48 LIBERATION OF ENERGY 



ment in depth, in tensile strength and in resistance to shear. This 

 experiment indicates that the thickness of the layer under the 

 influence of surface forces is at least of the order of 1,000 molecules, 

 its value depending on the eccentricity of the equipotential 

 surfaces of the molecules. The greater the eccentricity of the 

 fields of force about the molecules {i.e. the greater their polarity) 

 the thicker will be the oriented layer. 



Utilisation of Surface Energy. — As we have seen, the energy 

 developed at the surface is considerable. If some means could be 

 devised whereby this energy could be freed from the surface, or, 

 what comes to the same thing, if its intensity could be altered, 

 then it might be utilised to bring about changes in matter. Such a 

 potential power might have considerable significance in physiology. 

 The tissues of which organs consist and the cells that compose the 

 tissues abound in surfaces or interfaces where one liquid phase 

 subjoins another similar or dissimilar phase. Alterations in surface 

 tension, quite apart from gross energy changes, play a large part in 

 physiological processes, as we shall see later. 



Alterations in Surface Tension. 



A. Pure Liquids. — (1) Whatever alters the intrinsic energy of 

 the liquid will produce a corresponding alteration of the energy on 

 the surface. The attractive force between molecules of a liquid 

 (or gas) varies from absolute zero to the critical temperature 

 directly as a constant and inversely as the square of the distance 

 between the molecules. That is, increase of temperature will tend 

 to lower surface tension. In other words, surface tension has a 

 negative temperature coefficient. 



This means of varying surface energy is not of great interest to 

 the biologist, as it implies alterations of temperature which to be 

 significant have to be considerably more than is compatible with 

 life. 



(2) The electrical state of a surface layer is of interest in this 

 connection. Electrons accumulate on the aqueous side of a water- 

 air interface and they tend to cause the surface to expand. If 

 they are increased in number their mutually repellent power will 

 actually overcome the contracting power of Newtonian gravity, 

 and the surface will increase in curvature, i.e., expand, and the 

 surface tension be lowered. Examine the surface, for instance, of a 

 globule of mercury in water. The water molecules on this surface 

 are arranged with their polar ends, i.e., OH radicle, in the water 

 and the H"^ ion pressed against the mercury. A double electrical 

 layer thus exists. The metal, by virtue of the closely adherent H^ 

 ions, takes on a positive charge on its surface, while just external 



