ABSORPTIVE POWER OF SOILS. 77 



that of the water, then the colouring matter is com- 

 pletely withdrawn from the water ; if the attraction of 

 both is equal, a division takes place, and the attraction 

 is only partial. 



The materials attracted by the charcoal retain all 

 their chemical properties, and continue unaltered, mere- 

 ly losing their solubility in water ; yet very slight cir- 

 cumstances, increasing in the least degree the attractive 

 force of the water, are sufficient again to withdraw from 

 the charcoal the materials absorbed by it, and which 

 simply coat its surface. By a slight addition of alkali 

 to the water the colouring matter may be discharged 

 from the charcoal which has been used to decolorise the 

 fluid, and by treatment with alcohol, the quinine or 

 strychnine absorbed by charcoal from a fluid may be 

 again extracted. 



'The arable soil possesses, in these respects, the same 

 properties as charcoals. Diluted liquid manure, of deep 

 brown colour and strong smell, filtered through arable 

 soil, flows off colourless and inodorous ; and not merely 

 does it lose its smell and colour, but the ammonia, 

 potash, and phosphoric acid which it holds in solution, 

 are also more or less completely withdrawn from it by 

 the soil, and this in a far greater degree than by char- 

 coal. The rocks which by disintegration give rise to 

 arable soil, if reduced to a fine powder, are just as little 

 possessed of this power as pounded coal. On the con- 

 trary, contact with pure water or water containing car- 

 bonic acid, deprives many silicates of potash, soda, and 

 other constituents, a clear proof that the former cannot 

 possibly withdraw the latter from the water. There is 

 no perceptible connection between the composition of a 

 soil and its power of absorbing potash, ammonia, and 

 phosphoric acid. A soil abounding in clay, with a 

 small proportion of lime ii\it, possesses this absorptive 

 power in the same degree as a lime soil with a small 

 admixture of clay ; but the amount of humus substances 

 will alter the absorptive relation. 



By a closer observation we perceive that the absorp- 

 tive power of arable soil differs in proportion to its 



