a low ignition point. When made at higher temperatures it
is much more dense, and its ignition point is considerably higher.
Charcoal burns when heated in air, usually without the formation of
flame, although a flame is apparent if the temperature be raised. It
is characterized by its power of absorbing gases; thus, according to
J. Hunter [_Phil. Mag._, 1863 (4), 25, p. 363], one volume of charcoal
absorbs (at 0 deg. C. and 760 mm. pressure) 171.7 ccs. of ammonia,
86.3 ccs. of nitrous oxide, 67.7 ccs. of carbon monoxide, 21.2 ccs. of
carbon dioxide, 17.9 ccs. of oxygen, 15.2 ccs. of nitrogen, and 4.4
ccs. of hydrogen [see also J. Dewar, _Ann. Chim. Phys._, 1904 (8), 3,
p. 5]. It also has the power of absorbing colouring matters from
solution. Charcoal is used as a fuel and as a reducing agent in
metallurgical processes.

The element carbon unites directly with hydrogen to form acetylene
when an electric arc is passed between carbon poles in an atmosphere
of hydrogen (M. Berthelot); it also unites directly with fluorine,
producing, chiefly, carbon tetrafluoride CF4. It burns when heated in
an atmosphere of oxygen, forming carbon dioxide, and when heated in
sulphur vapour it forms carbon bisulphide (q.v.). When heated with
nitrogenous substances, in the presence of carbonated or caustic
alkali, it forms cyanides. It combines directly with silicon, at the
temperature of the electric furnace, yielding _carborundum_, SiC; and
H. Moissan has also shown that it will combine with many metals at
the temperature of the electric furnace, to form carbides (q.v.).

The specific heat of carbon varies with the temperature the following
values having been obtained by H.F. Weber (_Jahresberichte_, 1874, p.
63):--

+-----------------+-----------------+-------------------+
| Diamond. | Graphite. |Porous wood carbon.|
+-------+---------+-------+---------+--------+----------+
| t deg.| Sp. Ht. | t deg.| Sp. Ht. | t deg. | Sp. Ht. |
+-------+---------+-------+---------+--------+----------+
| -50.5 | 0.0635 | -50.3 | 0.1138 | 0-23 | 0.1653 |
| -10.6 | 0.0955 | -10.7 | 0.1437 | 0-99 | 0.1935 |
| +10.7 | 0.1128 | +10.8 | 0.1604 | 0-223 | 0-2385 |
| 85.5 | 0.1765 | 61.3 | 0.1990 | | |
| 206.1 | 0.2733 | 201.6 | 0.2966 | | |
| 606.7 | 0.4408 | 641.9 | 0.4454 | | |