PROPERTIES OF MATTER 21 



through |)l;mls. The xarious circuhuory sysicnis ;irc ( onsidered in 

 more tlctiiil in later chapters. 



Biological Functions 



The ))riiKipaI biological lunctions of gases iall into two categories. 

 Gases are required in the utilization of foodstuffs and might thus 

 be considered as foods themselves, or they are the by-products of food 

 utilization and biological processes in general. Different species take 

 up dillcrcnt gases, including oxygen, carbon dioxide, hydrogen, nitro- 

 gen, hydrogen sulfide, and methane. Gases eliminated include carfjon 

 dioxide, oxygen, nitrogen, hydrogen, and meihanc — in other words, 

 many of the same gases. In certain cases a given individual may (on- 

 sume one gas, while under different conditions the same gas may be 

 gixcn off and another taken up. 



In another somewhat incidental but important function, carbon 

 dioxide and ammonia dissolve in biological fluids to form buffers, 

 \'aluable in controlling the acidity of those fluids. Gases are employed 

 by swimming and floating species in providing the buoyancy necessary 

 to prevent sinking and resultant immobility. Finally, air is a mediimi 

 of movement possessing sufficient substance for the flight of insects, 

 birds, and bats. 



LIQUIDS 



In the liquid state the particles are in continuous motion and move 

 ratlier freely throughout the liquid. However, the particles are much 

 closer together than those in a gas, and there is much more interaction 

 between them. Freedom of motion is nevertheless great enough to 

 permit relative movement of layers of liquid. Thus liquids possess the 

 ability to flow and to take the shape of the container. On the other 

 hand, liquids do not fill containers regardless of size, as do gases, be- 

 cause the individual molecules seldom leave the neighfjorhood of their 

 fellows and escape from the lirjuid. At a given temperature, the mole- 

 cules of a liquid maintain a given average distance from adjacent 

 molccides regardless of the size of the containing system. Nor is this 

 sj)a( ing very nuich affected by pressure. Even temperature variations 

 produce but slight and gradual changes until the liquid boils or 

 freezes. 



Vapor Pressure and Boiling 



In any substance the molecules are in motion, and they possess an 

 average velocity characteristic of the temperature. An increase in 



