EXHALATION OF VAPOR BY TREES. 189 
When we recall the effect produced on the soil even of 
a thick wood by a rain-fall of one inch, we find it hard to believe 
that two hundred and forty times that quantity, received by the 
ground between early spring and autumn, would not keep it in 
a state of perpetual saturation, and speedily convert the forest 
into a bog. 
No such power of absorption of moisture by the earth from 
the atmosphere, or anything approaching it, has ever been 
shown by experiment, and all scientific observation contradicts 
the supposition. Schiibler found that in seventy-two hours 
thoroughly dried humus, which is capable of taking up twice 
its own weight of water in the liquid state, absorbed from the 
atmosphere only twelve per cent. of its weight of humidity ; 
garden-earth five and one-fifth per cent. and ordinary cultiva- 
ted soil two and one-third per cent. After seventy-two hours, 
and, in most of his experiments with thirteen different earths, 
after forty-eight hours, no further absorption took place. Wil- 
helm, experimenting with air-dried field-earth, exposed to air 
in contact with water and protected by a bell-glass, found that 
the absorption amounted in seventy-two hours to two per cent. 
and a very small fraction, nearly the whole of which was taken 
up in the first forty-eight hours. In other experiments with 
carefully heat-dried field-soil, the absorption was five per cent. 
in eighty-four hours, and when the water was first warmed to 
secure the complete saturation of the air, air-dried garden- 
earth absorbed five and one-tenth per cent. in seventy-two 
hours. 
In nature, the conditions are never so favorable to the absorp- 
tion of vapor as in these experiments. The ground is more 
compact and of course offers less surface to the air, and, espe- 
cially in the wood, it is already in astate approaching saturation. 
Hence, both these physicists conclude that the quantity of aque- 
ous vapor absorbed by the earth from the air is so inconsider- 
able “that we can ascribe to it no important influence on vege- 
tation.” * 
* WILHELM, Der Boden und das Wasser, pp. 14, 20. 
