104 TEXTBOOK OF PLANT PHYSIOLOGY 



dispersed particles are much smaller, usually not exceeding 0.1^. 

 Accordingly, they are more stable than coarse mixtures. The 

 particles of colloidal mixtures, called micellae, since the time of 

 Naegeli, are, however, considerably larger than molecules. They 

 represent solid or liquid aggregates which, due to their small dimen- 

 sions, possess in aggregate an immense surface, separating them 

 from the dispersion medium. Consequently, in colloidal systems 

 an important role is played by surface phenomena, especially by 

 adsorption. 



It is this enormous increase of the surface which determines the 

 greater part of the specific properties of colloidal systems. How 

 rapidly the surface increases with an augmented degree of disper- 

 sion and diminished size of the separate particles may be seen, for 

 instance, from the well-known calculations of Wolfgang Ostwald. 

 If we take a cube with sides of 1 cm., its surface will be 6 sq. cm. 

 If this cube is divided into smaller cubes with sides of 1 mm., their 

 number will be 10 3 , with a total surface of 60 sq. cm. In dimin- 

 ishing the sides to 0. 1m, the maximum size of colloidal particles, we 

 obtain from the initial cube 10 15 small cubes with a total surface of 

 60 sq. m. If the sides of the cube are diminished to O.OOl^i, the 

 minimum size of colloidal particles, the number of small cubes 

 will reach 10 21 , with a total surface of 6,000 sq. m. If these calcu- 

 lations are applied to the very fine animal charcoal, which is used 

 in the sugar industry for decolorizing sugar solutions, we shall see 

 that 1 cu. m. of charcoal with grains 1/x in diameter gives a total 

 adsorbing surface of 6 sq. km. It might be thought that in true 

 solutions with a still higher degree of dispersion and still smaller 

 dimensions of the particles, molecules, or ions, the total sum of the 

 surfaces ought to be greater than in colloidal systems, and that cor- 

 respondingly the processes connected with the increase of surface 

 area ought to be more marked. This, however, is not observed, 

 since with such a degree of dispersion every ion and every mole- 

 cule is bound to water particles, which form as it were a shell 

 around them. Consequently, the dispersed systems containing 

 molecules and ions are without an active surface. The maximum 

 manifestation of surface forces is connected not with maximum 

 dispersion, but with a certain intermediate degree of it, which 

 characterizes the colloidal systems. 



Depending on whether the micellae are liquid or solid aggre- 

 gates, two types of colloidal substances are distinguished, emul- 



