32 INTRODUCTION 



atoms grouped together. Sometimes the ion consists of a great 

 number of atoms, as when such a molecule as stearic acid dis- 

 sociates we have ions of hydrogen and ions of C^H^O^ In 

 general, if the ion is very large, its movement is relatively slow, 

 and it shows less pronounced chemical properties. 



Now, although the migration of the ions to the electrodes has 

 been known for a very long time, the important fact that the 

 separation of a substance into its ions is not brought about 

 primarily by the electric current was ascertained later. It is 

 not the passage of the current that splits up the electrolyte, but 

 rather it is because the substance has already been split by the 

 solvent into its ions that it conducts the current. When we 

 dissolve an electrolyte, say sodium chloride, in water, many of 

 the molecules split into the cation Na, and the anion Cl. If 

 the solution is very dilute, the dissociation may be complete, and 

 we have no molecules of NaCl in our solution at all, but merely 

 the two sorts of ions in rapid motion. If the solution is more 

 concentrated, a larger proportion remains undissociated, although 

 the total number of ions may be much greater. What the 

 electric current does in passing through such a solution is to 

 cause a migration of ions toward the respective poles, where 

 they accumulate ; as a result, the solution between the poles 

 contains fewer ions than it should and the molecules undergo con- 

 tinuous dissociation until they have finally disappeared, for the 

 ions are all collected about the poles as fast as formed, and 

 finally the solution becomes free from both molecules and ions 

 except in the vicinity of the poles. So complete is this migra- 

 tion that the most accurate method of quantitative estimation 

 of many metals, such as copper, is this electrolytic method, by 

 which we can cause all the copper ions to become attached to 

 the inner surface of a weighed platinum dish, and after washing 

 away the solution and drying we can determine accurately the 

 amount of copper that has been attached to the dish. 



It is the act of solution, then, and not the electric current, 

 that causes ionization, and so every solution of an electrolyte, 

 such as a physiological salt solution, or sea water, or urine, or 

 any secretion of the body, contains a greater or less number of 

 free ions. In y salt solution, which is nearly the same concen- 

 tration as physiological salt solution (its strength is 0.58 per 

 cent.), the amount of dissociation is so great that 84 per cent, 

 of the molecules of NaCl have been changed into the ionic 

 form and but 16 per cent, remain as molecules at room temper- 

 ature. Evidently, since it is the solvent that causes the dis- 

 sociation, the nature of the solvent will make a great difference 



