PHYSICAL CHARACTERS OF THE SOIL. 157 
pacted 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, we may proceed to a somewhat extend- 
ed notice of the properties of the soil as influencing fertil- 
ity. These divide themselves into physical characters— 
those which externally affect the growth of the 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 pa!pable 
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 ch:r- 
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. 
. Power of fixing Solid Matters from their Solutions, 
Permeability to Liquid Water. Capillary Power. 
Changes of Bulk by Drying, ete. 
Adhesiveness. 
. Relations to Heat. 
DIMM 
