COLLOIDS 



55 



tion, but are still visible when the ultrainicroscopic illumination is 

 used, are called submicrons. They have a dimension between 0.1 p and 

 1 pp (0.000,001 mm.),* and they constitute the colloids. Particles smaller 

 than 1 pp are called amicrons, this term being. used to include the mol- 

 ecules and ions present in molecular solutions. (The amicron of hydro- 

 gen is, for example, computed to be 0.067 to 0.159 pp, and that of water 

 vapor, 0.113 pp.) This classification of dissolved substances according 

 to the size of the particles and molecules shows the relationship of one 



Fig. 14. Diagram from W. Ostwald showing the relative size of various particles and colloidal 

 dispersoids compared with a red blood corpuscle and an anthrax bacillus. 



class of substances to others. An idea of the relative sizes of colloidal 

 particles and molecules in comparison with such familiar objects as a 

 blood corpuscle and an anthrax bacillus is given in Fig. 14. The fluid 

 in which the "particle" is suspended is called the dispersion medium, or 

 external phase, and the particle itself the dispersoid, or internal phase. 

 It is the enormous development of surface which determines the dif- 



*M 0.001 mm., and /*/* = 0.000,001 mm. 



