power of reorganising, or
of assimilating to the nature of their own organisms, certain of the
substances elaborated by plants, and destined to become food for
animals.

SECTION II.

COMPOSITION OF ORGANIC SUBSTANCES.

_Elements of Organic Bodies._--The number of distinct kinds of
substances--each distinguishable from all the others by the peculiarity
of its properties, taken as a whole--is exceedingly great, yet all
these substances are resolvable into a very small number of bodies.
As an illustration, I shall take a well-known substance, common
green copperas, or, as the chemists term it, protosulphate of iron.
By submitting this compound to the process termed chemical analysis,
two other kinds of matter may be obtained from it, namely, oxide of iron
and oil of vitrol, or sulphuric acid. If we continued this process--if
we submitted the acid and the oxide to analysis--we could separate the
former into sulphur and oxygen, and the latter into iron and oxygen.
Now, by these means we could demonstrate the compound nature of
copperas; we could prove that it was _proximately_ composed of sulphuric
acid and oxide of iron; and, _ultimately_, of iron, sulphur, and oxygen.

Iron, sulphur, and oxygen, are elementary, or simple bodies. They cannot
be decomposed; they cannot be analysed. Torture them as we will in our
crucibles; expose them as we please to the highest temperature of a wind
furnace, or to the more intense heat evolved by a powerful galvanic
battery; subject them to the influence of any agent, or force, or
process we may choose, and still they will yield nothing but iron,
sulphur, and oxygen: hence these undecomposable bodies are regarded as
_elements_, or simple substances. So far as our knowledge extends, there
are about sixty-six of these undecomposable bodies, of which about one
half occurs in but exceedingly minute quantities, and a considerable
number of the others exists in comparatively small amounts. As by far
the greater proportion of compounds is made up of two or more of about
a dozen elementary bodies, it would at first sight appear as if the
distinct kinds of compounds which exist, or which may be called into
existence by the chemist, must be limited to, at most, a realisable
number; but the fact is there is no practical limit to the variety of
substances which may be artificially formed. Every difference in the
mode of the arrangement of the constituent atoms of a compound, causes
its metamorph