48 ELEMENTARY AGRIGTJLTURAL CHEMISTRY 



potassium, ammonium, or phosphoric acid, though the sulphuric 

 acid may be present in the filtrate in the form of calcium sul- 

 phate. The substances which exert this retentive power in the 

 soil are believed to be : 



1. The humus ^ which, in addition to acting as an acid, has 

 lie absorptive power characteristic of extremely porous sub- 

 stances — e.g., charcoal. 



2. Hydrated double silicates, probably transition bodies, 

 produced by the weathering of felspar, &c., possessing a com- 

 position similar to the minerals known as zeolites (so called 

 from a Greek word meaning, to boil/' and applied to them 

 because of their frothing up — due to the escape of steam — 

 when they are heated before the blowpipe). In many cases 

 if one metal be absorbed an equivalent quantity of another 

 metal (often calcium or magnesium) is given up to take its 

 place, and is carried away by the drainage water^ 



3. Ferric and Aluminium Hydrates. — These substances can 

 combine with phosphoric acid, forming the very insoluble 

 phosphates of iron or aluminium ; they can also retain lime, 

 potash and ammonia, probably owing to the fact that they 

 have weak acidic functions. The bases, however, are not 

 retained very tenaciously, and can be removed by prolonged 

 washing with water. 



Phosphoric acid is retained most tenaciously by nearly all 

 fioils, and the loss of this substance in the drainage is usually 

 insignificant. 



The dissolved matter in soil water is distributed in two 

 ways : 



1. By diffusion — i.e., the motion of the dissolved substance 

 from one part of the solvent to another. This process, 

 which tends to make the liquid uniform in concentration, 

 takes place more rapidly with some substances than others. 

 Colloidal bodies, or those which resemble glue or gum in 

 character, have the lowest rate of difi'usion, whilst small diflfe- 

 rences are shown by the various acid radicals and the metals 

 of salts ; thus chlorides diffuse more rapidly than nitrates or 



