114 PROTOPLASM 



as we have seen, this may be due to gaseous molecules rather 

 than to colloidal particles. 



Solid-in-liquid systems are of great variety and importance. 

 Our j&rst example of the colloidal state was a solid-in-liquid 

 system, viz., the muddy waters of the Mississippi. Solid-in- 

 liquid suspensions are met with in the laboratory in the form of 

 dispersions of metals, such as gold, silver, copper, etc., in water. 

 It was a dispersion of gold in water which Faraday, in 1886, 

 showed to the Royal Society. The solid-in-liquid systems 

 are the chief representatives of the suspension colloids which 

 in many respects differ so prominently from the gluelike 

 (jelly-forming) colloids. 



Solid-in-solid systems are met with in nature as blue rock salt 

 (sodium in sodium chloride), black diamond (minute diamond 

 crystals separated by amorphous carbon or graphite), and 

 precious stones. The last owe their color, in part, to impurities 

 coUoidally dispersed in them, e.g., the emerald to chromium 

 and the topaz to iron. Such systems are produced commercially 

 as colored glass; ruby glass is metallic gold dispersed in glass. 

 It is not a difficult experiment to make solid-in-solid colloidal 

 systems and imitate the precious and semiprecious stones in 

 nature. If a small amount of a dilute solution of gold chloride 

 is added to crystals of any convenient salt, such as sodium chlo- 

 ride or potassium bromide, and the mixture brought into a state 

 of flux by heating in a porcelain crucible and allowed to cool 

 slowly, a colloidal dispersion of gold in the crystalline salt results. 



The tremendous importance of colloidal chemistry can be 

 realized from the fact that not only natural processes such as 

 weather conditions, muddy river water, the formation of deltas, 

 and soil problems in agriculture but also commercial processes 

 such as the manufacture of artificial silk, photographic nega- 

 tives, paint, gelatin, cheese, medicinal and other emulsions, dyes, 

 paper, and ceramics, whether involving the casting of a brick 

 from cheap clay or the molding of the finest porcelain from 

 kaolin, are all problems in colloidal chemistry. But such sub- 

 stances are not our chief concern here, and therefore it is not in 

 their colloidal behavior that we are primarily interested. A 

 basic knowledge of colloid chemistry is necessary in order to 

 interpret the properties of protoplasm. Superficially, proto- 

 plasm is a fine emulsion, a liquid-in-liquid system. In its finer 



