for some time becomes coated with a dark film of carbon. It is not
acted on at ordinary temperatures by most reagents, but at a higher
temperature it combines directly with many of the elements, forming
compounds called _carbides_. When heated in the presence of sufficient
oxygen it burns, forming carbon dioxide.

~Uses of carbon.~ The chief use of amorphous carbon is for fuel to furnish
heat and power for all the uses of civilization. An enormous quantity of
carbon in the form of the purer coals, coke, and charcoal is used as a
reducing agent in the manufacture of the various metals, especially in
the metallurgy of iron. Most of the metals are found in nature as
oxides, or in forms which can readily be converted into oxides. When
these oxides are heated with carbon the oxygen is abstracted, leaving
the metal. Retort carbon and coke are used to make electric light
carbons and battery plates, while lampblack is used for indelible inks,
printer's ink, and black varnishes. Bone black and charcoal have the
property of absorbing large volumes of certain gases, as well as smaller
amounts of organic matter; hence they are used in filters to remove
noxious gases and objectionable colors and odors from water. Bone black
is used extensively in the sugar refineries to remove coloring matter
from the impure sugars.

~Chemistry of carbon compounds.~ Carbon is remarkable for the very large
number of compounds which it forms with the other elements, especially
with oxygen and hydrogen. Compounds containing carbon are more numerous
than all others put together, and the chemistry of these substances
presents peculiarities not met with in the study of other substances.
For these reasons the systematic study of carbon compounds, or of
_organic chemistry_ as it is usually called, must be deferred until the
student has gained some knowledge of the chemistry of other elements. An
acquaintance with a few of the most familiar carbon compounds is,
however, essential for the understanding of the general principles of
chemistry.

~Compounds of carbon with hydrogen,--the hydrocarbons.~ Carbon unites with
hydrogen to form a very large number of compounds called _hydrocarbons_.
Petroleum and natural gas are essentially mixtures of a great variety of
these hydrocarbons. Many others are found in living plants, and still
others are produced by the decay of organic matter in the absence of
air. Only two of them, methane and acetylene, will be discuss