I have not much faith in the science of ignorant men; for the
foundations of all knowledge are laid in books; and those only who have
read and studied much can possess any considerable store of wisdom. But
there are philosophers among laboring swains, whose quaint observations
and solutions of nature’s problems are sometimes worthy of record. With
these men of untutored genius I have had considerable intercourse, and
hence I oftener quote them than the learned and distinguished, whom I
have rarely met. The ignorant, from want of knowledge, are always
theorists; but genius affords its possessor, how small soever his
acquisitions, some glimpses of truth which may be entirely hidden from
the mere pedant in science. My philosophic friend, a man of genius born
to the plough, entertained a theory in regard to the atmosphere, which,
though not strictly philosophical, is so ingenious and suggestive that I
have thought an account of it a good introduction to this essay.
My friend, when explaining his views, alluded to the well-known fact
that plants growing in an aquarium keep the water supplied with
atmospheric air—not with simple oxygen, but with oxygen chemically
combined with nitrogen—by some vital process that takes place in the
leaves of plants. As the lungs of animals decompose the air which they
inspire, and breathe out carbonic-acid gas, plants in their turn
decompose this deleterious gas, and breathe out pure atmospheric air.
His theory is that the atmosphere is entirely the product of vegetation,
and that nature has no other means of composing it; that it is not
simply a chemical, but a vital product; and that its production, like
its preservation, depends entirely on plants, and would be impossible
without their agency. But as all plants united are not equal in bulk to
the trees, it may be truly averred that any series of operations or
accidents that should deprive the earth entirely of its forests would
leave the atmosphere without a source for its regeneration.
The use of the foliage of trees in renovating the atmosphere is not, I
believe, denied by any man of science. This theory has been proved to be
true by experiments in vital chemistry. The same chemical appropriation
of gases and transpiration of oxygen is performed by all classes of
vegetables; but any work in the economy of nature assigned to vegetation
is the most effectually accomplished by trees. The property of foliage
that requires carbonic-acid gas for its breathing purposes, and causes
it to give out oxygen, is of vital importance; and it is hardly to be
doubted that a close room well lighted by the sun would sustain its
healthful atmosphere a longer time, if it were filled with plants in
leaf, but not in flower, and occupied by breathing animals, than if the
animals occupied it without the plants.
But there is another function performed by the foliage of trees and
herbs in which no chemical process is involved,—that of exhaling
moisture into the atmosphere after it has been absorbed by the roots.
Hence the humidity of this element is greatly dependent on foliage. A
few simple experiments will show how much more rapidly and abundantly
this evaporation takes place when the soil is covered with growing
plants than when the surface is bare. Take two teacups of equal size and
fill them with water. Place them on a table, and insert into one of them
cuttings of growing plants with their leaves, and let the other stand
with water only. In a few hours the water will disappear from the cup
containing the plants, while that in the other cup will not be sensibly
diminished. Indeed, there is reason to believe that gallons of water
might be evaporated into the air by keeping the cup containing the
cuttings always full, before the single gill contained in the other cup
would disappear. If a few cuttings will evaporate a half-pint of water
in twelve hours, we can imagine the vast quantity constantly exhaled
into the atmosphere by a single tree. The largest steam-boiler in use,
kept constantly boiling, would not probably evaporate more water than
one large elm in the same time.
We may judge, from our experiment with the cuttings, that a vastly
greater proportion of moisture would be exhaled into the atmosphere from
any given surface of ground when covered with vegetation, than from the
same amount of uncovered surface, or even of standing water. Plants are
indeed the most important existing agents of nature for conveying the
moisture of the earth into the air. The quantity of transpiring foliage
from a dense assemblage of trees must be immense. The evaporation of
water from the vast ocean itself is probably small compared with that
from the land which it surrounds. And there is reason to believe that
the water evaporated from the ocean would not produce rain enough to
sustain vegetation, if by any accident every continent and island were
deprived of its trees. The whole earth would soon become a desert. I
would remark, in this place, that trees are the agents by which the
superfluous waters of the ocean, as they are supplied by rivers emptying
into it, are restored to the atmosphere and thence again to the surface
of the earth. Trees pump up from great depths the waters as they ooze
into the soil from millions of subterranean ducts ramifying in all
directions from the bed of the ocean.
[Illustration: LEAF OF HOLLY.]