24 INTRODUCTION TO GENERAL PHYSIOLOGY 



sodium chloride by means of t\vo carbon plates immersed in the 

 solution, the current enters by one plate and leaves by the other. 

 Faraday called the two plates " electrodes," that by which the current 

 enters being the "anode" that by which it leaves the " cathode" 

 Since the current flows from the anode to the cathode, the former 

 has the higher potential, or is electro-positive to it. Now we find 

 that chlorine is attracted to the positive pole, and is present around 

 it in solution. Sodium goes to the negative pole, and can be 

 collected if mercury is present to dissolve and remove it ; otherwise, 

 it reacts with water to form the hydroxide. But if the sodium 

 atoms are attracted to the negative pole, it must be because they 

 have an opposite, or positive, charge. Correspondingly, the chlorine 

 atoms must be electro-negative. Since there is every reason lo 

 believe that atoms owe their chemical nature to their constitution 

 as electrons, or unit electrical charges, with a positive nucleus, it 

 is clear that if an electron is added or removed, making the atom 

 negative or positive to what it was before, the chemical properties 

 will be altered. A chlorine atom with an extra electron is not the 

 element chlorine, nor is sodium with an electron removed the 

 same thing as the metal sodium. It is only when the sodium and 

 chlorine "ions" as Faraday named them, on account of their 

 movements to the poles, lose their electrical charges by contact 

 with the opposite charges on the poles, that they are converted into 

 the ordinary elements. In Faraday's terminology, the sodium ion 

 is the cation, because it wanders to the cathode ; the chlorine ion is 

 anion, because it goes to the anode. If the current is allowed to pass 

 long enough, all the sodium chloride is decomposed by electrolysis, 

 and if we imagine that the last remaining molecule arrives as such 

 at either electrode and is not decomposed until it arrives there, 

 one of its constituent ions must be left free, and must pass through 

 the solution to the other pole. If this were so, it would exist 

 during its passage as an atom with a charge, that is, an ion. Hence 

 we must admit the possibility of the existence of free ions in the 

 solution, and it is natural to suppose that they are present as such 

 before the electrical current is sent through ; so that what this 

 current does is to carry those of opposite sign to the appropriate 

 electrode. This is the statement made by the theory of electrolytic 

 dissociation. The ions into which an electrolyte is dissociated in 

 water are the elements of which we are in search. To repeat, a 

 solution of sodium chloride or other electrolyte is already decom- 

 posed into its constituent ions, to a greater or less extent, before 

 any electrical current passes through it, and what the current does 

 is merely to attract the oppositely charged ions to the poles of 

 opposite charge to themselves and deprive thern^f their charges. 

 We see also that the passage of electricity from one pole to the 



