EMULSOIDS 



305 



china clay, have been similarly used on account of the large 

 surfaces which they present. 



From what has been said with regard to the structure of 

 gels and the assumption that they present a sort of network 

 with a considerable development of internal surface, it is easy to 

 find an explanation of the use of isinglass for clearing a turbid 

 solution or for the fact that colouring matter may be extracted 

 from a solution by precipitating gelatinous aluminium hydrox- 

 ide in it. 



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 colloidally dissolved impurities by the electro- 

 lyte. 



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 connexion 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 



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

 f Cf. van Bemmelen : " Z. anorg. Chem.," 1900, 23, 321. 

 2O 



