ncertainty, as their peculiar "resinous" character makes them very
difficult to study by the usual methods of chemical investigations.

Resins are divided into two classes: (_a_) the balsams, and (_b_) the solid
or hard resins. Canada balsam and crude turpentine are familiar examples of
the first class. They consist of resinous substances, dissolved in or mixed
with fluid terpenes. Ordinary resin, or _colophony_, consists chiefly of a
monobasic acid having the empirical formula C_{20}H_{30}O_{2}, known as
sylvinic acid, whose exact structure is not known. Its sodium salt is used
as the basis for cheap soaps.

The hard resins are amorphous substances of vitreous character, which
consist of very complex aromatic acids, alcohols, or esters, combined with
other complicated structures, known as _resenes_, whose definite chemical
nature is not yet known. Among the hard resins are many substances which
are extensively used in the manufacture of varnishes, such as copal, amber,
dammar, sandarach, etc.

There are also resinous substances, such as asaf[oe]tida, myrrh, gamboge,
etc., which are mixtures of gums (see Chapter VI) and true resins. Some of
these have considerable commercial value for medicinal or technical uses.

PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE OF ESSENTIAL OILS

No theory has yet been advanced concerning the possibility of the use of
essential oils and resins by plants in their normal metabolic processes.
The very great diversity in their chemical nature makes it impossible that
they should all be considered as having the same physiological function,
if indeed any of them actually have any such function.

It is evident that those aromatic compounds which occur as normal
secretions of plants and which give to the plants their characteristic
odors may act either as an attraction to animals which might utilize the
plants as food and so serve to distribute the seed forms, or as a repellent
to prevent the too rapid destruction of the leaves, stems, or seeds of
certain species of plants whose slow-growing habits require the
long-continued growth of these portions of the plant for the perpetuation
of the species. The presence of these compounds in larger proportions in
those species of conifers, etc., which grow in tropical regions, in
competition with other rapid-growing vegetation, suggests the latter
possibility. It must be admitted, however, that their presence in such
cases may be the result of