PHYSICAL FORCES INVOLVED IN BIOLOGICAL ACTIVITIES 159 



the attraction. The two atoms, however, remain electrically neutral. 

 A ny free atom or combination of atoms which carries a charge of electricity 

 is called an ion. When sodium chloride (NaCl) is resolved into the two 

 atoms sodium (Na) and chlorine (Cl) in solution and each contains an 

 electric charge, the process is called ionization or electric dissociation. 

 The sodium chloride (NaCl) or like substance is called an electrolyte. 

 The positive ions are known as cations and the negative as anions. 



Cathode 



FIG. 101. Movement of Electric Current and Ionization. 



In the dissociation of sodium chloride (NaCl) in water a negative 

 lit of electricity in the form of a valence electron goes with the chlorine 

 (Cl) and a positive unit of electricity in the form of a valence electron 

 is thus established with the sodium (Na). These electrons are further 

 attached to molecules of water together with which are formed the 

 electrically charged ions. The loss of the negative valence electron 

 by the sodium (Na) when it passes with the chlorine atom (Cl) leaves 

 the sodium atom (Na) positive or, in other words, the withdrawal of the 

 negative electron from the sodium (Na) disestablishes the neutrality 

 of the sodium (Na) by making it positive and that of the chlorine (Cl) 

 by making it negative. The result creates a flow of electricity because 

 there is a difference in the potential of the two atoms. The potential 

 determines the flow of electricity in much the same manner as pressure 

 determines the flow of a liquid. 



Such resistance as is offered by a solution to a current of electricity 

 may be measured by the Wheatstone Bridge. 



