CHAPTER XIV. 



ABSORPTION BY SOILS OF SOLIDS FROM SOLUTIONS. 

 ABSORPTION OF GASES. AIR OF THE SOILS. 



ABSORPTION OF SOLIDS FROM THEIR SOLUTIONS. 



JUST as solids have the power of condensing gases upon 

 their surfaces, to an extent proportional to that surface, and 

 therefore to the state of fine division : so fine powders have the 

 power of withdrawing from solutions solids held in solution, 

 to an extent varying with the nature of the substance dissolved, 

 and the absorbing solid. The most commonly-known mani- 

 festation of this principle is that sea-water filtering through 

 the sands of the shore, will at a certain distance become sensi- 

 bly less brackish, and finally so nearly fresh as to be capable of 

 domestic use. 1 The extent to which this occurs is in a measure 

 proportional to the fineness of the sand, and to the amount of 

 clay present in it. This is a clearly physical effect, independent 

 of any chemical action whatever; for it occurs equally with 

 quartz sand, charcoal, glass, limestone, or other rock powders 

 having no chemical effect upon the substance dissolved or 

 upon the liquid dissolving it. Very large amounts of water are 

 often required to remove all the soluble matter thus " ad- 

 sorbed." 



Decolorising Action. One of the commonest applications 

 of this principle is the decolorization of colored solutions by 

 means of finely pulverized charcoal. This property of char- 

 coal, as is well known, is extensively utilized in the arts, and 

 particularly in the refining of sugar; the charcoal used in this 

 case being preferably bone charcoal ("bone black"), which 

 on account of its state of extreme fineness, and separation by 

 the earthy particles with which it is associated, is more effective 

 than any other form. It is rendered still more effective, how- 



1 In many cases this decrease of salinity is probably due to a slow influx of fresh 

 water from landward ; but very often it cannot be thus explained. 



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