REFRIGERATION BY EXHALATION. 
175 
or exudation, after having surrendered to the plant the small 
proportion of matter required for vegetable growth which it 
held in solution or suspension.* The hygrometrical equilib¬ 
rium is then restored, so far as this : the tree yields up again 
the moisture it had drawn from the earth and the air, though 
it does not return it each to each ; for the vapor carried oft* by 
transpiration greatly exceeds the quantity of water absorbed by 
the foliage from the atmosphere, and the amount, if any, car¬ 
ried back to the ground by the roots. 
The evaporation of the juices of the plant, by whatever 
process effected, takes up atmospheric heat and produces re¬ 
frigeration. This effect is not less real, though much less 
sensible, in the forest than 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 ah' diffuses 
through it, at the same time, a medium which powerfully 
resists the escape of heat from the earth by radiation. Visible 
vapors or clouds, it is well known, prevent frosts by obstruct- 
* Ward’s tight glazed cases for raising, and especially for transporting 
plants, go far to prove that water only circulates through vegetables, and 
is again and again absorbed and transpired by organs appropriated to these 
functions. Seeds, growing grasses, shrubs, or trees planted in proper earth, 
moderately watered and covered with a glass bell or close frame of glass, 
live for months and even years, with only the original store of air and 
water. In one of Ward’s early experiments, a spire of grass and a fern, 
which sprang up in a corked bottle containing a little moist earth intro¬ 
duced as a bed for a snail, lived and flourished for eighteen years without 
a new supply of either fluid. In these boxes the plants grow till the en¬ 
closed air is exhausted of the gaseous constituents of vegetation, and till 
the water has yielded up the assimilable matter it held in solution, and dis¬ 
solved and supplied to the roots the nutriment contained in the earth in 
which they are planted. After this, they continue for a long time in a 
state of vegetable sleep, but if fresh air and water be introduced into the 
cases, or the plants be transplanted into open ground, they rouse them¬ 
selves to renewed life, and grow vigorously, without appearing to have suf¬ 
fered from their long imprisonment. The water transpired by the leaves 
is partly absorbed by the earth directly from the air, partly condensed on 
the glass, along which it trickles down to the earth, enters the roots again, 
and thus continually repeats the circuit. See Aus der Natur , 21, B. S. 537. 
