4 PLANT PHYSIOLOGY 



the microscope. Artificial emulsions are readily obtained by 

 shaking vigorously two fluids that as a rule do not mix or by 

 diluting with water an alcoholic solution of a substance that is 

 insoluble in water. 



In this manner, Perrin prepared the emulsion of mastic 

 (a resin) on w^hich he carried out his famous investigations on 

 Brownian movements. In true solutions, dissociation attains 

 its highest point, and the substance is dispersed in the mass of 

 the solvent in the form of separate molecules or even ions. At 

 such a degree of dispersion, the system becomes very stable, and 

 the dissolved substances can be separated from the solvent only 

 with the greatest difficulty and at the expense of a considerable 

 amount of energy. Such a separation may be observed in the 

 process of freezing or evaporating of solutions; the resistance to 

 separation is expressed in the lowering of the freezing point and 

 the increase of the boiling point. 



Colloidal systems occupy an intermediate position between 

 coarse mixtures and true solutions. The degree of dispersion in 

 colloids is considerably greater than in coarse mixtures, and the 

 dispersed particles are much smaller, usually not exceeding O.l/i- 

 Accordingly, they are more stable than coarse mixtures. The 

 particles of Naegeli (the micelles, or micellae) are, however, con- 

 siderably larger than molecules. They represent solid or liquid 

 aggregates that, owing to their small dimensions, have in aggre- 

 gate an immense surface, separating them from the dispersion 

 medium. Consequently, in colloidal systems an important role 

 is played by surface phenomena, especially by adsorption. 

 There are substances that possess such large molecules that even 

 their true molecular solutions display properties of colloidal 

 systems. Proteins, lipoids, and several other organic compounds 

 belong to this group of substances. 



Representing dispersed systems with relatively large par- 

 ticles, solutions of colloidal substances called ''colloidal solu- 

 tions," or ''sols," differ considerably from true solutions. A most 

 conspicuous difference is their relation to finely porous plant or 

 animal membranes, such as bladder or parchment paper. Col- 

 loidal sols cannot pass through membranes, as their large micelles 

 are retained in the pores, while true solutions may pass readily. 

 Upon this difference depends the well-known method of dialysis, 

 by which colloids may be separated from crystalloids. A vessel 



