V] OF SURFACE ENERGY 355 



no\v' call the potential energy* of the system; and we know, as a 

 fundamental theorem of dynamics, as well as of molecular physics, 

 that the potential energy of the system tends to a minimum, and 

 finds in that minimum its stable equihbrium. 



We see in our last equation that the term Mcq is irreducible, save 

 by a reduction of the mass itself. But the other term may be 

 diminished (1) by a reduction in the area of surface, S, or (2) by 

 a tendency towards equahty of e and ^q , that is to say by a diminu- 

 tion of the specific surface energy, e. 



These then are the two methods by which the energy of the 

 system will manifest itself in work. The one, which is much the 

 more important for our purposes, leads always to a diminution of 

 surface, to the so-called "principle of minimal areas"; the other, 

 which leads to the lowering (under certain circumstances) of surface 

 tension, is the basis of the theory of Adsorption, to which we shall 

 have some occasion to refer as the modus operandi in the develop- 

 ment of a celi-wall, and in a variety of other histological phenomena. 

 In the technical phraseology of the day, the '^capacity factor" is 

 involved in the one case, and the "intensity factor" in the otherf. 



Inasmuch as we are concerned with the form of the cell, it is the 

 former which becomes our main postulate: telhng us that the 

 energy-equations of the surface of a cell, or of the free surfaces of 

 cells in partial contact, or of the partition-surfaces of cells in contact 

 with one another, all indicate a minimum of potential energy in the 

 system, by which minimal condition the system is brought, ipso 

 facto, into equihbrium. And we shall not fail to observe, with 

 something more than mere historical interest and curiosity, how 



* The word Energy was substituted for the old vis viva by Thomas Young early 

 in the nineteenth century, and was used by James Thomson, Lord Kelvin's brother, 

 about 1852, to mean, more generally, "capacity for doing work." The term potential, 

 or latent, in contrast to actual energy, in other words the distinction between " energy 

 of activity and energy of configuration," was proposed by Macquorn Rankine, and 

 suggested to him by Aristotle's use of 8vva/j.L$ and evepyeia; see Rankine's paper 

 On the general law of the transformation of energy, Phil. Soc. Glasgow, Jan. 

 5, 1853, cf. ibid. Jan. 23, 1867, and Phil. Mag. (4), xxvii, p. 404, 1864. The phrase 

 potential energy was at once adopted, but kinetic was substituted for actical by 

 Thomson and Tait. 



t The capacity factor, inasmuch as it leads to diminution of surface, is responsible 

 for the concrescence of droplets into drops, of microcrystals into larger units, for 

 the flocculation of colloids, and for many other similar "changes of state." 



