176 SPECIFIC HEAT. 
evolved in the vital processes of vegetation, to raise the thermo- 
metric mean of wooded countries for that season, and, of course, 
for the year. The determination of this point is of much greater 
importance to vegetable physiology than the question of the 
winter temperature of trees, because a slight increment of heat 
in the trees of a forest might so affect the atmosphere in con- 
tact with them as to make possible the growing of many plants 
in or near the wood which could not otherwise be reared in 
that climate. 
The evaporation of the juices of trees and other plants is 
doubtless their most important thermoscopic function, and as 
recent observations lead to the conclusion that the quantity of 
moisture exhaled by vegetables has been hitherto underrated, 
we must ascribe to this element a higher value than has been 
usually assigned to it as a meteorological influence. 
The exhalation and evaporation of the juices of trees, by 
whatever process effected, take up atmospheric heat and _pro- 
duce a proportional refrigeration. This effect is not less real, 
though to common observation less sensible, in the forest tha 
in meadow or pasture land, and it cannot be doubted that the 
local temperature is considerably affected by it. But the 
evaporation that cools the air diffuses through it, at the same 
time, a medium which powerfully resists the escape of heat 
from the earth by radiation. Visible vapors, fogs and clouds, 
it is well known, prevent frosts by obstructing radiation, or 
rather by reflecting back again the heat radiated by the earth, 
just as any mechanical screen would do. On the other hand, 
fogs and clouds intercept the rays of the sun also, and hinder 
its heat from reaching the earth. The invisible 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 uncondensed moisture than 
after passing through a dry atmosphere. Hence the reduction 
of temperature by the evaporation of moisture from vegetation, 
