THE ABSORPTIVE PROPERTIES OF SOILS 363 



so. In other words, the action of a calcium salt in solu- 

 tion on the absorbed potassium combination is less than 

 the action of a dissolved potassium salt on the absorbed 

 calcium combination. Thus it comes about that under 

 similar conditions of temperature, volume, and concen- 

 tration of the solution, the quantity of calcium or of 

 sodium or of magnesium that goes into solution when 

 colloidal silicates are treated with a solution of a potassium 

 salt is greater than the quantity of potassium that would 

 go into solution if the same silicates were treated with a 

 solution containing the salts of any of these other bases. 



265. Absorptive power of colloidal silicates. — The 

 quantity of a substance that a certain weight of a colloidal 

 silicate can absorb increases with the strength of the 

 solution of the substance presented for absorption, be- 

 cause the final solution can remain stronger and conse- 

 quently its solvent power for that particular substance 

 is less. The point of equilibrium between the fixing 

 power of the colloid and the solvent action of the solvent 

 therefore varies with the strength of the solution. 



The nature of the acid with which a base is combined 

 likewise has an influence on the quantity of the base 

 absorbed. A base combined with a weak acid is ab- 

 sorbed in greater amount than the same base combined 

 with a strong acid. This is presumably because the 

 stronger acid remaining in solution has a greater solvent 

 action. 



266. Absorption by colloids versus absorption by 

 zeolites. — The early conception of the phenomenon of 

 fixation in soils was naturally a chemical one and was 

 founded on the chemical knowledge of that day. The 

 fact that the substitution of bases in the solutions passed 

 through the soil was in chemically equivalent quantities, 



