294 THE FORMS OF TISSUES [ch. 



changes of temperature or of electric charge. The condition of 

 minimum potential energy in the system, which is the condition of 

 equihbrium, will accordingly be obtained by the utmost possible 

 diminution in the area of the surfaces in contact. When we have 

 three bodies in contact, the case becomes a little more complex. 

 Suppose for instance we have a drop of some fluid. A, floating on 

 another fluid, B, and exposed to air, C. The whole surface energy 

 of the system may now be considered as divided into two parts, 

 one at the surface of the drop, and the other outside of the same ; 

 the latter portion is inherent in the surface BC, between the mass 

 of fluid B and the superincumbent air, C ; but the former portion 

 consists of two parts, for it is divided between the two surfaces AB 

 and AC, that namely which separates the drop from the surrounding 

 fluid and that which separates it from the atmosphere. So far as 



the drop is concerned, then, equihbrium depends on a proper 

 balance between the energy, per unit area, which is resident in 

 its own two surfaces, and that which is external thereto : that is 

 to say, if we call Ejjf. the energy at the surface between the two 

 fluids, and so on with the other two pairs of surface energies, the 

 condition of equilibrium, or of maintenance of the drop, is that 



If, on the other hand, the fluid A happens to be oil and the fluid 

 B, water, then the energy per unit area of the water-air surface 

 is greater than that of the oil-air surface and that of the oil-water 

 surface together ; i.e. 



E > E + E 



Here there is no equilibrium, and in order to obtain it the water-air 

 surface must always tend to decrease and the other two interfacial 

 surfaces to increase ; which is as much as to say that the water 

 tends to become covered by a spreading film of oil, and the water- 

 air surface to be abolished. 



