42 THE MECHANISM OF LIFE 



consider the molecules in the middle of a liquid, each molecule 

 is subjected to the cohesive attraction of molecules on every 

 side, attractions which neutralize one another. At the surface 

 of the liquid, however, there are quite other conditions of 

 equilibrium. There each molecule is drawn downwards 

 towards the centre of the liquid, and there is no compensating 

 attraction in an opposite direction. The resultant pressure 

 is normal to the surface of the liquid, and is mechanically 

 equivalent to an elastic membrane which tends to diminish 

 the surface, and hence the volume of the liquid. We may 

 therefore regard this surface tension as acting the part of a 

 veritable physical membrane. 



There is a still further differentiation of the surface of a 

 liquid. When the liquid is not a simple one, but complex 

 as in a solution, we find that the concentration of the solute 

 is greater at the surface than in the interior. This is the 

 so-called phenomenon of "adsorption," which is another cause 

 for the production of a physical membrane covering the 

 surface of a liquid. 



Substances in a colloidal state have a great tendency to 

 form these chemical or physical membranes at the point of 

 contact between the colloidal solute and the solvent. This 

 is probably the reason why the coagulum of a colloidal liquid 

 usually presents a vacuolar or spongy structure. 



