176 
CONDENSATION OF ABSORBED VAPOR. 
ing radiation, or rather by reflecting back again the heat 
radiated by the earth, just as any mechanical screen would 
do. On the other hand, clouds intercept the rays of the sun 
also, and hinder its heat from reaching the earth. The invis¬ 
ible vapors given out by leaves impede the passage of heat 
reflected and radiated by the earth and by all terrestrial 
objects, but oppose much less resistance to the transmission of 
direct solar heat, and indeed the beams of the sun seem more 
scorching when received through clear air charged with uncon¬ 
densed moisture than after passing through a dry atmosphere. 
Hence the reduction of temperature by the evaporation of 
moisture from vegetation, though sensible, is less than it would 
be if water in the gaseous state were as impervious to heat given 
out by the sun as to that emitted by terrestrial objects. 
The hygroscopicity of vegetable mould is much greater than 
that of any mineral earth, and therefore the soil of the forest 
absorbs more atmospheric moisture than the open ground. The 
condensation of the vapor by absorption disengages heat, and 
consequently raises the temperature of the soil which absorbs 
it. Yon Babo found the temperature of sandy earth thus 
elevated from 20° to 27° centigrade, making a difference of 
nearly thirteen degrees of Fahrenheit, and that of soil rich 
in humus from 20° to 31° centigrade, a difference of almost 
twenty degrees of Fahrenheit.* 
Balance of Conflicting Influences . 
We have shown that the forest, considered as dead matter, 
tends to diminish the moisture of the air, by preventing the 
sun’s rays from reaching the ground and evaporating the 
* Wilhelm, Der Boden und das Wasser , p. 18. It is not ascertained in 
what proportions the dew is evaporated, and in what it is absorbed by the 
earth, in actual nature, but there can be no doubt that the amount of water 
taken up by the ground, both from vapor suspended in the air and from 
dew, is large. The annual fall of dew in England is estimated at five 
inches, but this quantity is much exceeded in many countries with a 
clearer sky. “ In many of our Algerian campaigns,” says Babinet, “ when 
