466 THE FORMS OF TISSUES [ch. 



be obtained, caeteris paribus, by the utmost possible reduction of 

 the surfaces in contact. 



When we have three bodies in contact with one another the same 

 is true, but the case becomes a Httle more complex. Suppose a 

 drop of some fluid, A, to float on another fluid, J5, while both are 

 exposed to air, C Here are three surfaces of contact, that of the 

 drop with the fluid on which it floats, and those of air with the one 

 and other of these two; and the whole surface-energy, E, of the 

 system consists of. three' parts resident in these three surfaces, 



Fig. 151. 



or of three specific energies, -S^^, -E'^ic ^bc- "^^^ condition of 

 equihbrium, or minimal potential energy, will be reached by con- 

 tracting those surfaces whose specific energy happens to be large 

 and extending those where it is small — contraction leading to the 

 production of a "drop,'f and extension to a spreading "film." 

 Floating on water, turpentine gathers into a drop, olive-oil spreads 

 out in a film; and these, according to the several specific energies, 

 are the ways by which the total energy of the system is diminished 

 and equihbrium attained. 



A drop will continue to exist provided its own two surface-energies 

 exceed, per unit area, the specific energy of the water-air surface 

 around: that is to say, provided (Fig. 151) 



^AB + ^AC > ^BC 



But if the one fluid happen to be oil and the other water, then the 

 combined energy per unit-area of the oil-water and the oil-air 

 surfaces together is less than that of the water-air surface : 



E > E 4- E 



Hence the oil-air and oil-water surfaces increase, the air-water 

 surface contracts and disappears, the oil spreads over the water, 

 and the "drop" gives place to a "film," In both cases the total 

 surface-area is a minimum under the circumstances of the case, and 

 always provided that no external force, such as gravity, complicates 

 the situation. 



