4 COLLOID CHEMISTRY AND THE 



molecules. As all substances dissolve up to their solubility- 

 value, that is, they become subdivided to molecular size, a 

 colloidal dispersion only becomes possible when the solubility 

 value is exceeded. So that difficultly soluble substances lend 

 themselves most readily for transformation into the colloidal 

 state (e.g., metals, many metallic sulphides and hydroxides, 

 fats and resins, with water as dispersion medium). If, on the 

 other hand, it is desired to obtain salts or other substances 

 soluble in water in the colloidal state, methyl alcohol, acetone, 

 etc., must be used as dispersion medium. Of course, it is also 

 essential that the division of the particles must be sufficiently 

 fine for the particles to remain free swimming and evenly 

 distributed throughout the bulk of the liquid ; otherwise we are 

 dealing with a slime, a coarse suspension, or an emulsion. 



Such fine dispersions, or, as we can now call them, colloidal 

 solutions, pass easily through a filter. Their real nature and 

 the difference between them and true molecular solutions was 

 first made clear by the use of the ultramicroscope of Zsigmondy 

 and Siedentopf, which made the degree of dispersion optically 

 apparent and permitted a closer determination of the size of 

 the particles. According to R. Zsigmondy, we can distinguish 

 by their mean linear' dimensions coarse suspensions with 

 particles of o-iju, diameter, and colloidal solutions with par- 

 ticles of o-ijLt/i- diameter. The latter pass without a break 

 into the various molecular solutions as the size of particle 

 decreases. 



Wolfgang Ostwald is the author of the very suitable terms 

 dispersion medium (continuous phase) and disperse phase for the 

 component parts of a colloidal system. The degree of dispersion 

 (dispersity) is the quotient of the total surface and the total 

 volume of the dispersed particles. 



It is easy to see that there are fundamentally two general 

 methods of preparing a colloidal solution of a substance : 

 (i) dispersion methods, in which large particles or aggregates are 

 broken up, as in electrical dispersion, partial solution of coarse 

 precipitates or peptisation ; and (2) condensation methods, in 

 which a molecular solution is caused to become super-saturated, 

 either by sufficient dilution or the addition of certain substances 



