on Mixtures of Alcohol and Water. 11 



of the pressure. In support of this there is the long-known 

 fact that a copper ferrocyanide membrane is permeable 

 to alcohol, though not in an osmotic sense * ; and from 

 experiments recently performed by the writer, there i* 

 evidence that, under mechanical pressure, alcohol gets through 

 the membrane more easily than water. This removes the 

 argument based on the larger molecular volume of alcohol, 

 which might be put forward in favour of a sieve-like action. 

 However, with all this in support of the alcohol passing- 

 outwards, we find an increase of the internal pressure ; the 

 water, though present in so small quantity, gets through the 

 membrane. 



Any form of hydrate theory assumes an attraction between 

 the two kinds of molecules. This attraction is the result of 

 what has been called " residual affinity " |, a conception since 

 revived by Lodge % and Traube §. Its origin is most likely 

 t; chemical/' and its action must be one of the causes of which 

 osmotic pressure is the effect ; most probably it is the cause. 

 In consequence of this attraction there is mutual potential 

 energy between the solvent and the solution. This tends to 

 reach a minimum, and in doing so under the restrictions of 

 an osmotic experiment, an opposing pressure is set up which 

 hinders dilution and the consequent reduction of potential 

 energy of solution. 



I. Traube || has very recently put forward the theory that 

 difference between the surface-tensions of the solvent and 

 solution is the cause of the osmotic current ; and that the 

 latter will be from the liquid of weaker surface-tension. In 

 connexion with this there is the experiment at the end of 

 Table IV., where the current was from the weaker solution : 

 that is, from the one with the greater surface-tension. In 

 the same way, many similar experiments both with copper- 

 ferrocyanide membrane and animal-parchment membranes 

 are not in agreement with this theory. The only method of 

 finally testing such a theory would be to use a membrane 

 equally permeable in an osmotic sense to both liquids of the 

 mixtures. The existence of such a membrane seems highly 

 improbable. 



In any adequate theory of osmotic pressure, the part 

 played by the membrane must be taken into account. This 

 necessary part is that the membrane must absorb that liquid 



* Tamnian, Ann. Phys. Chem. [2] xxxiv. p. 309. 



t Pickering, Ber. Bent. Chem. Ges. xxiv. p. 3629. 



X 'Nature,' lxx. p. 176. 



§ Phil. Mas-, viii. (Aug. 1904) p. 162. 



|| Phil. Mag. viii, (Dec. 1904) p. 708. 



