THE NOMExNCLATURE OF COLLOIDAL SYSTEIVIS 35 



TABLE 9 GENERAL TYPES OF COLLOIDAL SYSTEMS 



Types 

 I. Solid-in-solid. . . 



a. Solid-in-llquid. . 



3. Solid-in-gas. . . . 



4. Liquid-in-solid. . 



5. Liquid-in-liquid 



6. Liquid-in-gas. . . 



7. Gas-in-solid. . . . 



8. Gas-in-liquid. . 



Examples 



Some alloys, certain types of colored glass, some precious 

 stones {e.g. black diamond). 



Many sols (see later). 



Smoke, fine dust clouds, certain fumes, "dust colloids." 



Certain minerals and gems {e.g. pearls). 



Many sols (see later). 



Clouds, fogs, and mists (sometimes, however, the water in 

 such systems is condensed on minute dust particles). 



Some minerals. An uncommon type of colloidal system. 



Some foams. An uncommon type of colloidal system. 



term to designate a certain group of substances, which seemed to be set apart 

 from other substances by several distinctive properties. When dispersed in 

 water these substances had a slow rate of diffusion, and failed to diffuse 

 through membranes of parchment paper. Furthermore they did not form 

 crystals. He applied the alternate term crystalloid to those crystal-forming 

 substances which, when in solution, diffused relatively rapidly and passed 

 readily through parchment membranes. We now know that, strictly speaking, 

 no such distinction can be made; the word colloid properly refers to a dis- 

 tinctive state of matter, and cannot be applied with accuracy to any one class 

 of substances. Theoretically any substance can, by proper manipulation, be 

 brought into the colloidal state, and actually this has been experimentally 

 accomplished for a large number of substances. 



Colloidal systems, as has already become evident, are composed of two 

 phases, a continuous phase, and a discontinuous phase, the latter composed of 

 discrete particles, each entirely separated from its fellows by the intervening 

 continuous phase. The continuous phase is commonly called the dispersion 

 medium, and the discontinuous phase the disperse phase. According to anotTier 

 terminology, applicable however only when the dispersion medium is a liquid, 

 each individual dispersed particle is called a micelle, while the continuous 

 phase of the system is called the intermicellar liquid. 



To Graham, also, we are indebted for the terms sol and gel. A sol is 

 a colloidal system which possesses the property of fluidity. Such systems can 



