80 Introductory Biology 



between molecules in a liquid or solid. In the atmosphere the several 

 kinds of molecules must move about one thousand times their own 

 diameters before colliding with other molecules. The motion of mole- 

 cules in a gas is greater than in a solid because in the latter they merely 

 vibrate back and forth because of the mutual attraction between adja- 

 cent molecules and probably the closer association of molecules. 



If a single molecule could possibly be completely isolated and remain 

 so, its kinetic energy would remain constant. However, the kinetic 

 energy of molecules is influenced by the kinetic energy of surrounding 

 molecules. When molecules increase their speed, they exert greater pres- 

 sure on other molecules, so that the average distances between them is 

 increased. Consequently, when heat is applied to certain substances, the 

 molecules increase their speeds and the substance expands. We measure 

 the amount of kinetic energy in terms of temperature. Likewise, con- 

 traction usually is the consequence of reduced molecular speed. If 

 molecules of two kinds are placed together, the two kinds tend to mix 

 with each other through the process called diffusion (L. diff under e, to 

 pour). If a drop of perfume volatilizes (becomes a gas) in a room, its 

 molecules will move and mix with the various molecules of the atmos- 

 phere and the odor will diflfuse so as to be detectable some distance away. 

 Our nose is affected by the molecules of the perfume so we detect the 

 odor. The odor is not detected immediately because it takes some time 

 for the perfume molecules to move toward us and the v-arious molecules 

 of the atmosphere offer resistance (because of collisions). The continual 

 bombardment of an enclosing wall or membrane by molecules exerts a 

 pressure which varies with the number of molecules, their movements, 

 temperature, etc. Likewise, a chemical substance will diffuse through 

 water in which it is placed. These phenomena of diffusion through gases, 

 liquids, and solids are common in the nonliving and living worlds. 



An atom is the smallest unit particle of an element capable of taking 

 part in a chemical reaction. There are as many different kinds of atoms 

 as there are elements (elementary substances), and vice versa. These 

 submicroscopic atoms are invisible. As a result of recent scientific studies 

 it is thought that atoms consist of smaller units which are arranged some- 

 what like a miniature solar system, with much of the atom supposedly 

 "empty" space. The units which compose atoms are: 



1. A large central atomic nucleus consisting of particles smaller but 

 heavier than the electrons. These nuclear particles are called (a) pro- 

 tons, which are charged with positive electricity, and (b) neutrons, which 

 are uncharged electrically (neutral). Each nuclear proton has the power 



