It has been already observed
that starch, sugar, and the other allied substances, may be considered
as compounds of carbon with water; and they might be supposed to owe
their origin to the carbonic acid losing the whole of its oxygen, and
direct combination then ensuing between the residual carbon and a
certain proportion of water; but this would imply that the latter
substance undergoes no decomposition, and though undoubtedly the
simplest view of the case, it is by no means the most probable. It is
much more likely that the carbonic acid is only partially decomposed,
half its oxygen being separated, and replaced by hydrogen, produced by
the decomposition of a certain quantity of water into its elements.
Thus, for instance, sugar may be produced from twelve equivalents of
carbonic acid and twelve equivalents of water, twenty-four equivalents
of oxygen being eliminated, as thus represented:

12 equivalents of carbonic acid, C_{12}O_{12}O_{12}
12 " water, H_{12}O_{12}
1 " sugar, and 24 of ox. C_{12}H_{12}O_{12} + O_{24}

It must not be supposed that we are in a condition to assert that sugar
is really produced in the manner here shown, the illustration being
given merely for the purpose of pointing out how it may be supposed to
occur, and on a similar principle it is possible to explain the
formation of most other vegetable compounds; and this subject has been
very fully discussed by the late Dr. Gregory, in his "Handbook of
Organic Chemistry." That water must be decomposed, is evident from the
fact, established by analysis, that the hydrogen of the plant generally
exceeds the quantity required to form water with its oxygen, so that
this excess at least must be produced by the decomposition of water. The
hydrogen of the volatile oils, many of which contain no oxygen, and that
of the fats, which contain only a small quantity, must manifestly be
obtained in a similar manner.

_Decomposition of Ammonia._--The nitrogenous or albuminous compounds of
vegetables must necessarily obtain their nitrogen from the decomposition
either of ammonia or nitric acid, experiment having distinctly shown
that they are incapable of absorbing it in the free state from the
atmosphere. It has been clearly ascertained that the albuminous
substances do not contain ammonia, and it is hence apparent that a
complete decomposition of that substance must take place in the plant.
No doubt carbonic acid and water take