The Plasmatic Membrane. 
1 81 
I.—The Physical Chemistry of Surfaces. 
The purely mechanical result of the special condition of the 
surface-layer of a mass of solid or liquid is the same as if vve 
imagine the existence of an elastic skin compressing the mass and 
tending to deform it until the skin attains the smallest surface 
possible under the circumstances. The skin thus exerts a surface- 
tension, and energy is liberated when the skin contracts, and 
absorbed when the skin is caused to extend. 
When this surface-tension has a negligible effect on the form 
of the mass, the substance is said to be a solid, but liquid if the 
surface-tension predominates over the resistance of internal 
friction, so that a particle otherwise unconstrained takes the 
spherical form, which gives the minimum possible surface for the 
volume. Of known liquids water has the highest surface-tension 
at ordinary temperatures, and as a purely physical factor the effect 
of this property enters into a number of biological phenomena. 
This physical state of the surface-layers of a liquid has, how¬ 
ever, a still more profound effect in the modification which it 
induces in the chemical processes taking place in these layers. 
Such surfaces are to be regarded as being the seat of a special 
form of energy, which is termed “ surface-energy,” and it is this 
surface-energy which is liberated on contraction and becomes latent 
in expansion of the surface. 
The presence of surface-energy in the superficial layer affects 
the chemical processes taking place there in a way quite analogous 
to the way in which temperature, or pressure, or electrical energy, 
affect chemical processes. This has been clearly formulated by 
Ostwald, who, in the last edition of his “ General Chemistry,” 
devotes a special section to what he calls “ Microchemistry,” 1 or 
the chemistry of small masses. The smaller a mass of matter, 
the greater is the proportion of its surface to its volume and the 
more the chemical processes conditioned by surface-energy 
dominate over the ordinary processes, which are conditioned by 
volume-energy and take place throughout the rest of its mass. 
When a drop of water has only a diameter of about 1 fj. (which is 
near the limit of microscopic vision), then even its density is per¬ 
ceptibly greater than unity and the surface energy effects are quite 
1 Third English Edition, translated by W. W. Taylor (MacMillan, 
1912). To such an act of piracy with regard to a term with a 
long established different use in biology we can only submit 
without protest, in respect for the chemical philosopher to 
whom science owes so much. 
