oxygen into itself, and the mixture of the
nitrous gas and oxygen, if you put water with
it, goes into the water. Mix nitrous gas and
air together in a jar over water, and the
nitrous gas takes away the oxygen, and then
the water sucks up the mixed oxygen and
nitrous gas, and that part of the air which
weakens the oxygen is left behind. Burning
phosphorus in confined air will also take all
the oxygen from it, and there are other ways
of doing the same thing. The portion of
the air left behind is called nitrogen. You
wouldn't know it from common air by the
look; it has no colour, taste, nor smell, and it
won't burn. But things won't burn in it,
either; and anything on fire put into it goes
out directly. It isn't fit to breathe,—and a
mouse, or any animal, shut up in it, dies. It
isn't poisonous, though; creatures only die in
it for want of oxygen. We breathe it with
oxygen, and then it does no harm, but good;
for if we breathed pure oxygen, we should
breathe away so violently, that we should soon
breathe our life out. In the same way, if the
air were nothing but oxygen, a candle would
not last above a minute."

"What a tallow-chandler's bill we should
have!" remarked Mrs. Wilkinson.

"'If a house were on fire in oxygen,' as
Professor Faraday said, 'every iron bar, or
rafter, or pillar, every nail and iron tool, and
the fire-place itself; all the zinc and copper
roofs, and leaden coverings, and gutters, and
pipes, would consume and burn, increasing the
combustion.'"

"That would be, indeed, burning 'like a
house on fire,'" observed Mr. Bagges.

"'Think,'" said Harry, continuing his
quotation, "'of the Houses of Parliament, or a
steam-engine manufactory. Think of an iron-
proof chest—no proof against oxygen. Think
of a locomotive and its train,—every engine,
every carriage, and even every rail would be set
on fire and burnt up.' So now, uncle, I think
you see what the use of nitrogen is, and
especially how it prevents a candle from
burning out too fast."

"Eh?" said Mr. Bagges. "Well, I will say
I do think we are under considerable obligations
to nitrogen."

"I have explained to you, uncle," pursued
Harry, "how a candle, in burning, turns into
water. But it turns into 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 colour
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 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 stupifies 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