PHYSICAL PROPERTIES OF SOIL. 225 



developed for three months. During these 92 days, 

 the evaporation would have amounted, at 12 hours 

 per day, to 2,852,000 lbs. The rain at the same 

 place during this period, was 8,333 inches or 43 

 4-10 of a pound to every square foot of surface, 

 equal per acre of 43,560 feet, to 1,890,504 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 1,000,000 of lbs. This ex- 

 cess arises from the decomposition of geine in the 

 soil, and consequent formation of water, by the ac- 

 tion of the living plant. If we allow the process to 

 go on, during 15 hours per day, then in 92 days, as 

 above, 3,565.000 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 plant are thus withdrawn, but a still larger portion, 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 un- 

 gathered crop, may add more than the amount thus 

 withdrawn from soil. That plants do form from car- 

 bonic 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. 



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