ELECTRIC CONDUCTIVITY, DISSOCIATION, IONIZATION 21 



tion proceeds, therefore, the conductivity of the digestion mixture pro- 

 gressively increases, and is a measure of the rate of digestion. 



Applications of the dissociation hypothesis in physiology concern the 

 explanation of such phenomena as the production of electric currents 

 during muscular, glandular, and nervous activity. The exact details of 

 the application are not as yet sufficiently understood to warrant our at- 

 tempting to do more than indicate the general lines along which the 

 problems are being investigated. Let us, for example, consider how the 

 current of action of muscle may be explained in terms of the dissociation 

 hypothesis. To do so we must delve a little further into physicochem- 

 ical research, when we shall find that there are two further facts . con- 

 cerning ionized molecules that must be of importance in connection with 

 our problem. The first is that the contribution which each ion makes to 

 the equivalent (or molecular) conductivity of a solution is independent 

 of the other ion with which it is associated; and the second, that ions 

 differ considerably in their conducting power. Since the univalent ions, 

 K., Na., CL', N0 3 ', carry charges of the same magnitude,* and yet all do 

 not conduct to the same degree, they must move at different velocities 

 through the solution. We are driven, therefore, to the conclusion that, 

 exposed to the same electric force, different ions have different mobili- 

 ties ; that is to say, when an electric current passes through a solution of 

 an electrolyte, the positively charged ions move towards the cathode at a 

 different rate from that at which the negatively charged ions move 

 towards the anode. Confirmation of this conclusion is obtained by exam- 

 ination of the concentration changes around the two electrodes of an 

 electrolytic cell. The actual velocity of each ion can be determined by 

 experimental means. 



*Thus Faraday showed that the amounts of the various ions liberated by electrolysis are in the 

 same ratio as their chemical equivalents. 



