OSIMOTIC PRESSURE 43 



clearly by the micelles of hydrophobic sols than by those of hydrophilic sols. 

 The viscosity of the liquid phase is also an important factor governing the 

 rapidity with which the dispersed particles move. The more viscous the 

 liquid, the more sluggish the movement of the particles. 



Brownian movement is caused by the kinetic activity of the molecules of 

 the solvent. Even the smallest particles in which Brownian movement can 

 be observed are very large in proportion to the size of the solvent molecules 

 which impinge upon them. A particle suspended in a liquid such as water 

 suffers a continual bombardment by the molecules of the liquid. If the 

 particle be relatively large, at any given moment it is bombarded on every side 

 by numerous molecules, moving in all possible directions, and at different 

 speeds. The effects of the individual impacts largely counteract each other, 

 however, and there is little or no movement of the particle. If the particle 

 be smaller, however, the results are quite different. At any given moment a 

 much smaller number of water molecules impinge upon the particle. The 

 resulting forces cease to be balanced and the sum total effect of the blows 

 which the particle sustains on some one side are greater than the effect of the 

 blows sustained on any other side. Hence the particle moves. The next 

 moment a greater impetus may be given to the particle from some other direc- 

 tion and the course of its movement is changed. In this way the highly er- 

 ratic movements of suspended particles known as Brownian movement origi- 

 nate. Increase in temperature increases the rate of Brownian movement be- 

 cause of an increase in the kinetic energy of the solvent molecules. This phe- 

 nomenon is the nearest approach we have to actual visible evidence of the 

 validity of the kinetic theory of matter. It almost brings before our eyes the 

 veritable "dance of the molecules." 



Osmotic Pressure. — As discussion in Chap. VIII shows the osmotic pres- 

 sure of solutions is a colligative property; that is one which depends on the 

 proportion of solute particles (molecules or ions or both) to solvent molecules, 

 regardless of the kind of solute particles. A sol should also possess an osmotic 

 pressure since theoretically a micelle has the same effect upon the magnitude 

 of the osmotic pressure as a molecule or an ion. A brief discussion should 

 aid in making this point clear. Large molecules have the same theoretical 

 effect upon osmotic pressure as small molecules. The individual molecules 

 of some compounds are large enough to fall within the colloidal range of 

 sizes. They are both molecules and micelles at one and the same time. Such 

 a micelle should have just as much of an effect on osmotic pressure as a mole- 

 cule. Actually the osmotic pressures of sols never exceed a small fraction of 

 an atmosphere and are therefore much less than the osmotic pressure of any 

 but the most extremely dilute solutions (Chap. VIII). The osmotic pressure 



