4o8 THE COLLOIDAL STATE 



The purification of sewage by means of alum followed by 

 alkali likewise depends on the adsorption of impurities by the 

 colloidal gelatinous aluminium hydroxide, and also upon the 

 precipitation of coUoidally dissolved impurities by the elec- 

 trolyte. 



The deodorizing and generally purifying effect of the soil 

 is likewise probably due largely to the adsorption by porous 

 or colloidal constituents of such soil. 



A very striking case of selective adsorption is to be found 

 in the power which seaweeds * have of extracting iodine from 

 the surrounding sea water, although the amount of this element 

 in sea water is extremely small ; again, in spite of the enor- 

 mous preponderance of sodium over all other metals in sea 

 water, the plant takes up practically none of this, but takes 

 instead potassium, which is present in much smaller quantity. 



Many natural phenomena can be attributed to the same 

 cause. For example, the power possessed by soils rich in 

 clay or humus to retain soluble potassium salts or phosphates 

 which would otherwise be washed away by rain. 



The hydrated aluminium magnesium and sodium silicates, 

 known as Zaeolites, which are contained in clays are colloids 

 and they react by double decomposition with the potassium 

 salts which may be applied as manures, and, while retaining 

 the potash, set free a corresponding quantity of lime or soda.f 



In this connection it may be mentioned that the affinity of 

 colloids, such as humus and clay, for certain dyes, such as 

 methyl violet or malachite green, has been employed as a 

 rough means of detecting or estimating the proportion of 

 these substances in a soil. For this purpose a quantity of the 

 soil is shaken up with the dye solution in a cylindrical vessel ; 

 on settling, the heavier particles sink to the bottom, and a 

 band of the dyed soil constituents is formed on the surface, 



Thermodynamical considerations, coupled with experi- 

 mental measurements, show the fact that true adsorption 

 takes place according to well-defined mathematical laws which 

 enable one to decide definitely whether a certain phenomenon 



* Cameron : " J. Biol. Chem.," 1914, 18, 335. 



t Cf. van Bemmelen : " Z. anorg. Chem.," 1900, 33, 321. 



