82 



DISPERSE SYSTEMS 



ment is independent of the chemical nature of the particles, but 

 depends on three factors, viz. (a) the size of the particle, (b) the 

 temperature, and (c) the viscosity of the dispersion medium. The 

 rate is increased by decrease in the mass of the particle, by increase 

 in temperature or by decrease in the viscosity of the medium. 

 The movement persists, never changing, once equilibrium has 

 set in. It has been observed in granite and in other rocks in small 

 pockets of liquid, which they must have occluded for millions of 

 years. 



Direct observation of the absolute motion of the particles is 

 very difficult, although differences in motion are easily perceptible. 

 This difficulty has been overcome by the application of the 

 cinematograph to the microscope. A glance at Fig. 16, obtained 



Fig. 16. — Movements of two particle.s of iiidia-ruhber latex in colloidal solution, recorded 

 by cinematograph and ultramicroscope. (Henri.) 



in this way, shows that a particle oscillates apparently in a 

 haphazard fashion about a certain mean position during a short 

 interval of time. Any alteration in the kinetic energy of the 

 dispersing medium, of course, produces alterations in the mean 

 velocity of the particles — e.g. increase of temperature increases 

 velocity. When the viscosity of the colloidal solution is increased 

 by the formation of a gel, the particles aggregate in one way or 

 another, their mean free path is reduced, and consequently their 

 motion is reduced in amplitude ; the resistance to movement is 

 increased, and so their velocity may become smaller and smaller 

 till they stop altogether. This phenomenon has been studied in 

 order to find out something about tlie structure of gels and will be 

 referred to later. The velocity may also be modified by alterations 

 in the hydration of the particles. Ramsay considers that the 



