248 PHYSICAL PROPERTIES OF SOIL. 



for three months. During these 92 days, the evaporation 

 would have amounted, at 12 hours per day, to 2852000 lbs. 

 The rain at the place during this period, was 8.333 inches or 

 43to pounds to every square foot of surface, equal per acre 

 of 43560 feet, to 1890504 lbs. The amount of evaporation 

 during the time in which the tree was in full leaf exceeds 

 that of the actual fall of rain, by nearly 1000000 of lbs. 

 This excess arises from the decomposition of geine in the 

 soil, and consequent formation of water, by the action of the 

 living plant. If we allow the process to go on, during 15 

 hours per day, then in 92 days, as above, 3565000 lbs. of 

 water would be evaporated. One may easily understand 

 how exhausting a process must be vegetation, where every 

 year all above ground is cut and carried away. Not only 

 the geine, whose carbon and water have become parts of the 

 plant, is thus withdrawn, but a still larger portion disappears 

 as water and carbonic acid. In forests, the annual fall of 

 leaves and wood, in fields, the^, ungathered crop, may add 

 more than the amount thus withdrawn from soil. That plants 

 do form from carbonic acid and water, a great amount of 

 vegetable matter, is by all admitted. This amount in dry 

 or green crops turned in, increases the geine of soil. 



There is yet another view of the effect of the conversion 

 of geine into water. Allowing, as has been asserted, that all 

 land, forest or cultivated, produces annually about the same 

 amount of carbon, then the amount of water transpired 

 above from woodland in 15 hours, is nearly equal to dissolv- 

 ing one-half of the geine, to produce that amount, leaving 

 the balance to be derived from air. An acre of woodland 

 produces, it is said, annually, about 1783 English pounds of 

 carbon. If water dissolves only ^ j^o P^^^ ^^ ^^^ weight of 

 humus, or geine, then 3565000 dissolves 1426 lbs., which, at 



