecules are 

 closer together. In a solid, such as iron, the molecules are crowded together in 

 patterns. Solids have fixed shape and volume. 



The behavior of water molecules is very exceptional. Down to 39° F. they 

 draw closer together; between 39° F. and 32° F. they move apart. Thus, ice 

 expands and floats, forming a protecting cover to the animals beneath it (Fig. 

 5.17). 



Surface film. Surface films are composed of molecules that are attracted 

 only by those at and close to the area where one substance comes in contact 

 with another, such as water and air (Fig. 2.10). Molecules below the surface 

 are attracted equally from all directions. Surface film occurs on all bodies of 

 water and forms the boundary of such units as soap bubbles and raindrops. It 

 is important in the lives of many small aquatic animals. Certain insects, such 

 as the water striders, forage on the upper face of surface films that bend but 

 do not break with the pressure of their feet (Fig. 2.10). Snails glide over the 

 underface of the film and hydras are often buoyed up against it. 



SURFACE FILM OF WATER 



A o-o-o-o-o 



/ \ / \ / \ 



o o o o 



o o 

 o o 



B 



Fig. 2.10. The surface film of water. A, In surface film molecules of water are 

 attracted only by those at the surface or just below it. B, Molecules below the sur- 

 face are attracted evenly from all directions by other molecules. C, Hydras rest 

 against the surface film in the topmost water where oxygen is plentiful. D, Water- 

 striders skim over the surface film of quiet water and their feet make the dimples 

 that cast shadows on the brook bed. 



