PHYSICAL. CUAKAOTEKS OF THE SOIL. 157 



pactt'd gravel, often underlying a fairly fruitful soil. It 

 is the soil reverting to rock. The particles once disjointed 

 are being cemented together again by the solutions of 

 lime, iron, or alkali-silicates and humates that descend from 

 the surface soil. Such a stratum often separates the sur- 

 face soil from a deep gravel bed, and peat swamps thus 

 exist in basins formed on the most porous soils by a thin 

 layer of moor-bed-pan. 



With these general notions regarding the origin and 

 characters of soils, Ave may proceed to a somewhat extend- 

 ed notice of the properties of the soil as influencing fertil- 

 ity. These divide themselves into physicnl characters — 

 those which externally affect the growth of llie plant ; 

 and chemical characters — those which provide it with food. 



CHAPTER IV. 



PHYSICAL CHARACTERS OF THE SOIL. 



The physical characters of the soil are those which con- 

 cern the form and arrangement of its visible or palpable 

 particles, and likewise include the relations of these parti- 

 cles to each other, and to air and water, as well as to the 

 forces of heat and gravitation. Of these physical char- 

 acters we have to notice : 



1. The Weight of Soils. 



2. State of Division. 



3. Absorbent Power for Vapor of Water, or Hygro» 

 scopic Capacity. 



4. Property of Condensing Gases. 



5. Power of fixing Solid JMatters from their Solutions. 



6. Permeability to Liquid Water. Capillary Power. 



7. Changes of Bulk by Drying, etc. 



8. Adhesiveness. 



9. Relations to Heat, 



