the C of the smoke. It is used for making printers' ink, India
ink, etc. A very pure variety is obtained from natural gas.

Bone-black, or animal charcoal, is obtained by distilling bones,
i.e. by heating them in retorts into which no air is admitted.
The C is the charred residue.

Gas Carbon is formed in the retorts of the gas-house. See page
182. It is used to some extent in electrical work.

46. Coke is the residue left after distilling soft coal. It is
tolerably pure carbon, with some ash and a little volatile
matter. It burns without flame. 47. Mineral Coal is fossilized
wood or other vegetable matter. Millions of years ago trees and
other vegetation covered the earth as they do to-day. In certain
places they slowly sank, together with the land, into the
interior of the earth, were covered with sand, rock, and water,
and heated from the earth's interior. A slow distillation took
place, which drove off some of the gases, and converted vegetable
matter into coal. All the coal dug from the earth represents
vegetable life of a former period. Millions of years were
required for the transformation; but the same change is in
progress now, where peat beds are forming from turf.

Coal is found in all countries, the largest beds being in the
United States. From the nature of its formation, coal varies much
in purity.

Anthracite, or hard coal, is purest in carbon, some varieties
having from 90 to 95 per cent. This represents most complete