l compounds, but each consists of a mixture
of hydrocarbons, the boiling points of which lie within certain limits.

2. _Coal tar._ This product is obtained in the manufacture of coal gas,
as already explained. It is a complex mixture and is refined by the same
general method used in refining crude petroleum. The principal
hydrocarbons obtained from the coal tar are benzene, toluene,
naphthalene, and anthracene. In addition to the hydrocarbons, coal tar
contains many other compounds, such as carbolic acid and aniline.

~Properties of the hydrocarbons.~ The lower members of the first two
series of hydrocarbons mentioned are all gases; the succeeding members
are liquids. In some series, as the methane series, the higher members
are solids. The preparation and properties of methane and acetylene have
been discussed in a previous chapter. Ethylene is present in small
quantities in coal gas and may be obtained in the laboratory by
treating alcohol (C_{2}H_{6}O) with sulphuric acid:

C_{2}H_{6}O = C_{2}H_{4} + H_{2}O.

Benzene, the first member of the benzene series, is a liquid boiling at
80 deg..

The hydrocarbons serve as the materials from which a large number of
compounds can be prepared; indeed, it has been proposed to call organic
chemistry _the chemistry of the hydrocarbon derivatives_.

~Substitution products of the hydrocarbons.~ As a rule, at least a part of
the hydrogen in any hydrocarbon can be displaced by an equivalent amount
of certain elements or groups of elements. Thus the compounds CH_{3}Cl,
CH_{2}Cl_{2}, CHCl_{3}, CCl_{4} can be obtained from methane by
treatment with chlorine. Such compounds are called _substitution
products_.

~Chloroform~ (CHCl_{3}). This can be made by treating methane with
chlorine, as just indicated, although a much easier method consists in
treating alcohol or acetone (which see) with bleaching powder.
Chloroform is a heavy liquid having a pleasant odor and a sweetish
taste. It is largely used as a solvent and as an anaesthetic in surgery.

~Iodoform~ (CHI_{3}). This is a yellow crystalline solid obtained by
treating alcohol with iodine and an alkali. It has a characteristic odor
and is used as an antiseptic.

ALCOHOLS

When such a compound as CH_{3}Cl is treated with silver hydroxide the
reaction expressed by the following equation takes place:

CH_{3}Cl + AgOH = CH_{3}OH + AgCl.

Similarly C_{2}H_{5}Cl will give C_{2}H_{5}OH and AgCl. The compounds
CH_{3}