66 DISPERSE SYSTEMS 



This table shows how a molecular solution of particles of 

 0-1/xyU radius which has no effective surface acquires an effective 

 surface of 12,600 square metres when the particles are increased 

 in size sufficiently to give them an effective surface, i.e. to bring 

 them into the colloidal realm. These figures demonstrate the 

 extraordinary adsorbing surface of a small amount of matter 

 highly dispersed. In connection with the theory of surface 

 tension Wo. Ostwald has introduced the term specific surface to 

 denote the ratio of surface to volume or SjV. In a sphere 



O O 



S = 4-n-r 2 and V = -*-r 3 : therefore - = -. It has been found in 



V r 



physical chemistry that adsorption to a surface becomes an im- 

 portant factor when the specific surface reaches a value of about 

 10,000. It has also been noticed that when the specific surface 

 becomes greater than 6 xlO 7 or thereby, i.e. when the material is 

 so finely subdivided that it is in molecular solution, adsorption 

 phenomena cannot be detected. 



Further, it has been demonstrated that colloids, as a rule, carry 

 a definite charge, some positive some negative. The sign of the 

 charge depends both on the nature of the colloid and on the 

 nature of the medium in which it is. 



(1) As colloids have extremely low osmotic pressures they are a 

 suitable medium for the storage of potential energy. Carbohydrates 

 may be stored as starch or glycogen, both colloids, and changed 

 readily into maltose or glucose, which are crystalloids. 



(2) The salts adsorbed by a colloid are so rendered osmotically 

 inactive, but may be set free again by alteration of the colloidal 

 electric charge. 



(3) Some colloids imbibe water and compress it. A hydrated 

 gel (jelly) has therefore a store of hydraulic pressure within it. 

 Gels may thus be regarded as the great reservoirs of energy in 

 the body (Part II.). 



A great deal has been done to elucidate the nature of colloids. 

 A full discussion of that work is out of place here, but a brief 

 account of the results may be acceptable. 



Protoplasm maybe considered as a watery solution of crystalloids 

 and colloids. The division is due to Graham, the pioneer in 

 colloidal research. As the result of a large series of investigations 

 on the rates of diffusion of various substances in water, he was 

 led to divide all substances into two classes, e.g. Crystalloids, 

 which have a high rate of diffusion and which crystallise from 



