"I said that there was carbon or charcoal in all common lights, so
there is in every common kind of fuel. If you heat coal or wood away
from the air, some gas comes away, and leaves behind coke from coal,
and charcoal from wood; both carbon, though not pure. Heat carbon
as much as you will in a close vessel, and it does not change in the
least; but let the air get to it, and then it burns and flies off in
carbonic acid gas. This makes carbon so convenient for fuel. But it is
ornamental as well as useful, uncle. The diamond is nothing else than
carbon."

"The diamond, eh! You mean the black diamond."

"No: the diamond, really and truly. The diamond is only carbon in the
shape of a crystal."

"Eh? and can't some of your clever chemists crystalize a little bit of
carbon, and make a Koh-i-noor?"

"Ah, uncle, perhaps we shall, some day. In the mean time I suppose we
must be content with making carbon so brilliant as it is in the flame
of a candle. Well; now you see that a candle-flame is vapor burning,
and the vapor, in burning, turns into water and carbonic acid gas. The
oxygen of both the carbonic acid gas and the water comes from the air,
and the hydrogen and carbon together are the vapor. They are distilled
out of the melted was by the heat. But, you know, carbon alone can't
be distilled by any heat. It can be distilled, though, when it is
joined with hydrogen, as it is in the wax, and then the mixed hydrogen
and carbon rise in gas of the same kind as the gas in the streets, and
that also is distilled by heat from coal. So a candle is a little gas
manufactory in itself, that burns the gas as fast as it makes it."