In the arid region, sanding of the surface is sometimes resorted to for the prevention of the evaporation which brings alkali salts to the surface. But the necessity of repeating this dressing annually unless cultivation can be omitted, restricts the use of this expedient to narrow limits. The sanding of the surface of cranberry plantations in swamps or bogs in the northern parts of the humid region doubtless owes its efficacy largely, if not chiefly, to the retention of moisture, while at the same time it prevents the consolidation of the surface, so as to render tillage unnecessary. 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 manifestation of this principle is that sea-water filtering through the sands of the shore, will at a certain distance become sensibly less brackish, and finally so nearly fresh as to be capable of domestic use.[98] 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 “adsorbed.” [98] 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. _Decolorizing Action._—One of the commonest applications of this principle is the decolorization of colored solutions by means of finely pulverized charcoal. This property of charcoal, 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, however, by the extraction of these earthy particles (calcic carbonate 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.