60 THE: soil, SOLUTION 



lime or ferric oxide. Or it may be a condensation or concentra- 

 tion of the dissolved substance on or about the surface of the 

 absorbing medium, a phenomenon known as adsorption. To 

 prove the existence of adsorption definitely and conclusively in 

 any given case is always difficult, if ever possible, but the exist- 

 ence of this phenomenon is the most logical explanation of many 

 observations, and is generally admitted by chemists and physicists 

 at the present time. 1 It is by adsorption, probably, that potash 

 and ammonia are held by the soil when added in fertilizers. 



That absorption is dependent in some manner upon the solu- 

 bility of the dissolved substance in the particular solvent em- 

 ployed would seem to be obvious. But what the relation may 

 be, if it exists at all, is not known. For instance, silk absorbs 

 picric acid from solutions in water and alcohol but not from solu- 

 tions in benzene, although the solubility of picric acid in benzene 

 lies between its solubility in water and in alcohol. 2 



The absorption of any given dissolved substance from differ- 

 ent solvents is markedly different. Most soils absorb methylene 

 blue from aqueous solutions with great avidity, but washing out 

 the absorbed dye with water is an extremely tedious and unsatis- 

 factory process, although the dye can be readily and more or 

 less completely removed from the soil by alcohol. As might be 

 anticipated from this, it is known that the presence of one dis- 

 solved substance affects the absorption of another, but in what 

 way can not, generally, be anticipated, although it would seem 

 that the importance of this subject for manurial practice would 

 invite further research. 



From the same solution, different absorbents remove a dis- 



1 A clear and apparently indisputable case of adsorption has been 

 noted by Patten (Some surface factors affecting distribution, Trans. Am. 

 Electrochem. Soc., 10, 67-74 (1906). On adding powdered quartz to an 

 aqueous solution of gentian violet, there is a distribution of the dye 

 between the water and the quartz. A microscopic examination of the 

 latter showed that the dye was concentrated in thin layers upon the sur- 

 face of the quartz grains, from which it could be washed with water, no 

 change in the quartz grains being noticeable. 



2 Absorption of dilute acids by silk, by James Walker and James R. 

 Appleyard, Jour. Chem. Soc., 69, 1334-1349 (1896). 



