246 PHYSICAL PROPERTIES OF SOIL. 



water speedily and equally? Is its power of retaining water 

 equal? As a general fact, it may be stated, that the soil 

 which absorbs fastest and most, evaporates slowest and least. 

 Geine evaporates least in a given time. The power of evapo- 

 ration is modified by the consistence of soil; by a different 

 degree of looseness or compactness of soil. Garden mould, 

 for instance, dries faster than clay. As it has been already 

 shown, that the power of being warmed is much modified by 

 moisture, so the power of a soil to retain water makes the 

 distinction of a hot or cold, wet or dry soil. In all the rela- 

 tions to moisture, as to heat, geine exercises the greatest 

 influence. 



295. Connected with this power of absorption of moisture, 

 is the very important relation of soil to gas. All soil absorbs 

 oxygen gas, when damp, never when dry. Of the ingredi- 

 ents of soil, geine forms the only exception to this rule. 

 That absorbs oxygen, whether it be wet or dry. Geine has 

 this power in the highest degree, clay next ; frozen earths 

 not at all. A moderate temperature increases the absorp- 

 tion. 



When earths absorb oxygen, they give it up unchanged. 

 They do not combine with it. They merely induce on the 

 absorbed moisture power to imbibe oxygen. But when 

 ^eine absorbs oxygen, one portion of that combines with its 

 carbon, producing carbonic acid, which decomposes silicates, 

 and a second portion of oxygen combines with the hydrogen 

 of the geine, and produces water. Hence, in a dry season 

 well manured soils, or those abounding in geine, sufler very 

 little. The power of geine to produce water, is a circum- 

 stance of soil almost wholly overlooked. It is one whose 

 high value will appear by a comparison with the quantity of 

 water produced by a fresh-ploughed, upturned sward, with 

 that from the same soil undisturbed. The evaporation from 



