14 THE COLLOIDAL STATE [ch. 



The colloidal phases so far dealt with can be tabulated as follows': 

 disperse continuotis 



liquid solid gels 



solid liquid .su«ponsoids 



liquid liquid eiiudsoids 



Some of the properties of colloidal solutions may now be considered. 

 A point that has already been emphasized in the previous chapter is 

 that the surface of particles in the colloidal state is very great in pro- 

 portion to their mass. Such particles, moreover, unlike ions and small 

 molecules in true solution, possess the properties of the surfaces of matter 

 in mass, as, for instance, those connected with surface tension, electrical 

 charge, etc., and these are especially marked on account of the propor- 

 tionately large surfaces involved. Other properties are their inability, 

 as a rule, to exert an osmotic pressure, to raise the boiling point, and to 

 lower the freezing point of water. Some of the metallic suspensoids are 

 characterized by their colour, this being red, purple or blue as in the case 

 of gold sols. 



An apparatus, by means of which the colloidal state can be demon- 

 strated ocularly, is the ultramicroscope. This is a special form of micro- 

 scope in which a powerful beam of light is directed upon a colloidal solution, 

 which is then seen to contain a number of particles in rapid motion. 

 When analyzed by special methods, this motion has been found to be 

 identical with that shown by much larger, though still microscopic, 

 particles, which has been termed Brownian movement. 



Expt. 7. Demonstration of Brownian movement of microscopic particles. Mount a 

 little gamboge in water and examine under the high power of a microscope. The 

 particles will be seen to be in rapid motion. 



It has been shown that Brownian movement is the outcome of the 

 movement of the molecules of the liquid in which the particles are 

 suspended. This movement is one of the factors which keeps the sol 

 stable and prevents the particles from "settling" as in the case of a 

 true suspension. 



Another factor tending to keep the sol stable is the electrical charge 

 borne by the particles. It is commonly known that there is usually a 

 ditfcrence of potential between the contact surfaces of phases. If the 

 1 There are also the following combinations (Bayliss, 1) : 



