CORRESPONDENCE OF MOLECULAR STATES 221 



center forms two balanced systems of potential energy. 

 For a molecule at the surface as in Fig. 24 it is evident 

 that the systems are unbalanced, and hence it would 

 move inward. (The systems which a surface mole- 

 cule forms with other molecules on the surface appear 

 in balanced pairs, and there 

 is no tendency to move 

 along the surface.) The 

 p.e. of the liquid molecules 

 is therefore decreased by 



u J A! FIG. 24. 



such an inward motion 



until the surface is as small as is consistent with the 

 volume which it must contain. The ratio of surface 

 area to volume thus reduces to a minimum, and hence 

 liquid drops are spherical unless affected by gravity. 

 This phenomenon is known as "surface tension. " 



When a liquid "wets" a solid in contact with it, the 

 molecules of the liquid and those of the solid form 

 systems for which the potential energy is 

 greater, other things being equal, than it 

 FIG 25 * s f r ^ e systems of molecules of the liquid 

 alone. The liquid molecules tend to 

 spread out over the surface of the solid, reducing as 

 much as possible their separation from those of the 

 solid. Conversely if the liquid does not wet the solid 

 it is unaffected by the latter. Thus a drop of mercury 

 in contact with clean glass assumes the oblate 1 sphe- 

 roidal form shown in Fig. 25. 



At the critical temperature at which liquefaction is 



1 The drop of mercury approaches the spherical form until a fur- 

 ther increase in sphericity would cause an increase in gravitational 

 p.e. greater than the decrease in molecular p.e. 



