144 A STUDY OF THE ABSORPTION SPECTRA. 



theory of solvation, as far as aqueous solutions are concerned, is now on 

 approximately a quantitative basis. 



The question arises, in this connection, why is a true and adequate 

 theory of solutions of such importance not only for physical or general 

 chemistry, but for so many branches of science? The answer is to be found 

 in the importance of solutions, in the broad sense of that term, for the 

 natural sciences. The one reason above all others why physical chemistry 

 has reached out into so many branches of science, is that it deals scientifi- 

 cally with solutions. Into what branch of science do solutions not enter? 

 Chemistry is essentially a science of solutions. Physics depends upon solu- 

 tions for many of its more important developments. Geology deals with 

 the results of solution not only in the sedimentary rocks, but in the fused 

 magmas. We might almost say without solution no geology. 



When we turn to the biological sciences we find many of them funda- 

 mentally connected with the science of solutions. This is especially true 

 of physiology, as Loeb has shown. It is almost equally true of pharma- 

 cology; and solutions are of fundamental importance for physiological 

 chemistry, physiological botany, and pathology. Indeed, about the only 

 branch of natural science that seems to be independent of solutions is 

 astronomy. 



We can see from the above why a theory that accounts for the prop- 

 erties of solutions in general is of fundamental importance for the develop- 

 ment of the natural sciences. 



A word as to the relation between the solvate theory and the theory of 

 electrolytic dissociation, lest some one should suppose that they are antag- 

 onistic. The theory of electrolytic dissociation, as has been pointed out, 

 simply says that molecules of electrolytes in the presence of a dissociating 

 solvent are broken down more or less into ions. It does not raise any 

 question as to whether the ions are or are not combined with any of the 

 solvent. It has been shown that while this theory is necessary to account 

 for the properties of solutions, and is accepted without question by prac- 

 tically all chemists of reputation, it is not sufficient to account for the 

 properties especially of concentrated solutions. 



We must go farther than recognize dissociation and determine its 

 magnitude. We must find out the condition of the ions in solution after 

 they are formed, and of the undissociated molecules. This the theory of 

 solvation aims to do. It attempts to answer the question whether the 

 molecules or ions are combined with any part of the solvent, and, if so, 

 with how much. The theory of solvation thus supplements the theory 

 of ionization, and when the former is upon as good a quantitative basis 

 as the latter, we shall have a satisfactory theory of solution. 



