128 NATURE AND PROPEETIES OF SOILS 



is distributed througii a second, which may also be a gas, a 

 liquid, or a solid. The material in the finely divided state 

 is called the dispersed phase, while the matter containing it 

 is designated as the continuous or dispersive medium. A 

 very good example of a colloidal system occurs when very 

 fine clay particles (solids) are suspended in water (liquid) 

 or when an emulsion of oil and water is formed, the oil under 

 certain conditions becoming the dispersed material, hetero- 

 geneously disposed. The particles of material in a colloidal 

 state in these cases are so small that they will not sink as long 

 as conditions are stable. Moreover, they exhibit the Brownian 

 movement,^ the oscillations increasing very rapidly as the 

 size decreases. Such particles are molecular complexes and 

 the solution is heterogeneous. In this respect a colloidal solu- 

 tion differs from a true solution, which is homogeneous, the 

 particles being molecules and often ions. 



69. Size of colloidal particles. — The size of the particles 

 of matter in a colloidal state vary with the material and with 

 the conditions of formation. The diameters of material in a 

 colloidal state are considered to range from 100 \i yi^ (.0001 

 m.m.) to 1 ^ ^i (.000001 m.m.). Above 100 \i \i suspended 

 material is usually sinkable, while below 1 fx [i the particles 

 generally become single molecules and a true solution is at- 

 tained. Theoretically it would seem possible to pass from 

 a suspension to a true solution without a break by a progres- 

 sive subdivision of particles. There seems to be a discontinu- 

 ity, however, between the colloidal state and a true solution. 

 As the molecular complexes subdivide, they at last go into 

 solution and may reprecipitate as coarser complexes, thus 



^ SmaU particles, even those well within the range of ordinary micro- 

 scopic vision, exhibit, when suspended in a liquid, an oscillating motion 

 around a central position. This movement, which is called the Brownian, 

 is inversely proportional to the size of the particle. It is probably due 

 to the bombardment of the molecules and ions of the liquid in which 

 the particle is suspended. The Brownian movement is very slow for 

 particles of a diameter of .001 mm. 



*A micron (/*)=: .001 mm. or 10-^ mm. A millimicron (l/4/t) = 

 .000001mm. or 10-^ mm. 



