60 PHENOMENA, ATOMS, AND MOLECULES 



of introducing definite small amounts of oxygen by diffusion through a 

 heated silver tube. In this way known numbers of oxygen atoms may be 

 placed on the surface of a tungsten filament thus forming negative dipoles. 

 These attract the positive caesium dipoles which tend to form clusters 

 around them, forming a kind of 2-dimensional colloidal distribution of 

 caesium. We hope by the detailed study of such systems to learn the effects 

 not only of repulsive but also of attractive forces between adatoms. Ex- 

 periments are also in progress to study the adsorption of caesium on 

 tungsten surfaces containing known numbers of adsorbed thorium atoms. 



In such ways as these, together with the methods involving the spread- 

 ing of oil films on water, it becomes possible to determine the equations of 

 states of 2-dimensional gases, with nearly the same degree of accuracy as in 

 the case of 3-dimensionals, but with the further advantage that the con- 

 ditions are especially favorable for the determination of the mechanism of 

 all of the processes involved. 



Types of Adsorption. 



The classical kinetic theory of gases, as for example in the treatment of 

 viscosity by Sutherland and of the continuity of the transition from gas 

 to liquid by van der Waals, led to the recognition of attractive forces 

 between molecules at large distances and of strong repulsive forces at short 

 distances. The chemist has long known that exceptionally large attractive 

 and repulsive forces are associated with the chemical union between ad- 

 jacent atoms, which he represents by the valence bond (primary valence) 

 and that weaker forces, which he has called secondary valence forces, 

 usually correspond to other types of chemical combinations. The physicist 

 has studied the forces between charged particles, such as ions and dipoles, 

 and more recently has discovered the interchange forces, which are best 

 explained in terms of quantum mechanics. 



In the structure of matter there can be no fundamental distinction 

 between chemical and physical forces : it has been customary to call a force 

 chemical, when it is more familiar to chemists, and to call the same force 

 physical, when the physicist discovers an explanation of it. There are also 

 usually no sharp dividing lines between attractive and repulsive forces or 

 between the various kinds of attractive or repulsive forces. Nevertheless a 

 recognition of various types of forces responsible for the stability of matter 

 proves useful in classifying natural phenomena. Thus in a qualitative way 

 we may consider the following types of forces (29) : 



( 1 ) Coulomb forces between ions or ions and electrons varying with 

 1/r^ where r is the distance between ions. The best examples of these forces 

 are in salt-like substances. 



(2) Forces between dipoles which vary with l/r'* and depend on the 

 orientation of the dipole. 



