76 DISPERSE SYSTEMS 



rocks in small pockets of liquid, which they must have occluded 

 for millions of years. But equilibrium must first be reached. 



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. 8, obtained 

 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 



FJG. 8. Movements of two particles of india-rubber latex in colloidal solution, 

 recorded by cinematograph and ultra-microscope. (Henri.) 



velocity. The velocity may also be modified by alterations in 

 the hydration of the particles. Ramsay considers that the 

 particles in pure water do not touch one another at any time, each 

 particle being surrounded by a liquid layer. This layer is de- 

 stroyed by the addition of salts, which thus cause negative 

 acceleration. 



To use a somewhat homely illustration, the colloidal particle 

 may be likened to a morsel of bait dropped into the water of a 

 river estuary. The moment that it reaches the water it is pushed 

 to and fro by a multitude of hungry small fish. The velocity 

 and amplitude of the oscillatory movements of the bait depend 

 principally on the size of the bait and on the energy with which 

 it is attacked. 



