ADSORPTION 287 



retain traces of electrolytes, and why so many precipitates, in 

 spite of continued washing, persistently retain small quantities 

 of substances which they have carried down with them from 

 solution. 



The following experiment, due to Linder and Picton,* 

 illustrates this phenomenon : — 



If, to a colloidal solution of arsenic sulphide, obtained by 

 passing a current of sulphuretted hydrogen through arsenious 

 acid dissolved in water, a little barium chloride be added, the 

 arsenic sulphide is precipitated. This precipitate, if filtered 

 and washed thoroughly to free it from barium salts, will still 

 be found to contain traces of barium if dissolved in concen- 

 trated hydrochloric acid with a little potassium chlorate and 

 tested for barium in the usual way. 



Again, the mutual precipitation of colloids of opposite 

 electrical sign,f resulting in the formation of a coagulum 

 consisting of both constituents, is probably only another 

 example of adsorption. 



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. 



Another very striking case of selective adsorption is to be 

 found in the power which sea-weeds 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 enormous 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. 



ENZYME ACTION OF COLLOIDS. 



Associated with this enormous development of surface 

 there is, of course, a corresponding development of surface 



* Linder and Picton : " J. Chem. Soc, Lond.," 1895, 67, 66. 

 f Cf. van Bemmelen: "Z. anorg. Chem.," 1900, 23, 360. 



