INTRINSIC PRESSURE OF A LIQUID 47 



in the surface area. This state of strain is called the surface 

 tension of the fluid, and by release of this strain work may be 

 done. That there is apparently an elastic skin over fluids at 

 their junction with air is easily demonstrated. (Physicists call 

 the junction of a fluid with any other substance an interface. 

 They write of fluid-air, fluid-gas, fluid-fluid interfaces.) At a 

 water-air interface substances heavier than water can be sup- 

 ported. A clean needle floats on water. Water-beetles, etc., move 

 freely on the surface. Resistance to deformation is greater on the 

 surfacethan in the body of the liquid (cf. Searle's torsion balance). 

 How can this energy be utilised ? How can S.T. be either raised 

 or lowered ? Whatever alters intrinsic pressure will, of course, alter 

 surface tension. The magnitude of the intrinsic pressure depends 

 inversely on the kinetic energy of the molecules. The less the 

 kinetic energy the less the mean free path of the molecules, and 

 the less the distance between the molecules. Intrinsic pressure 



varies as -= 2 . This variation of S.T. with K.E. does not concern 



d 



the biologist, as alterations in K.E. imply alterations of tempera- 

 ture, and we have seen that only slight alterations in temperature 

 are compatible with life. 



It is possible to alter surface tension without altering intrinsic 

 pressure by altering the electric charge on the surface layer. 

 According to the Helmholtz-Lippmann theory, at any surface 

 there are two electrical layers equal in amount but opposite in sign. 



1 . On the surface molecules of the fluid. 



2. On the immediate external layer of the surrounding medium 



air, glass, etc. 



Therefore, any alteration in the amount of the charge on the 

 liquid layer will produce an alteration in the surface tension. 

 If the charge on the surface is increased, the K.E. of the mole- 

 cules (i.e. repulsion of like signs) will be increased, " d " will be 

 increased and S.T. will be diminished. Conversely, an increase 

 in S.T. is brought about by decreasing the electrical charge. 

 The S.T. cannot, of course, be increased beyond that which the 

 superficial layer would have in the absence of any charge or 

 double layer. This alteration may be brought about directly or 

 indirectly; 



I. Directly. Lippmann's capillary electrometer. If a capillary 

 tube be taken and a drop of mercury placed in it, the mercury 

 will run down and some will gather as a drop at the tip. If now 

 the tip of the capillary be immersed in dilute H 2 SO 4 , the Hg 



