72 



DISPERSE SYSTEMS 



TABLE IX 



Increase in Surface of a Sphere when its Radius is Decimally 



Divided 



introduced the term specific surface to denote the ratio of surface 

 to vohime or SjV. In a sphere S = 4-TTr^ and T" = ^nr^ : therefore 



— ^ = '-= - . It has been found in physical chemistry that 



V 477/"* r 



adsorption to a surface becomes an important 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 X IC^ approx., i.e. when the material is so finely subdivided 



that it is in molecular solution, adsorption phenomena cannot 



be detected. 



Crystalloids and Colloids. Protoplasm, composed of proteins, 

 lipides, carbohydrates, organic and inorganic salts and a large 

 amount of water, is enclosed within a plasma-membrane, which 

 permits of the free passage of water, certain salts and other sub- 

 stances, but not of protein and similar complexes. If a mixture 

 of, say, albumin, starch, glucose, common salt and water were 

 enclosed in a parchment bag suspended in water, the glucose and 

 salt would pass through the membrane into the external water, 

 while the albumin and starch would remain within the bag. 

 Substances which pass readily through membranes like parch- 

 ment are termed crystalloids, and the albumin-like substances 

 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 saturated solutions, and 

 colloids, which diffuse very slowly and in general have a gluey 

 consistency. " They appear," he writes, " like different worlds of 



