thing made by the burning of the candle that
changes the color of the lime-water. That is a gas, too, and you can
collect it, and examine it. It is to be got from several things,
and is a part of all chalk, marble, and the shells of eggs or of
shell-fish. The easiest way to make it is by pouring muriatic or
sulphuric acid on chalk or marble. The marble or chalk begins to hiss
or bubble, and you can collect the bubbles in the same way that you
can oxygen. The gas made by the candle in burning, and which also is
got out of the chalk and marble, is called carbonic acid. It puts out
a light in a moment; it kills any animal that breathes it, and it is
really poisonous to breathe, because it destroys life even when mixed
with a pretty large quantity of common air. The bubbles made by beer
when it ferments, are carbonic acid, so is the air that fizzes out of
soda-water, and it is good to swallow though it is deadly to breathe.
It is got from chalk by burning the chalk as well as by putting acid
to it, and burning the carbonic acid out of chalk makes the chalk
lime. This is why people are killed sometimes by getting in the way of
the wind that blows from lime-kilns."

"Of which it is advisable carefully to keep to the windward." Mr.
Wilkinson observed.

"The most curious thing about carbonic acid gas," proceeded Harry, "is
its weight. Although it is only a sort of air, it is so heavy that
you can pour it from one vessel into another. You may dip a cup of it
and pour it down upon a candle, and it will put the candle out, which
would astonish an ignorant person; because carbonic acid gas is as
invisible as the air, and the candle seems to be put out by nothing. A
soap-bubble or common air floats on it like wood on water. Its weight
is what makes it collect in brewers' vats; and also in wells, where
it is produced naturally; and owing to its collecting in such 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. Als