ibility of rebreathing
respired air until it has been purified and restored to its natural and
healthful state; for carbonic acid, the vitiating product of
respiration, although immediately _fatal_ to _animals_, constitutes the
very _life_ of _vegetation_. When brought in contact with the upper
surface of the green leaves of trees and plants, and acted upon by the
direct solar rays, this gas is decomposed, and its carbon is absorbed to
sustain, in part, the life of the plant, by affording it one element of
its food, while the oxygen is liberated and restored to the atmosphere.
Vegetables and animals are thus perpetually interchanging kindly
offices, and each flourishes upon that which is fatal to the other. It
is in this way that the healthful state of the atmosphere is kept up.
Its equilibrium seems never to be disturbed, or, if disturbed at all, it
is immediately restored by the mutual exchange of poison for aliment,
which is constantly going on between the animal and vegetable worlds.
This interchange of kindly offices is constantly going on all over the
earth, even in the highest latitudes, and in the very depths of winter;
for air which has been respired is rarefied, and, when thrown from the
lungs, _ascends_, and is thus not only out of our reach, whereby we are
protected from respiring it a second time, but this (to us) deadly
poison falls into the great aerial current which is constantly flowing
from the polar to the tropical regions, where it is converted into
vegetable growth. The oxygen which is exhaled in the processes of
tropical vegetation, heated and rarefied by the vertical rays of the
sun, mounts to the upper regions of the atmosphere, and, falling into a
returning current, in its appointed time revisits the higher latitudes.
So wisely has the Divine Author ordered these processes, that air, in
its natural state[15] in any part of the world, does not contain more
than _one half of one per cent._ of carbonic acid gas, although, as
already stated, air which has been once respired contains _eight and a
half per cent._ of this gas, which is at least seventeen times its
natural quantity.

[15] It would be difficult to say whether carbonic acid gas is in the
atmosphere constitutionally, or accidentally, or both.--_Dr. Wm. A.
Alcott's Health Tracts._

There are other agencies than carbonic acid gas which in civic life
render the atmosphere impure. Of this nature is carbureted hydrogen gas,
which is