DISCUSSION OF EVIDENCE. 193 



THE SOLVATE THEORY AND THE THEORY OF ELECTROLYTIC DISSOCIATION. 



When Arrhenius proposed the theory of electrolytic dissociation, 

 the question was not even raised as to the condition of the ions in the 

 solution, except that they behave as if they existed independently of 

 one another in solution. The theory simply said that molecules of 

 acids, bases, and salts, in the presence of a dissociating solvent like 

 water, break down to a greater or less extent into charged parts called 

 ions, the cations or positively charged parts being electrically equiva- 

 lent to the anions or negatively charged parts. The cations are usually 

 simple metallic atoms carrying one or more unit charges of positive 

 electricity. The cation might, however, be more or less complex, as 

 illustrated by ammonium and its substitution products. The anion is 

 usually complex, consisting of a larger or smaller number of atoms. 

 It may, however, be an atom carrying negative electricity, as in the 

 case of the halogen acids and their salts. 



The degree of dissociation is determined by the nature of the acid, 

 base, or salt. Strong acids and bases are greatly dissociated. Indeed, 

 the degree of dissociation determines their strength. Nearly all of the 

 salts are strongly dissociated compounds, there being, however, some 

 exceptions, as, notably, the halogen salts of mercury, cadmium, and 

 zinc. There are, however, considerable differences in the amounts to 

 which salts in general are dissociated at the same dilution. 



The quantitative evidence furnished by Arrhenius and others for the 

 theory of electrolytic dissociation is so convincing that few chemists 

 of any prominence, who have carefully examined the evidence, have 

 ever doubted the general validity of the theory; and the theory has 

 become substantiated by such an abundance of subsequently discovered 

 facts that it has now become a law of nature and a fundamental law 

 of chemical science. 



Arrhenius saw and pointed out clearly the importance of ions for 

 chemistry ; Ostwald and his pupils have shown that chemistry is essen- 

 tially a science of the ion, molecules for the most part being incapable 

 of reacting chemically with molecules ; and Nernst has proved that the 

 ion is the active agent in all forms of primary cells. 



The theory of electrolytic dissociation, as already stated, does not 

 raise the question as to the relation between the ion and the solvent. 

 At the time that the theory was proposed, chemists did not know, and 

 probably had no means of finding out, whether the ion is or is not com- 

 bined with the solvent in contact with it. The solution of this problem 

 remained for subsequent work. 



Some of the many lines of evidence that ions and certain molecules 

 are combined with a larger or smaller number of molecules of the sol- 

 vent, and in many cases with a very large number of molecules of the 

 solvent, have been recently discussed briefly by Jones in an article in 



