Allotropy, or allotropism, is the quality which an element often
has of appearing under various forms, with different properties.
The forms of C are a good illustration.

42. Diamond is the purest C; but even this in burning leaves a
little ash, showing that it is not quite pure. It is a rare
mineral, found in India, South Africa, and Brazil, and is the
hardest and most highly refractive to light of all minerals.
Boron is harder. [Footnote: B, not occurring free, is not a
mineral.] When heated in the electric arc, at very high
temperatures, diamond swells and turns black. 43. Graphite, or
Plumbago, is One of the Softest Minerals.--It is black and
infusible, and oxidizes only at very high temperatures, higher
than the diamond. It contains from 95 to 98 per cent C. Graphite
is found in the oldest rock formations, in the United States and
Siberia. It is artificially formed in the iron furnace. Graphite
is employed for crucibles where great heat is required, for a
lubricant, for making metal castings, and, mixed with clay, for
lead-pencils. It is often called black-lead.

44. Amorphous Carbon comprises the following varieties.

Charcoal is made by heating wood, for a long time, out of contact
with the air. The volatile gases are thus driven off from the
wood; what is left is C, and a small quantity of mineral matter
which remains as ash when the coal is burned.

45. Lamp-black is prepared as in Experiment 26, or by igniting
turpentine (C1OH16), naphtha, and various oils, and collecting