h places it causes the
deaths we so often hear about of those who go down into them without
proper care. It is found in many springs of water, more or less; and a
great deal of it comes out of the earth in some places. Carbonic acid
gas is what stupefies the dogs in the Grotto del Cane. Well, but how is
carbonic acid gas made by the candle?"

"I hope with your candle you'll throw some light upon the subject," said
Uncle Bagges.

"I hope so," answered Harry. "Recollect it is the burning of the smoke,
or soot, or carbon of the candle that makes the candle-flame bright.
Also that the candle won't burn without air. Likewise that it will not
burn in nitrogen, or air that has been deprived of oxygen. So the carbon
of the candle mingles with oxygen, in burning, to make carbonic acid
gas, just as the hydrogen does to form water. Carbonic acid gas, then,
is carbon or charcoal dissolved in oxygen. Here is black soot getting
invisible and changing into air; and this seems strange, uncle, doesn't
it?"

"Ahem! Strange, if true," answered Mr. Bagges. "Eh? well! I suppose it's
all right."

"Quite so, uncle. Burn carbon or charcoal either in the air or in
oxygen, and it is sure always to make carbonic acid, and nothing else,
if it is dry. No dew or mist gathers in a cold glass jar if you burn dry
charcoal in it. The charcoal goes entirely into carbonic acid gas, and
leaves nothing behind but ashes, which are only earthy stuff that was in
the charcoal, but not part of the charcoal itself. And now, shall I tell
you something about carbon?"

"With all my heart," assented Mr. Bagges.

"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 crystallize a little bit of
carbon, and make a Koh-i-noor?"

"Ah, uncle, perhaps we shall, some day. In the mean ti