they are all exactly the same substance. The diamond is the purest
carbon, and occurs in the crystalline form known as a regular
octahedron; the diamond is one of the hardest substances known, and is
therefore, utilized for cutting glass; it has also a very high specific
gravity, namely, 3.5, which means that it is three and a half times
heavier than water, and it is far heavier than any of the other
allotropic modifications of carbon. Graphite or plumbago, the second
form in which carbon occurs, is widely distributed in nature, and the
finer qualities are known as black lead, although no lead enters into
their composition, as they are composed of carbon almost as pure as the
diamond; the specific gravity of graphite is only 2.3. Charcoal, the
third allotropic modification of carbon, is by far the most common, and
is formed by the natural or artificial disintegration of organic matters
by heat; we thus have formed wood charcoal, animal charcoal, lamp-black,
and coke, all produced by artificial means, and we may also class with
these coal, which is a natural product, and which contains from 85 to 95
per cent. of pure carbon.

Wood charcoal is made by heating wood in closed vessels or in large
masses, when all the hydrogen, oxygen, and nitrogen are expelled in
the gaseous state, and the carbon is left mixed with the mineral
constituents of the wood; this form of carbon is very porous and light,
and is used in a number of industrial processes.

Animal charcoal, as its name implies, is the carbonaceous residue left
on heating any animal matters in a retort; and contains, in addition to
the carbon, a large proportion of phosphates and other mineral salts,
which, however, can be extracted by dilute acids. Animal charcoal
possesses to a remarkable degree the property of removing color from
solutions of animal and vegetable substances, and it is used for this