588 WYCKOFF: forces between atoms in souds 



Adsorption ^"^ — There will be fields of force upon the surfaces 

 of all solids and all liquids. The molecules of the gas surround- 

 ing the solid likewise possess stray fields of greater or less im- 

 portance. One of two things will happen to those gas mole- 

 cules which strike the surface of the solid (or liquid) . 



1. If the molecule is moving so fast that its energ}^ exceeds 

 that necessary to hold together the two fields, it will be reflected 

 from the surface with a loss in energy depending upon the stray 

 forces. 



2. Otherwise it will condense and be held upon the surface. 

 The molecule may by reason of collisions from other molecules 

 acquire enough energy to enable it again to leave the surface. 

 The average life and the number of atoms upon the surface at 

 any one time will depend upon the balance set up between these 

 opposing tendencies. As a result of the condensation (adsorp- 

 tion) a new surface, really one of the adsorbed substance, is 

 produced. 



From this point of view adsorption is seen to depend upon the 

 following factors : 



1. The adsorbed substance. The greater the outside fields of 

 the molecules, the higher will be the adsorption. 



2. The adsorbing substance. Solids of the valency and polar 

 (electrolyte) types should have large surface fields and should 

 adsorb strongly. Those solids, the particles of which are held 

 together by stray fields only (the organic type), will adsorb to 

 only a slight extent if at all. 



Solubility. — Solubility results from the entangling of the fields 

 of force of the solute and solvent. The process is influenced by: 



I. The intensity of the residual forces of the solvent. When 

 these forces are large, the degree of association of the liquid will 

 furnish a rough measure of the forces. With weaker non- 

 associating liquids the boiling point, combined with the weight 

 of the molecule, gives the desired information. 



2. The intensity of the residual forces of the solute. The vari- 

 ous kinds and degrees of these have been discussed under a con- 

 sideration of various typical solids. 



*2 I. Langmuir. Phys. Rev. 8: 149-176. 1916; Journ. Amer. Chera. Soc. 40: 

 1361-1403. 1918. 



