ism, instead of in practically
constant proportions in only a few species, as they do.

ALKALOIDS

These are a group of strong vegetable bases whose nitrogen atom is a part
of a closed-ring arrangement.

As a rule, alkaloids are colorless, crystalline solids, although a few are
liquids at ordinary temperatures. They are generally insoluble in water,
but easily soluble in organic solvents. Being strong bases, they readily
form salts with acids, and these salts are usually readily soluble in
water.

Alkaloids are usually odorless; although nicotine, coniine, and a few
others, have strong, characteristic odors. Most of them have a bitter
taste, and many of them have marked physiological effects upon animal
organisms, so that they are extensively used as narcotics, stimulants, or
for other medicinal purposes.

Most of the alkaloids contain asymmetric carbon atoms and are, therefore,
optically active, usually levorotatory, although a few are dextrorotatory.

The alkaloids are precipitated out of their solutions by various solutions
of chemical compounds, known as the "alkaloidal reagents": iodine dissolved
in potassium iodide solution gives a chocolate-brown precipitate; tannic
acid, phosphotungstic acid, phosphomolybdic acid, and mercuric iodide
solutions give colorless, amorphous precipitates; while gold chloride and
platinic chloride solutions give crystalline precipitates, many of which
have sharp melting points and can be used for the identification of
individual alkaloids. There are a great many specific color reactions for
individual alkaloids, which are important to toxicologists and pharmacists,
but which it would not be desirable to consider in detail here.

The alkaloids are conveniently divided into groups, according to the
characteristic closed-ring arrangements which they contain. The several
closed-ring arrangements which are found in common alkaloids, and upon
which their grouping is based, may be illustrated by the following
formulas:

H H H
H{2}_C---CH_{2} | \ /
| | C C
| | // \ / \
H{2}_C CH_{2} HC CH H_{2}C CH_{2}
\ / | | | |
N HC CH H_{