Theory of the Ideal Dilute Solution. 525 



cases. It need hardly be stated that ample experimental 

 evidence exists to disprove this deduction. Thus dilute 

 solutions of both alcohol in water, and water in alcohol, are 

 formed with contraction of total volume. 



In conclusion a few remarks on this important question of 

 the kinetic explanation of Raoult's law will not be out of place. 

 The fundamental difficulty is to reconcile the varied volume 

 relationships shown by different ideal solutions with the 

 uniformity exhibited in their compliance with Raoult's law. 

 Thus in the case of a dilute solution of alcohol in water con- 

 taining one molecule of alcohol in a thousand molecules of 

 water, the number of water molecules per unit volume is 

 reduced *3 per cent, by the addition of the alcohol, while the 

 corresponding reduction in the case of a similar solution of 

 water in alcohol is only about "023 per cent. Yet in both 

 cases the reduction of the partial vapour pressure of the 

 solvent is '1 per cent. Dr. Tinker's theory, in its application 

 to the ideal dilute solution, amounts to the statement that this 

 discrepancy between the proportional reduction in the volume 

 concentration of the solvent and the reduction in the molar 

 fraction, disappears if instead of considering the total 

 volumes of solvent and solute, we consider the " free space " 

 in each. It is evident from equations (A) and (B) that this 

 explanation necessitates the universal validity of the relation* 



6=(V I -6 1 )-(V 2 -6 2 ), 



which, as we have seen, cannot be generally true. 



The failure of Dr. 'Tinker's theory is due to the fact that 

 it takes no account of the change in the intermolecular 

 forces produced by the addition of the solute. The fulfilment 

 of Raoult's law is due to the combined action of two separate 

 factors (1) the diminution of the volume concentration of the 

 solvent molecules, (2) the alteration of the intermolecular 

 forces acting on the solvent molecules. Both these factors 

 vary very much in different solutions, but must somehow be 

 mutually adjusted so as to produce the fulfilment of Raoult's 

 law. I hope, in a future communication, to show how this 

 exact adjustment is brought about. 



I am, 



High Kilburn, York. Yours faithfully, 



Sept. 11, 1917. S. A. Shorter. 



* Dr. Tinker does not appear to have noticed that this relation (p. 435 

 of his paper) and the assumption of the possibility of a relation such as 

 (V 2 — b 2 +e) = lO(Yi-b 1 ) (p. 434) are inconsistent with each other. 



Phil. Mag. S. 6. Vol. 34. No. 204. Dec 1917. 2 P 



