EVAPORATION AND CONDENSATION 309 



In a diamond crystal the carbon atoms form an endless chain of benzol 

 rings, and the stability of this chemical group is probably the cause of the 

 hardness, high melting-point and low vapor pressure of solid carbon. There 

 is every reason to assume that all forces acting between the atoms of solid 

 bodies are qualitatively of the same nature. The inert gases crystallize 

 at low temperatures, so that even with these substances the atoms are 

 surrounded by force fields which are only quantitatively different from 

 those of more chemically active substances. 



In the case of the diamond the arrangement of the atoms corresponds 

 to the tetravalent character of the carbon atom, but in most crystals the 

 valency of the atoms appears to be divided between several other atoms. 

 Thus in the sodium chloride crystal the single valency of the sodium is 

 divided between six chlorine atoms. 



The principal characteristics of chemical forces are that they act only 

 through very short distances and that they show in a marked degree the 

 phenomena of saturation. Thus chemical forces act only between adjacent 

 atoms, and even then only when the distances between the atoms are of the 

 order of magnitude of the diameter of the atoms. When an atom of oxygen 

 has taken up two hydrogen atoms it loses its attraction for other hydrogen 

 atoms : it becomes chemically saturated. However, from the fact that water 

 molecules become associated (H20)n and readily combine with substances 

 as water of crystallization, we may conclude that the presence of two 

 hydrogen atoms around an oxygen atom does not entirely saturate the 

 chemical forces. Werner's theory of valence recognizes the existence of 

 residual valences. 



In a crystal the forces of attraction must be balanced by repulsive 

 forces. We cannot assume that the repulsive forces are exerted only during 

 collisions between adjacent atoms but. because of the work of Einstein,^^ 

 Lindemann ^^ and others, must assume that the atoms oscillate about 

 positions of equilibrium in which the attractive and repulsive forces 

 balance. 



The atoms on the surface of a crystal must also be held by attractive 

 and repulsive forces, so that they oscillate about equilibrium positions, 

 but these atoms must be looked upon as being in an unsaturated state. 

 There is every reason, therefore, for the existence of very strong attrac- 

 tive forces within a short distance of the surface of solid bodies. The 

 phenomena of adsorption may be looked upon as a result of these un- 

 saturated chemical forces. This will be discussed in more detail in Part II. 

 of this paper. 



^^ Ann. Phys., S4, 170, 1911, and 35, 679, 1911. 

 ^" Phys. Ztschr., ji, 609, 1910. 



