68 PHENOMENA, ATOMS, AND MOLECULES 



acid or in molten salts or metals, there are probably no group molecules 

 at all ; the atoms being held together by secondary valences. 



A drop of liquid is thus regarded as a single large molecule. In fact, 

 the whole earth, including the oceans (but excluding the atmosphere) may 

 be looked upon as a single molecule. 



The most essential difference between liquids and solids is the mobility 

 of the former. The chemist is already familiar with a mobility between 

 different parts of a molecule and has given the name tautomerism to this 

 phenomenon. The mobility of liquids is therefore a result of tautomeric 

 re-arrangements of the atoms. 



Evaporation, condensation, freezing, melting, solution, adsorption, and 

 surface tension, and even viscosity, are thus chemical phenomena, and 

 chemical knowledge already available may be directly applied in their study. 



This theory has been applied with marked success in the study of the 

 adsorption of gases by solids. Adsorbed gases are regarded as being chemi- 

 cally combined (either primary or secondary valence) with the atoms 

 forming the surface of the solid. The adsorbed film thus usually consists 

 of a single layer of atoms or group molecules, which forms a continuation 

 of the structure of the solid. The effect of such films on the catalytic 

 activity of surfaces (as, for example, platinum) depends entirely upon the 

 nature and the arrangement of the atoms forming the actual surface of 

 the solid. 



In a similar way a theory of surface tension is now proposed in which 

 the structure of the surface layer of atoms is regarded as the principal 

 factor in determining the surface tension (or rather surface energy) of 

 liquids. This theory is supported in the most remarkable way by all avail- 

 able published data on the surface tension of organic liquids. 



According to this theory, the group molecules of organic liquids arrange 

 themselves in the surface layer in such a way that their active portions are 

 drawn inwards, leaving the least active portion of the molecule to form 

 the surface layer. By "active portion" of a molecule is meant a portion 

 which is characterized by a strong stray field (residual valence). Chemical 

 action may be assumed to be due to the presence of electromagnetic fields 

 surrounding atoms. Surface tension (or surface energy) is thus a measure 

 of the potential energy of the electromagnetic stray field which extends 

 out from the surface layer of atoms. The molecules in the surface layer of 

 the liquid arrange themselves so that this stray field is a minimum. 



The surface energy of a liquid is thus not a property of the group mole- 

 cules, but depends only on the least active portions of the molecules and 

 on the manner in which these are able to arrange themselves in the surface 

 layer. 



In liquid hydrocarbons of the paraffin series, the molecules arrange 



