268 SOILS. 



ever, by the extraction of these earthy particles (calcic carbon- 

 ate and phosphate) by means of acid; for by removal of the 

 earthy particles, the surface of the charcoal is greatly increased, 

 and its decolorizing as well as its absorbing power increases 

 accordingly. 



While in one and the same substance the decolorizing effect 

 is more or less directly proportional to the fineness of the parti- 

 cles, corresponding to increased surface, it is nevertheless true 

 that in this case, as in that of the absorption of gases, there are 

 specific differences between different powders; so that for ex- 

 ample no other substance can replace charcoal in the decoloriz- 

 ing effect which it produces upon colored solutions. It must 

 not, however, be supposed that there is any special reason why 

 coloring matters, as such, should be taken up by preference. 

 Coloring matters are of all kinds of chemical composition, and 

 have in common only the fact that a relatively small amount 

 produces a very strong coloring effect ; hence their name, and 

 hence also the apparently extraordinarily strong effect pro- 

 duced upon them by charcoal. 



This effect is not, however, by any means greater than it is 

 in the case of many other compounds which are colorless. 



Complex Action of Soils. The powdery ingredients of 

 soils, of course, share this power with all other powders. In 

 the case of soils, however, the action is almost always much 

 more complex than in that of charcoal, because solutions that 

 are passed through the soil are apt to act chemically upon one 

 or the other of its ingredients, usually resulting in a partial 

 exchange of ingredients between the soil and the solution ; 

 one or more of the constituents of the solution being retained 

 by the soil, while one or more of the (basic) soil constituents 

 pass into the solution, in combination with its acidic ingredi- 

 ents. 



Thus when a very dilute (|- or i<%) solution of potassic chlorid is 

 filtered through almost any soil, the first portions passing through will be 

 practically free from potash, but will contain the chlorids of calcium 

 and magnesium. But as more of the solution is passed through, potash 

 passes also ultimately without absorption. In addition to the zeolitic 

 and clay portions of the soil, the humus is very effective in absorbing 



