something else besides
that; there is a stream of hot air going up from it that won't condense
into dew; some of that is the nitrogen of the air which the candle has
taken all the oxygen from. But there is more in it than nitrogen. Hold a
long glass tube over a candle, so that the stream of hot air from it may
go up through the tube. Hold a jar over the end of the tube to collect
some of the stream of hot air. Put some lime-water, which looks quite
clear, into the jar; stop the jar, and shake it up. The lime-water,
which was quite clear before, turns milky. Then there is something 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
of 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 suc