the Standpoint of the Kinetic Theory. 557 



N + n to N, and that this will also be the ratio of the vapour- 

 pressures, as required by the above formula. The argument 

 is, of course, identical with that used by Poynting to show 

 that each molecule of solute must destroy the mobility of a 

 molecule of solvent ; but, whereas he was led to assume the 

 regular formation of labile monohydrates, the theory given 

 above merely postulates that the mobility of a solvent molecule 

 is destroyed when its place in the surface of the liquid is 

 occupied by a molecule of the solute. 



Surface Structure or Liquids. 



It will at once be noticed that in its simplest form the kinetic 

 theory of osmotic pressure would indicate that the pressures 

 calculated from the gas-formula might be subject to a small 

 correction for the volume changes accompanying dissolution. 

 Whether such corrections are necessary can only be deter- 

 mined by experiment, but the evidence now available points 

 to a very close agreement between the values observed and 

 calculated for dilute solutions. If this identity should be 

 confirmed, it will be possible to deduce from observations of 

 osmotic pressure some information in reference to the surface 

 structure of liquids, since if the agreement is exact there 

 must be an equally exact replacement of solvent by solute in 

 the surface of the liquid. Thus in view of the different 

 masses of the molecules that may be dissolved in the same 

 solute and yield identical osmotic pressures, there must be a 

 considerable spacing between the actual molecules in the 

 surface. Again, it may be noted that this exact replacement 

 does not take place in the interior of the liquid where the 

 molecular volumes of different solutes differ widely, and 

 when calculated in the conventional way may even have a 

 negative value. It is, however, by no means improbable 

 that the marshalling of the molecules on the frontiers of the 

 liquid may be governed by a stricter discipline than that 

 which prevails in the interior, and that the surface molecules 

 may even be forced to conform to the exact regulations which 

 govern the replacement of molecules in solid solutions or 

 isomorphous mixtures. 



Formation of Complexes. 



The formation in the solution of loose complexes of solvent 

 with solute or of solvent molecules with one another has not 

 yet been referred to, but presents no difficulty in the develop- 

 ment of the theory. Such complexes are usually formed 

 without any large change of volume, and under the stricter 



