Moreover, its atoms possess in a special
degree, unknown in the case of other ele-
ments, the power of combining with each
other, so that long chains or rings are formed.
Compounds containing as many as two hun-
dred or more atoms are known to exist.
Another important fact about carbon is its
position with respect to other elements with
which it may combine. The chemical ele-
ments can be arranged in a series, such that
those at opposite ends have the greatest
"affinity" for each other. In other words,
they combine together with the setting free



14 PHYSIOLOGY OF FOOD

of large amounts of energy. Oxygen and
hydrogen, for example, are a long way apart
in this arrangement, and they combine with
great violence. Now, carbon is somewhere
about the middle, so that it can combine with
either hydrogen or with oxygen. In the
latter case, we have the universal product of
combustion of carbon compounds as it takes
place in living organisms, as well as in the
non-living world. This is carbon dioxide or
carbonic acid, as it used to be called, since it
forms an acid when dissolved in water. In
this process of combustion, energy is given
off. On the other hand, carbon can be made
to combine with hydrogen, giving rise to the
well-known paraffin oils and similar hydro-
carbons. But, in this case, energy must be
supplied from outside. These compounds of
carbon and hydrogen, when burnt, give out
large quantities of energy, as is familiar to
all, at all events in one of its forms, that of
heat. We see thus that carbon serves as
an admirable means for the transference of
energy in vital processes.
