TRANSACTIONS OF SECTION B. — PRESIDENTIAL ADDRESS. 349 



Section B.— CHEMISTRY. 

 President of the Section. — Professor J. Walker, D.Sc, F.R.S. 



THUESDAY, AUGUST 31. 

 The President delivered the following Address : — 



Theories of Solutions. 



Twenty-one years ago the Chemistry Section of the British Association at its 

 meeting in Leeds was the scene of a great discussion on the nature of solutions. 

 It was my first experience of a British Association meeting, and I well remember 

 the stimulating effect of the lively discussion on all who took part in it. To-day, 

 speaking from the honourable position of President of the Section, I conceive 

 I can do no better than indicate the position of the question at. the present 

 time. And this appears to me the more appropriate as our science has had this 

 year to mourn the departure of van't Hoff, the founder of the modern theory 

 of solution, whose name will remain one of the greatest in theoretical chemistry 

 —in time to come, it will, I think, be considered almost the greatest. He had 

 expressed the hope that he might, attend this meeting as he did that twenty- 

 one years ago. The hope is not fulfilled : his activity is merged in the final 

 equilibrium of death. But his ideas are part and parcel of the chemical equip- 

 ment of every one of us, and we know that whatever form the fundamental 

 conceptions of chemistry may assume, the quantitative idea of osmotic pressure 

 will be to the theory of solution what the quantitative idea of the atom is to 

 chemical composition and properties. For I must emphasise the fact that 

 chemistry is essentially a quantitative science, and no chemical theory, no 

 partial chemical theory even, can be successful unless its character is quantita- 

 tive. To quote the words of Lord Kelvin : ' I often say that when you can 

 measure what you are speaking about, and express it in numbers, you know 

 something about it ; but when you cannot measure it, when you cannot express it 

 in numbers, your knowledge is of a meagre and unsatisfactory kind ; it may be 

 the beginning of knowledge, but you have scarcely in your thoughts advanced to 

 the stage of science.' 



A general theory of solutions must be applicable to all solutions — to those 

 in which solvent and solute exist in practically mere intermixture, as well to 

 those in which solute and solvent are bound together in what we cannot sharply 

 distinguish from ordinary chemical union. Between these extremes all grades 

 of binding between solvent and solute exist, and it may be well to give a few 

 examples illustrating the various types of solution. 



Where no affinity exists between solvent and solute, the solution is practically 

 of the same type as a mixture of two gases which are without chemical action on 

 each other. The solute is merely diluted by the solvent and retains its properties 

 unchanged. An example of this type of solution may be found in the solution 

 of one saturated hydrocarbon in another, say of pentane in hexane. On mixing 

 the two liquids there is no evidence of union between them, the volume of the 

 mixture is practically the sum of the volume of the components, the heat of 



