omplex organic acid.

The gums are translucent, amorphous substances, whose solutions in water
are levorotatory. They are precipitated out of solution by alcohol and by
lead subacetate solution.

Gums are extremely difficult to hydrolyze, the laboratory process of
hydrolysis usually requiring from eighteen to twenty-four hours of
continuous boiling with acids for its completion. Because of this
difficulty of hydrolysis, gums are practically indigestible by animals and
of little use as food.

The following common examples will serve to illustrate the general nature
of these compounds.

=Gum arabic=, found in the exudate from the stems of various species of
Acacia, is a mixture of the calcium, magnesium, and potassium salts of a
diaraban-tetragalactan-arabic acid. Arabic acid has the formula
C_{23}H_{38}O_{22}, and one molecule of this acid serves as the nucleus for
the union of eight galactose and four arabinose groups, linked together in
some unknown way. The formula for the compound, exclusive of the metallic
elements with which it is loosely united is C_{91}H_{150}O_{78}. This gives
some idea of its complexity.

When boiled with nitric acid, it is oxidized to mucic, saccharic, and
oxalic acids. It gives characteristic reactions with alum, basic lead
acetate, and other common reagents.

Gum arabic comes on the market as a brittle, glassy mass, which is used in
the preparation of mucilages, and as a carrier for essential oils, etc., in
certain toilet preparations.

Recent investigations have shown that the so-called "meta-pectic acid,"
which is often found in sugar beets and interferes with the process of
sugar manufacture, is identical with gum arabic in composition and
properties.

=Gum tragacanth= is the soluble portion of the natural gum which is found
in several species of _Astragalus_. It constitutes only 8 to 10 per cent of
the total gum-like material which is present, the remainder being composed
of insoluble gummy substances of unknown composition. The soluble gum
consists of calcium, potassium, and magnesium salts of an acid which, when
hydrolyzed, yields several molecules of arabinose, six of galactose, and
one of geddic acid (an isomer of arabic acid). It is said to be produced by
the metamorphosis of the medullary rays under unfavorable conditions of
growth. It comes on the market in globular masses of amorphous material,
and is used in the manufacture of cosmetics, etc.

=Wound gum= is frequentl