of it are used in the manufacture of the _aniline or coal-tar
dyes_, which include many important compounds.

~Carbolic acid~ (C_{6}H_{5}OH). This compound, sometimes known as
_phenol_, occurs in coal tar, and is also prepared from benzene. It
forms colorless crystals which are very soluble in water. It is strongly
corrosive and very poisonous.

~Naphthalene and anthracene.~ These are hydrocarbons occurring along with
benzene in coal tar. They are white solids, insoluble in water. The
well-known _moth balls_ are made of naphthalene. Large quantities of
naphthalene are used in the preparation of _indigo_, a dye formerly
obtained from the indigo plant, but now largely prepared by laboratory
methods. Similarly anthracene is used in the preparation of the dye
_alizarin_, which was formerly obtained from the madder root.

THE ALKALOIDS

This term is applied to a group of compounds found in many plants and
trees. They all contain nitrogen, and most of them are characterized by
their power to combine with acids to form salts. This property is
indicated by the name alkaloids, which signifies alkali-like. The salts
are soluble in water, and on this account are more largely used than the
free alkaloids, which are insoluble in water. Many of the alkaloids are
used in medicine, some of the more important ones being given below.

~Quinine.~ This alkaloid occurs along with a number of others in the bark
of certain trees which grow in districts in South America and also in
Java and other tropical islands. It is a white solid, and its sulphate
is used in medicine in the treatment of fevers.

~Morphine.~ When incisions are made in the unripe capsules of one of the
varieties of the poppy plant, a milky juice exudes which soon thickens.
This is removed and partially dried. The resulting substance is the
ordinary _opium_ which contains a number of alkaloids, the principal one
being morphine. This alkaloid is a white solid and is of great service
in medicine.

Among the other alkaloids may be mentioned the following: _Nicotine_, a
very poisonous liquid, the salts of which occur in the leaves of the
tobacco plant; _cocaine_, a crystalline solid present in coca leaves and
used in medicine as a local anaesthetic; _atropine_, a solid present in
the berry of the deadly nightshade, and used in the treatment of
diseases of the eye; _strychnine_, a white, intensely poisonous solid
present in the seeds of the members of the _Strychnos_ fami