g in plants is expelled by exposure to
the air or a gentle heat, the residual dry matter is found to be
composed of a considerable number of different substances, which have
been divided into two great classes, called the organic and the
inorganic, or mineral constituents of plants. The former are readily
combustible, and on the application of heat, catch fire, and are
entirely consumed, leaving the inorganic matters in the form of a white
residuum or ash. All plants contain both classes of substances; and
though their relative proportions vary within very wide limits, the
former always greatly exceed the latter, which in many cases form only a
very minute proportion of the whole weight of the plant. Owing to the
great preponderance of the organic or combustible matters, it was at one
time believed that the inorganic substances formed no part of the true
structure of plants, and consisted only of a small portion of the
mineral matters of the soil, which had been absorbed along with their
organic food; but this opinion, which probably was never universally
entertained, is now entirely abandoned, and it is no longer doubted that
both classes of substances are equally essential to their existence.

Although they form so large a proportion of the plant, its organic
constituents are composed of no more than four elements, viz.:--

Carbon.
Hydrogen.
Nitrogen.
Oxygen.

The inorganic constituents are much more numerous, not less than
thirteen substances, which appear to be essential, having been observed.
These are--

Potash.
Soda.
Lime.
Magnesia.
Peroxide of Iron.
Silicic Acid.
Phosphoric Acid.
Sulphuric Acid.
Chlorine.

And more rarely

Manganese.
Iodine.
Bromine.
Fluorine.

Several other substances, among which may be mentioned alumina and
copper, have also been enumerated; but there is every reason to believe
that they are not essential, and the cases in which they have been found
are quite exceptional.

It is to be especially noticed that none of these substances occur in
plants in the free or uncombined state, but always in the form of
compounds of greater or less complexity, and extremely varied both in
their properties and composition.

It would be out of place, in a work like the present, to enter into
complete details of the properties of the elements of which plants are
composed, which belongs strictly to pure