SURFACES OF MOLECULES 219 



H. B. Hardy (i) in 1912 and the writer (2) (3) in 1916 pointed out 

 that the fields of force around many chemical molecules must be very un- 

 symmetrical, and that this resulted in an orientation of the molecules at 

 the surfaces of liquids and in adsorbed films on solids. It was also found 

 that the shapes which the molecules could assume, and which were deter- 

 mined by their chemical structures, were of great importance in connection 

 with such properties as surface tension. 



Within recent years, particularly through the work of Debye and his 

 students, a great deal of knowledge has been accumulated regarding the 

 electrical forces in liquids and solutions which contain ions and dipoles 

 Particularly in the study of electrolytes has this work cleared up many of 

 the most serious difficulties of earlier theories. 



There is now no need of drawing distinctions between chemical and 

 physical forces. The chemist recognizes that many of the forces with which 

 he has been dealing are electrical in nature, for, after all, molecules are 

 built up of electrons and positive nuclei, and their interactions must neces- 

 sarily be electrical. However, the simple classical theory of electrical forces 

 based on Coulomb's law is wholly insufiicient to account for chemical prop- 

 erties. With charged particles obeying Coulomb's law only, the minimum 

 of potential energy is obtained when the positive and negative particles 

 coincide. There must be something corresponding to repulsive forces 

 holding the particles apart. 



REPULSIVE FORCES BETWEEN MOLECULES 



In many of the later developments of the kinetic theory where the 

 molecules were regarded as rigid spheres, the repulsive forces which must 

 counteract the attractive forces were assumed to act only during collisions — ■ 

 that is, at the instant of contact of the spheres (4). No explanation was 

 attempted of the internal forces within the molecule which could result in 

 this repulsion between molecules. 



In order to account for the compressibility of solids. Born and Lande 

 (5) have assumed that the repulsive force between molecules (or ions) 

 varies inversely as a power of the distance between the centers of the 

 molecules. Thus in the case of halide salts of the alkalies, such as NaCl, KT, 

 etc., they showed that the compressibility indicated that the repulsive 

 force varied inversely as the tenth power of the distance, while the attrac- 

 tive force resulting from the charges on the ions varied inversely as the 

 square, in accord with Coulomb's law. Born (6) also attempted to show 

 that the inverse tenth power law for repulsion could be deduced by assum- 

 ing that the electrons and the ions were arranged with cubic symmetry as 

 at the corners of a cube. This attempt, however, must be regarded as a 



