680 TRANSACTIONS OF SECTION I. , 



influence of similar molecules ; on the other side, they are exposed to the 

 influence of molecules of a nature chemically unlike their own or in a different 

 physical state of aggregation. The result of such asymmetric forces is that the 

 phase boundary is the seat of various forms of energy not present in the interior 

 of the phase. The most obvious of these is the surface energy due to the 

 state of tension existing where a liquid or a gas forms one of the phases. It 

 ■would lead us too far to discuss the mode of origin of this surface tension, 

 except to call to mind that it is due to the attractive force of the molecules for 

 one another, a force which is left partially unbalanced at the surface, so that 

 the molecules here are pulled inwards. The tension is, of course, only the 

 intensity factor of the surface energy, the capacity factor being the area of 

 the surface. We see at once that any influence which alters the area of the 

 surface alters also the magnitude of that form of energy of which we are 

 speaking. 



This is not the only way in which the properties of substances are changed 

 at phase boundaries. The compressibility of a solvent, such as water, are 

 altered, so that the solubilities of various substances in it are not the same 

 as in the interior of the liquid phase. It is stated by J. J. Thomson that 

 potassium sulphate is 60 per cent, more soluble in the surface film. The ways 

 in which the properties of a solvent are changed are sometimes spoken of as 

 ' lyotropic,' and they play an important part in the behaviour of colloids. We 

 meet also with the presence of electrical charges, of positive or negative sign. 

 Those are due, as a rule, to electrolytic dissociation of the surface of one phase, 

 in which the one ion, owing to its insolubility, remains fixed at the surface, 

 while the opposite ion, although soluble, cannot wander away further than 

 permitted by electrostatic attraction. Thus we have a Helmholtz double 

 layer produced. 



Before we pass on to consider how these phenomena intervene in physio- 

 logical processes, there is one fact that should be referred to on account of its 

 significance in connection with the contractile force of muscle. Surface tension 

 is found to decrease as the temperature rises, or, as it is sometimes put, it has a 

 negative temperature coefficient. This is unusual ; but, if we remember that 

 the interface between a liquid and its vapour disappears when the temperature 

 rises to the critical point and, with it, of course, all phenomena at the boundary 

 surface, the fact is not surprising that there is a diminution of these phenomena 

 as the critical temperature is approached. 



Perhaps that result of surface energy known as ' adsorption ' is the one in 

 which the conditions present at phase boundaries make themselves most 

 frequently obviou.s. Since the name has been used somewhat loosely, it is a 

 matter of some consequence to have clear ideas of what is meant when it is 

 made use of. Unless it is used to describe a definite fact, it can only be 

 mischievous to the progress of science. 



Permit me, then, first to remind you of that fact of universal experience, 

 known as the ' dissipation of energy,' which is involved in the second law of 

 energetics. Free energy — that is, energy which can be used for the performance 

 of useful work — is invariably found to diminish if the conditions are such that 

 this is possible. If we have, therefore, a system in which, by any change of 

 distribution of the constituents, free energy can be decreased, such a change of 

 distribution will take place. This is one form of the well-known 'Principle of 

 Carnot and Clausius.' 



Now, practically any substance dissolved in water lowers the surface tension 

 present at the interface between the liquid and another solid or liquid phase 

 with which i't is in contact. Moreover, up to a certain limit, the magnitude of 

 this effect is in proportion to the concentration of the solute. Therefore, as 

 was first pointed out by Willard Gibbs, concentration of a solute at an interface 

 has the effect of reducing free energy and will therefore occur. This is 

 adsorption. As an example, we may take the deposition of a dye-stuff on the 

 surface of char(^oal, from which it can be removed again, unaltered, by appro- 

 priate means, such as extraction with alcohol. Charcoal plus dye may, if any 

 satisfaction is derived from the statement, be called a compound. But, since 

 its chemical composition depends on the concentration of the solution in which 

 it was formed, it is much more accurate to qualify the statement by calling it an 



