362 WELLS: PHYSICAL VS. CHEMICAL FORCES 



to both physics and chemistry, according to which forces are 

 classified as (i) molar,"^ (2) molecular, (3) atomic, and (4) electronic. 

 There is little anthropomorphic in these words, and they center 

 the attention on the phenomena. The classification is quali- 

 tative in the sense that no quantitative relations of these forces 

 to energy have yet been defined. The electron theory of atomic 

 structure, however, dispels much of the vagueness surrounding 

 many of the forces, and raises into further prominence the con- 

 viction of Faraday that there is but one fundamental type of 

 force. 



Electronic forces may be defined as those which maintain 

 the negative or valence electrons and the positive nucleus in 

 equilibrium as a single system. Similarly, atomic forces may 

 be defined as those which maintain two or more atoms in equi- 

 librium as a single system ; molecular forces as those which main- 

 tain two or more molecules in equilibrium as a single system; 

 and molar forces those which maintain two or more masses in 

 equilibrium as a single system. Each group of forces may be 

 regarded as the residual fields of force remaining unsaturated in 

 the smaller systems constituting the components of the system 

 under consideration. 



For the description of certain phenomena the molar theory is 

 sufficient, but soon a stage is reached at which the phenomena 

 must be considered in more detail. The system considered is 

 then restricted to groups of molecules. This system is in its 

 turn further restricted to atomic groups, and so on. The im- 

 portant point is that each theory includes all those preceding as 

 special cases, as the result of simplifying approximations, or as 

 arising from statistical considerations. The more closely one 

 wishes to' examine the nature of the extended system the more 

 the point of view approaches that of the restricted system. The 

 key to the nature of molecular forces, for example, is to be found 

 in atomic considerations. This is the point particularly empha- 

 sized by Langmuir in his paper. 



2 Molar is derived from tlie Latin moles, a large mass. In the Century Dic- 

 tionary molar forces are defined as those producing motions between large masses, 

 molecular forces those between molecules, but which are insensible at sensible dis- 

 tance. 



