88 THE THEORY OF IONS 



colloids of the living body and the action of crystal- 

 loids upon them. The gels in living organisms are 

 proteins. 



The sols or liquid colloids play an important part 

 in the biological processes. Plants are able to derive 

 nourishment from absolutely pure crystalloidal 

 solutions. In contrast to them, animals are depen- 

 dent upon liquid colloidal food. The process of 

 digestion serves to prepare such nutrient sols for 

 absorption, and being absorbed they are mechani- 

 cally moved about and distributed through the 

 organism to the nourishing fluids. The globulins 

 are such sols : they are colloids held in solution by 

 the interaction of the colloid and a crystalloid. The 

 sols possess properties common to all suspensions of 

 very fine particles. The enormous surface tension 

 or free surface-energy of these particles and their 

 osmotic pressure bear a relationship to each other in 

 the cell which is reciprocal. The enzymes also have 

 a colloidal constitution ; and Bredig explains their 

 effects by the enormous surface tension of the par- 

 ticles. Such ferment-like action is possessed even 

 by the metallic surfaces of the particles in an in- 

 organic colloidal solution.* The metabolic changes 

 for ever going on in the living organism compel us 

 to study them as a dynamic process ; and to find the 

 right connexion between metabolic physiology and 

 physical chemistry is an important problem. 



The laws of colloidal chemistry govern the changes 

 that go on in living cells ; and, although these laws 

 may be modified by metabolism, the parallelism 



* Pauli's "Physical Chemistry in the Service of Medicine," 

 p. 34. 



