CRYSTALLOIDS AND COLLOIDS 49 



simpler organic compounds, such as the sugars. On 

 evaporation of the solvent they reappear, often in the 

 form of crystals. Substances which behave in this 

 way are known as crystalloids. The substances which 

 when mixed with water become dispersed in very fine 

 particles through the water, each particle consisting of 

 a number of molecules aggregated together, are called 

 colloids. It is to be noted that many simple substances 

 —even elements such as gold and platinum — may be 

 prepared in such a form that they behave as colloids 

 with water, i.e. they become dispersed in minute particles 

 through the liquid. We now know that we cannot sharply 

 divide substances into the two categories, crystalloids 

 and colloids, but speak of a substance, in a given 

 condition, behaving as "a crystalloid or as a colloid " 

 with water, or some other solvent. Nevertheless the 

 organic compounds with large molecules, e.g. poly- 

 saccharides, proteins, etc., commonly do behave as, 

 and are therefore commonly spoken of as, colloids. 



The aggregate of the dispersed particles of a colloid 

 is known as the disperse phase, the continuous liquid 

 medium in which they are dispersed as the continuous 

 phase of the colloid sol. The disperse phase does not 

 necessarily consist of solid particles, it may consist of 

 very minute liquid droplets, and then the sol is called 

 an emulsion sol, or an emulsoid, while if the disperse 

 phase is solid we have a suspension sol, or a suspensoid. 



Coarse particles or droplets, suspended in a liquid, 

 form ordinary suspensions or emulsions, and tend to 

 sink slowly to the bottom, or rise to the top, as they 

 are heavier or lighter than the liquid. Thus the fat 

 globules suspended in milk slowly rise to the surface 

 as cream. If we shake up a handful of soil in a beaker 

 of water, the coarser and heavier particles sink to the 



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