386 SCIENCE PROGRESS 



lithium chloride and for sugar, an element of uncertainty is 

 introduced in the measurements, and it seems desirable to 

 reserve judgment on the matter, pending further investigation. 



Combination between Solvent and Solute. The Hydrate 



Theory 



The question as to whether solution is a chemical or physical 

 process has long been one of the standing problems of chemistry. 

 According to the hydrate theory, already referred to, the solute 

 enters into chemical combination with the solvent, with forma- 

 tion of hydrates of varying complexity. The chief evidence 

 adduced in favour of this theory was that the curves repre- 

 senting the variation of the physical properties of such solutions 

 with dilution showed breaks at points corresponding with the 

 composition of various hydrates, and in many cases it was 

 possible to isolate the hydrates, With the development of 

 the electrolytic dissociation theory, although the possibility of 

 hydrate formation was not lost sight of, 1 most stress naturally 

 came to be laid on the independent existence of ions, and by 

 many the solvent was supposed to play a passive part, simply 

 acting as a medium for dissociation. In recent years, however, 

 more and more evidence has accumulated to show that the 

 solvent plays an essential part in the phenomenon. The 

 evidence in favour of the view that the ions, and in many cases 

 the undissociated molecules, are associated with molecules of 

 the solvent, will now be considered. 2 



(i) The first important contribution to this subject is due to 

 Werner, 3 as a result of his investigation of complex compounds 



1 Ostwald, Lehrbuch, ii. i, 801. 



2 It is important to realise clearly the distinction between the old hydrate 

 theory of Pickering and the electrolytic dissociation theory, including the possibility 

 of hydrated ions. On the basis of the former the molecules of the solute remain 

 intact in solution and the abnormal osmotic pressures shown by solutions of 

 electrolytes are regarded as being connected with hydrate formation ; on the 

 other hand, the fundamental point of the electrolytic dissociation theory is the 

 independent existence of the ions ; the increased osmotic pressure in solutions of 

 electrolytes is considered as being due to an increase in the number of particles of 

 solute owing to ionisation, and this number will not be affected if only one ion 

 enters into a complex with solvent molecules. The question of hydration, though 

 of great interest, is thus of secondary importance for the electrolytic dissociation 

 theory. 



3 For a summary of Werner's work, by himself, see Nenere Anschauungen 

 anf dem Gebicte der Atwrganischen Ckemie, Braunschweig, 1905. 



