groups are as follows:

METHANE SERIES
CH_{4} methane
C_{2}H_{6} ethane
C_{3}H_{8} propane
C_{4}H_{10} butane
C_{5}H_{12} pentane
C_{6}H_{14} hexane
C_{7}H_{16} heptane
C_{8}H_{18} octane

ETHYLENE SERIES
C_{2}H_{4} ethylene
C_{3}H_{6} propylene
C_{4}H_{8} butylene

BENZENE SERIES
C_{6}H_{6} benzene
C_{7}H_{8} toluene
C_{8}H_{10} xylene

ACETYLENE SERIES
C_{2}H_{2} acetylene
C_{3}H_{4} allylene

Only the lower members (that is, those which contain a small number of
carbon atoms) of the above groups are given. The methane series is the
most extensive, all of the compounds up to C_{24}H_{50} being known.

It will be noticed that the successive members of each of the above
series differ by the group of atoms (CH_{2}). Such a series is called an
_homologous series_. In general, it may be stated that the members of an
homologous series show a regular gradation in most physical properties
and are similar in chemical properties. Thus in the methane group the
first four members are gases at ordinary temperatures; those containing
from five to sixteen carbon atoms are liquids, the boiling points of
which increase with the number of carbon atoms present. Those containing
more than sixteen carbon atoms are solids.

~Sources of the hydrocarbons.~ There are two chief sources of the
hydrocarbons, namely, (1) crude petroleum and (2) coal tar.

1. _Crude petroleum._ This is a liquid pumped from wells driven into the
earth in certain localities. Pennsylvania, Ohio, Kansas, California, and
Texas are the chief oil-producing regions in the United States. The
crude petroleum consists largely of liquid hydrocarbons in which are
dissolved both gaseous and solid hydrocarbons. Before being used it must
be refined. In this process the petroleum is run into large iron stills
and subjected to fractional distillation. The various hydrocarbons
distill over in the general order of their boiling points. The
distillates which collect between certain limits of temperature are kept
separate and serve for different uses; they are further purified,
generally by washing with sulphuric acid, then with an alkali, and
finally with water. Among the products obtained from crude petroleum in
this way are the naphthas, including benzine and gasoline, kerosene or
coal oil, lubricating oils, vaseline, and paraffin. None of these
products are definite chemica