56 OUTLINES OF FORESTRY. 



soil, by effecting new combinations of its mineral 

 constituents. 



Some soils possess the very valuable property of 

 absorbing water vapor directly from the atmos- 

 phere and condensing it in their pores. This 

 property is exceedingly valuable during times of 

 extended droughts, when a considerable quantity 

 of vapor may be present in the air. Of all soils, 

 those containing the greatest quantity of vegetable 

 mould or humus possess this valuable property in 

 the greatest degree. Clayey soils also possess it 

 to a marked degree. 



Soils also possess the power of absorbing gases,. 

 Ordinarily, most soils contain of absorbed gases 

 a smaller proportion of oxygen and more carbonic 

 acid gas than the atmosphere. 



The ability of soils to absorb the sun's heat 

 varies with their color; as a rule, dark-colored 

 soils absorb the heat more rapidly than light- 

 colored soils. 



Darwin has shown that in certain localities the 

 common earthworm greatly aids in the formation 

 and physical character of the soil by extensive bur- 

 rowing and tunnelling. 



In all cases, however, the process by which the 

 soil is formed is a gradual one ; and since there is 



