nt for
the requirements of a luxuriant vegetation. At the Poles, on the
contrary, the summer is but two or three months long; here, however, it
is daylight all summer, and plants from continual growth develop
themselves in that short time.

It will be recollected that carbonic acid constitutes but about 1/2500
of the air, yet, although about one half of all the vegetable matter in
the world is derived from this source, as well as all of the carbon
required by the growth of plants, its proportion in the atmosphere is
constantly about the same. In order that we may understated this, it
becomes necessary for us to consider the means by which it is formed.
Carbon, by the aid of fire, is made to unite with oxygen, and always
when bodies containing carbon are burnt _with the presence of
atmospheric air_, the oxygen of that air unites with the carbon, and
forms carbonic acid. The same occurs when bodies containing carbon
_decay_, as this is simply a slower _burning_ and produces the same
results. The respiration (or breathing) of animals is simply the union
of the carbon of the blood with the oxygen of the air drawn into the
lungs, and their breath, when thrown out, always contains carbonic acid.
From this we see that the reproduction of this gas is the direct effect
of the destruction of all organized bodies, whether by fire, decay, or
consumption by animals.

[Explain some of the operations in which this reproduction
takes place.

How is it reproduced?]

Furnaces are its wholesale manufactories. Every cottage fire is
continually producing a new supply, and the blue smoke issuing from the
cottage-chimney, as described by so many poets, possesses a new beauty,
when we reflect that besides indicating a cheerful fire on the hearth,
it contains materials for making food for the cottager's tables and new
faggots for his fire. The wick of every burning lamp draws up the carbon
of the oil to be made into carbonic acid at the flame. All matters in
process of combustion, decay, fermentation, or putrefaction, are
returning to the atmosphere those constituents, which they obtained from
it. Every living animal, even to the smallest insect, by respiration,
spends its life in the production of this material necessary to the
growth of plants, and at death gives up its body in part for such
formation by decay.

Thus we see that there is a continual change from the carbon of plants
to air, and from air back to plants, or through them t