To see this we need only take into consideration carbon's relationship
to oxidation and reduction respectively. As it is natural for sulphur
to be in the reduced state, and for phosphorus to be in the oxidized
state, so it is in the nature of carbon to be related to both states
and therefore to oscillate between them. By its readiness to change
over from the oxidized to the reduced state, it can serve the plant in
the assimilation of light, while by its readiness to make the reverse
change it serves man and animal in the breathing process. We breathe in
oxygen from the air; the oxygen circulates through the blood-stream and
passes out again in conjunction with carbon, as carbon dioxide, when we
exhale. In the process whereby the plants reduce the carbon dioxide
exhaled by man and animal, while the latter again absorb with their
food the carbon produced in the form of organic matter by the plant, we
see carbon moving to and fro between the oxidized and the reduced
conditions.

Within the plant itself, too, carbon acts as functionary of the
alternation between oxidation and reduction. During the first half of
the year, when vegetation is unfolding, there is a great reduction
process of oxidized carbon, while in the second half of the year, when
the withering process prevails, a great deal of the previously reduced
carbon passes into the oxidized condition. As this is connected with
exhaling and inhaling of oxygen through carbon, carbon can be regarded
as having the function of the lung-organ of the earth. Logically
enough, we find carbon playing the same role in the middle part of the
threefold human organism.

Another indication of the midway position of carbon is its ability to
combine as readily with hydrogen as with oxygen, and, in these polar
combinations, even to combine with itself. In this latter form it