330 PHYSICAL CHEMISTRY [CH. XXIV. 



Surface Tension. The surface layer of a liquid possesses certain properties 

 which are not shared by the rest of it, for in the interior the arrangement of matter 

 is symmetrical round any point, whereas on the surface the surroundings consist of 

 liquid on one side only, while on the other side is solid, or gas, or it may be another 

 liquid. In a gas, the molecules are free from one another's attractive influence 

 and fly about freely with high velocity, producing pressure on the walls of the 

 containing vessel ; in a liquid, the mutual attractions of the molepules are great 

 enough to keep the substance together in a definite volume ; in order to separate 

 the molecules and convert the liquid into gas a large amount of energy is required- 

 the so-called latent heat of evaporation. ^The molecular attractions in a liquid are 

 thus very great, so that a molecule of the surface layer is strongly pulled inwards, 

 and this layer constitutes a stretched elastic skin, and the power thus exerted is 

 spoken of as surface tension. The effect of surface tension is most simply seen in a 

 free drop of liquid, such as a rain-drop, or a drop of oil immersed in a mixture of 

 alcohol and water of the same density. There is then nothing to prevent the 

 tension in the surface layer from contracting as much as possible, and the drop will 

 therefore assume a form in which its volume will have the smallest surface, that is, 

 the drop will assume the form of a sphere. 



Now animal cells are liquid, and when they are at rest, other forces being 

 absent, they also are spherical, and although they do not possess, as a rule, a 

 definite wall of cellulose or other hard substance such as vegetable cells have, 

 nevertheless the surface film, exercising the force called surface tension, plays the 

 part of an elastic skin, and is termed the plasmatic membrane. This membrane 

 plays an important physiological role. In the projection of pseudopodia, for 

 instance, variations in the surface tension must occur in different parts of the 

 circumference of the cell. Protoplasm, however, is not a simple liquid, but 

 contains substances of varying chemical composition, and substances which have 

 the power of diminishing surface tension always show a tendency to accumulate at 

 the surface. Hence the fats and lipoids which are powerful depressants of surface 

 tension are found probably in a state of an extremely fine emulsion more 

 abundantly in the plasmatic membrane than elsewhere in the cell. The interstitial 

 spaces between the fat globules are filled up with a watery colloid solution, 

 namely a protein solution. The theory of diffusion of dissolved substances 

 through membranes as applied to cells has been profoundly influenced by the 

 discovery of the composition of the plasmatic membrane. At one time it was 

 believed that diffusion of a colloid material was prevented by the pores of the 

 membrane being too small to allow large molecules to get through them ; it was 

 considered to act as a sort of sieve. But this cannot be the whole explanation, 

 and it is now held that solution affinities play the most important part ; that is to 

 say, a membrane is permeable to substances which are soluble in the material of 

 the membrane. Such solubility may imply the formation of actual chemical 

 unions, or more frequently the process is one of adsorption ; this latter process 

 comes specially into play when nutritive materials are assimilated by the cell by 

 means of the protein solution which occupies the interstices between the fat 

 globules. On the other hand, the permeability of the plasmatic membrane by 

 substances such as alcohol, chloroform, and ether, is mainly determined by the 

 solubility of these materials in the fatty or fat-like components of the membrane, 

 and this consideration is the foundation of the Meyer-Over ton theory of the narcotic 

 effect on cells which these volatile anaesthetics exercise. 



