158 PROTOPLASM 



in an optical section through the center of the Hquid {M, Fig. 93). 

 But a molecule at the surface has none of its own kind above it 

 and is therefore subjected to an unequal attraction {N, Fig. 93). 

 As a result, all surface molecules are pulled one way or the other; 

 if on to adjoining foreign matter, adsorption results; if into their 

 own mass, as usual at liquid-air interfaces, there results a closer 

 packing of the molecules at the surface than exists deeper within 

 the liquid. This condition at the surface brings about the forma- 

 tion of an elastic film which is capable of supporting a steel 

 needle, of pulling a free droplet into a sphere, and of holding up 

 a column of water in a capillary tube. 



/\/V OOqOoOoOo 

 O-CMn^O o o o o 



o o o o o 

 o o o o o 

 oooooooo 



Fig. 93. — Diagram of the distribution of molecules within and at the surface of a 



liquid. 



When water and oil are in contact, there may result two inter- 

 facial membranes (one of water and one of oil), a single composite 

 membrane, or a transitional zone from pure water on one side to 

 pure oil on the other. Which condition prevails cannot be said. 



That a spherical or circular shape is "striven" for by surface 

 membranes is due to the law of minimum surface which is a 

 corollary of the law of minimum energy. The second law of 

 thermodynamics states that all systems "strive" toward the 

 state of least free energy — they tend to run down. A droplet 

 assumes the shape of a sphere when free, because the sphere, 

 among all possible shapes, encloses maximum volume with 

 minimum surface. (There are some slight exceptions to the rule 

 that the circle and sphere enclose the maximum area or volume 

 with minimum circumference or surface. The surface-tension 

 film of a liquid in a tube has, in section, the shape of a catenary. 

 This is not the segment of a circle, but it is a minimum surface.) 



