SURFACES OF MOLECULES 229 



polar axis is twice as great as in the equatorial plane for a given value of r. 

 An analysis of experimental data, however, shows that in solutions of many 

 dipole substances in non-polar solvents, the polarization per molecule de- 

 creases as the concentration increases. Such a result could be explained 

 if the molecule instead of being a sphere were ellipsoidal with the equatorial 

 diameter less than 80 per cent of the polar diameter. The introduction of 

 such a concept would introduce far too great mathematical complications 

 to make an analysis worth. while. 



Perhaps the greatest objection to a mathematical treatment of the 

 properties of liquids in terms of power laws of force is that it becomes 

 practically impossible to take into account the efifects resulting from the 

 complicated shapes of molecules which must characterize organic sub- 

 stances, according to the structural formulas of the chemist. If now we 

 assume that the forces between molecules come into play only at their 

 areas of contact, we have simplified the problem mathematically to such an 

 extent that we can take into account our knowledge regarding the shapes 

 of the molecules which can be furnished us by the chemist. Of course, in so 

 doing we are making approximations, but the errors so introduced will, I 

 believe, usually be far less than those which are often made by the physicist 

 in dealing with these problems. I should like to outline now the ways in 

 which a conception of surface forces can be developed to give concrete 

 pictures and quantitative results for many problems which are too difficult 

 to handle by the method involving the power laws of force. 



In considering that the interactions between molecules occur at their 

 surfaces of contact, it is not necessary to assume that the nature of the 

 forces at any given part of the surface is characteristic wholly of the atom 

 imderlying the surface. The chemist knows, for example, that the properties 

 of an organic molecule are not simply the sum of the effects due to the 

 separate atoms in the molecule. If one of the hydrogen atoms in the methyl 

 group of acetic acid be replaced by a chlorine atom, the effect of the greater 

 charge on the kernel of the chlorine atom as compared with that of the 

 hydrogen atom which it replaces, is to displace the electrons in the carboxyl 

 group in the direction towards the chlorine atom. This displacement of the 

 pair of electrons which hold the nucleus of the hydrogen atom of the 

 carboxyl group causes the hydrogen nucleus to be held less firmly, and thus 

 makes it easier for the hydrogen nucleus to pass over to a water molecule 

 (in aqueous solutions of acetic acid) to form an OHs"" ion, which is the so- 

 called hydrogen ion characteristic of acids. We thus have a clear indication 

 of an alteration in surface forces which may extend over the whole surface 

 of the molecule as a result of the replacement of a hydrogen atom by a 

 chlorine atom. 



We may form a clearer conception as to the magnitude of these changes 



