THE CHEMISTRY AND PHYSICS OF THE CELL 31 



or non-utility of these substances as conductors of electrical 

 currents in the ordinary sense, but rather on the existence of 

 those properties which determine their conductive ability. 

 Electrical conductivity is an index of ionization, and upon 

 ionization depends the chief influence of the electrolytes upon 

 vital activities. 



Electrolytes and Non-electrolytes. Ionization. 

 If we attempt to pass a current of electricity through water, 

 we find that it meets a great resistance, and the purer the 

 water, the greater the resistance. In water as pure as can 

 possibly be obtained the resistance of a layer only one milli- 

 meter thick has been found equal to that of a copper wire of 

 equal cross-section long enough to reach around the earth one 

 thousand times. 1 The addition of the slightest quantity of salts, 

 acids, or alkalies increases the conductivity enormously, and in 

 any considerable amounts they make the solution an excellent 

 conductor of an electric current. On the other hand, by the 

 addition of sugar or alcohol to the water, the conductivity is in- 

 creased very little or not at all. What differences exist between 

 the soluble substances that do increase the conductivity of the 

 solution and those that do not ? 



If we dissolve in water in a platinum dish a small quantity 

 of an electrolyte, say copper sulphate, and pass a current of 

 electricity through it, using the platinum dish as the negative 

 electrode and inserting the positive electrode in the solution, it 

 will be found after a short time that there is no longer a blue 

 solution of copper sulphate in the dish, but rather that the 

 lining of the dish, where it is under the liquid, has become red 

 from the deposition of a thin layer of copper. If we reverse 

 the current, it will be found that the copper leaves the surface 

 of the dish to collect upon the electrode inserted in the water, 

 which is now the negative pole. This experiment illustrates 

 the ability of the copper to wander from one electrode to 

 another, and it is by this wandering that the electricity is carried 

 by the copper particles. The copper sulphate is dissociated 

 into its two parts : copper, carrying a positive charge which 

 goes to the negative pole or cathode, and is therefore called the 

 positive ion, or cation ; SO 4 , carrying a negative charge, wanders 

 to the anode, and is therefore called the negative ion or anion. 

 The individual particles which carry the charges are designated 

 as ions. It will be noted that the particles may or may not be 

 single atoms ; in the case of copper sulphate the cations 

 (copper) are atoms, but the anion, SO 4 , is formed by several 



1 Kohlrausch and Heydweiller, quoted by Cohen. 



