matter in former ages, which became
covered over with earthy material and were thus protected from rapid
decay. Under various natural agencies the organic matter was slowly
changed into coal. In anthracite these changes have gone the farthest,
and this variety of coal is nearly pure carbon. Soft or bituminous coals
contain considerable organic matter besides carbon and mineral
substances. When heated strongly out of contact with air the organic
matter is decomposed and the resulting volatile matter is driven off in
the form of gases and vapors, and only the mineral matter and carbon
remain behind. The gaseous product is chiefly illuminating gas and the
solid residue is _coke_. Some of the coke is found as a dense cake on
the sides and roof of the retort. This is called retort carbon and is
quite pure.

3. _Charcoal._ This is prepared from wood in the same way that coke is
made from coal. When the process is carried on in retorts the products
expelled by the heat are saved. Among these are many valuable substances
such as wood alcohol and acetic acid. Where timber is abundant the
process is carried out in a wasteful way, by merely covering piles of
wood with sod and setting the wood on fire. Some wood burns and the heat
from this decomposes the wood not burned, forming charcoal from it. The
charcoal, of course, contains the mineral part of the wood from which it
is formed.

4. _Bone black._ This is sometimes called animal charcoal, and is made
by charring bones and animal refuse. The organic part of the materials
is thus decomposed and carbon is left in a very finely divided state,
scattered through the mineral part which consists largely of calcium
phosphate. For some uses this mineral part is removed by treatment with
hydrochloric acid and prolonged washing.

5. _Lampblack._ Lampblack and soot are products of imperfect combustion
of oil and coal, and are deposited from a smoky flame on a cold surface.
The carbon in this form is very finely divided and usually contains
various oily materials.

~Properties.~ While the various forms of carbon differ in many properties,
especially in color and hardness, yet they are all odorless, tasteless
solids, insoluble in water and characterized by their stability towards
heat. Only in the intense heat of the electric arc does carbon
volatilize, passing directly from the solid state into a vapor. Owing to
this fact the inside surface of an incandescent light bulb after being
used