WILLIAM D. HARKINS 



163 



cal, then h/\ should be much larger, since there is no "light" end which can be lifted 

 into the surface. 



In entire confirmation of these ideas, h/\ is very small (0.18) for the lower al- 

 cohols which have highly unsymmetrical molecules, and much larger (0.50-0.60) for 

 highly symmetrical molecules, such as those of oxygen, nitrogen, or mercury. These 

 values are those found at a corresponding temperature of 0.7 (T = o.'jTc), but the same 

 general relations are followed at other temperatures. 



/o £0 JO 40 so 



Fig. 15. — The free interfacial energy (or interfacial tension) between organic liquids and water 



This is the only evidence thus far found which indicates strongly that the mole- 

 cules in the surfaces of pure liquids are oriented. 



3 . Evidence for orientation in relations of monomolecular films. — If a long-chain fatty 

 acid, such as stearic acid, is dissolved in hexane it spreads on water to form a dilute 

 film I molecule thick. This may be compressed between the movable barrier (back 

 of Fig. 17) and the floating barrier attached to a film balance (front of Fig. 17). When 

 the film is compressed until it is tightly packed, but still monomolecular, the area per 

 molecule is 19.3 A units of area (19.3 A^ = 19.3X10"'^ sq.cm.) (Fig. 18). The square 

 root of this area, 4.4 A, gives an idea of the diameter of the area occupied by the 



