74 



DISPERSE S Y STEMS 



Disperse systems may be classified according to the nature of 

 the contact surface between the phases. Taking the three states 

 of matter, solid, liquid and gaseous, five different kinds of contact 

 surface can be produced, as is indicated in the following table, in 

 which are also given examples of the various disperse systems. 



TABLE X 



Class III. (So-called Emulsoid) is of the most importance in 

 biology, but Class IV. (Suspensoid) has been most studied and is 

 of considerable industrial and therapeutic value. 



Colloids might be classified according to their degree of dispersion, 

 that is the ratio of total surface to volume, or the surface exposed to 

 each c.c. of the dispersed phase. This would give a continuous 

 series of systems ranging from a non-dispersed two-phase system 

 on the one hand to a homogeneous mixture of an ionised salt in 

 water, i.e. a true solution. Colloids may thus be regarded as 

 intermediate in this series, e.g., gold coin in water, gold dust 

 suspended in water, very fine gold dust suspended in water, range 

 of colloidal gold in water (Zsigmondy), solution of gold salt (un- 

 dissociated) and, finally, completely dissociated gold salt in aqueous 

 solution. 



This state is not peculiar to the metallic colloids, but, as has 

 been amply demonstrated by Von Weimarn, can be obtained 

 from such materials as NaCl, A1(0H)3 and silver salts. He has 

 enunciated a postulate called the law of corresponding states, 

 which is as follows : " The degree of dispersion and the general 

 physical appearance of precipitates are always the same irrespec- 



