THE ORIGIN OF GUMBOTIL iii 



sitely charged solvent. The stabiHty is greatest when this potential 

 difference is greatest; its instability, or its tendency to coagulate, 

 is greatest when this difference approaches zero. 



Electrolytic coagulation or precipitation is preceded by the ♦ 

 electrical adsorption of ions by the oppositely charged colloid 

 particles. The precipitate carries with it the adsorbed ion or its 

 salt, which in its adsorbed state is more or less difficultly removed 

 by washing. Thus certain salts like those of potassium and 

 ammonium are specifically and tenaciously held by the soil and 

 clays, while the more toxic, less firmly adsorbed sodium salts 

 are leached away. 



When present in traces the singly charged H+ and 0H~ ions 

 not only do not coagulate colloidal material but may even 

 increase the' stabiUty of similarly charged colloid particles. In 

 the hydrolytic decomposition of the alkaHne siHcates free 0H~ 

 ions are formed. These tend to increase the negative potential, 

 likewise the stability of the negative colloids. Thus in the alkaline- 

 soil solutions silicic acid and the colloidal hydrated siHcates are 

 kept to a slight extent, at least, in a state of pseudo-solution. 

 Under the influence of the free carbonic acid and of mineral acids 

 formed by adsorption cleavage of the dissolved salts there may be, 

 as in acid soils, a slight excess of H+ ions. These stabiKze those 

 colloidal "sols" containing positively charged particles. Thus 

 colloidal hydrated ferric oxide is, to a very slight extent at least, 

 rendered capable of transportation by the downward-moving 

 free-water. In the initial stages of the leaching of an original 

 silicate material, where the solution is distinctly alkaline, only the 

 soluble sahs and the transportable negative colloidal siHcic acid 

 are removed by leaching. Not until the alkahnity has disappeared 

 would it be possible for the positively charged colloidal ferric 

 hydroxide to exist in suspension. Iron in the colloidal form would 

 be, therefore, almost the last colloidal material to undergo leaching. 



A soil or clay colloid when once coagulated may again pass into 

 the soluble hydrosol condition.. Numerous experiments have 

 been made dealing with this particular problem. Van Bemmelen 

 has found that when finely divided clay is washed upon a filter the 

 loosely bound coagulating salts are washed away. Upon further 



