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ESTERDAY we found limestone to be rich in carbonic
acid, and we also found that, to release this gas from
the stone and obtain it by itself, all we need to do is
to apply to the stone another and a stronger acid,
preferably hydrochloric acid, a very cheap liquid and
one that has the additional advantage of always keeping
the surface of the stone clean. Our plan for to-day is
to extract from limestone this gas of burnt carbon.
The apparatus required will be the same as for the
preparing of hydrogen—that is to say, a two-necked
bottle of jar, if our laboratory can furnish this handy
utensil; or else, if our resources are more modest, a
bottle with a wide neck to hold a large stopper having
two holes running through it from top to bottom. Into
one of these holes will be fitted a straight glass tube
reaching to the lower part of the bottle, and through
it, by means of a small glass funnel or, lacking that,
a paper cone, the hydrochloric acid will be poured
little by little so that the effervescence
may not be rapid enough to make the foam rise too high
and run over. Through the second opening will pass a
bent tube to serve as conductor of the released gas.
 "Here is what we are after,—a plain bottle with a
large cork stopper having two holes through it. Into
this bottle I put a handful of broken limestone of the
hardest sort. If I had a piece of marble,—a
fragment from some old bureau-top, for
example,—things would go better; but, not having
any, I do the best I can with common limestone, its
only defect being a tendency to soil the liquid
somewhat with its impurities. I add water and adjust
the stopper with its two tubes. Of course the straight
tube, not the bent one, is pushed down into the liquid.
Now I pour in a little hydrochloric acid, and
immediately we see a commotion, a kind of boiling,
caused by the setting free of gas from the stone. From
this point things will go of themselves, and we need
only let the apparatus alone, confining our activities
to the occasional addition of a little acid to keep the
process from halting."
"Quick, quick, the bowl of water for collecting the
gas!" cried Emile, seeing his uncle abandon the
apparatus to itself in an apparently very careless
"The bowl of water is unnecessary here," his uncle
assured him; "we shall get our carbonic-acid gas quite
well without having to use that cumbrous thing."
"But the gas is just going to waste."
"We can afford to waste a little, so easy is it to make
more and so inexpensive the process. What does it take
to produce all we wish of this gas? Only a pennyworth
of acid and a stone picked up by the roadside.
Besides, there is a reason for
 my inattention to waste: there is air in the bottle and
I am allowing the carbonic acid to drive it out.
"Now it is done, or very nearly, and there is no more
air left in the apparatus—except, perhaps, a mere
trace. Accordingly, I introduce into a wide-mouthed
bottle the tube conducting the gas, and make sure it
reaches the bottom. In a few moments the bottle will
be full of carbonic acid."
"But it will get out of the bottle, for there's no
cork," objected Jules; "or at least it will mix with
the air in the bottle."
"You need fear nothing of the sort," replied his uncle.
"Carbonic acid is heavier than air. As fast as it
arrives, conducted by the tube that reaches almost to
the bottom of the receiving bottle, it accumulates and
forms an increasingly thick layer, which expels the
air, a lighter gas. This air comes out in an invisible
stream through the mouth of the bottle. If we had a
vessel full of oil and slowly poured in a stream of
water at the bottom, what would happen? The water,
being heavier than the
 oil, would accumulate in the bottom of the vessel, rise
gradually, and drive out the oil, the lighter liquid.
A similar process goes on when carbonic acid gas is
introduced at the bottom of an enclosed body of air."
"I understand," said Emile, "but I should like to ask
one question. With oil, I should know by the color
when it was all driven out and its place taken by
water; but here there's nothing to be seen, neither
carbonic acid gas nor air. How can we know, then, when
all the air is driven out and the bottle filled with
"What our eyes cannot tell us, a flame will help us to
see with our understanding. Carbonic-acid is a foe to
combustion; it will not keep the smallest flame alive.
I will light a piece of paper and thrust it into the
mouth of the bottle. If it continues to burn, there is
still a layer of air in the upper part of the bottle;
but if it goes out, there is nothing in the bottle but
carbonic-acid gas. Let us try it; now is the time.
Hardly is the lighted paper inside the neck of the
bottle when it is extinguished,—a sure proof that
carbonic-acid gas reaches up to the mouth of the
bottle. We have now a supply of this gas for further
experiment. I put the apparatus aside, as we have no
further use for it at present. When we need it again,
all we shall have to do will be to pour in more
hydrochloric acid and let it act on the limestone.
"Here, then, is our carbonic-acid gas. It is as
colorless, as transparent, as invisible as air. We
have just extracted it from limestone, where
chemi-  cal combination held great quantities of it captive
with a very narrow compass. A piece of stone hardly
bigger than a walnut will yield several liters of it.
We have just driven some of it our of the rock, and are
now going to drive it back and make it reenter the
composition of rock,—that is to say limestone,
powdered chalk. I pour some lime-water into the bottle
that is filled with carbonic-acid gas, close it tightly
with the palm of my hand, and shake it thoroughly. The
liquid turns white and thick like sour milk. We let it
stand a while, and flakes settle at the bottom in a
considerable layer. You know these white flakes as
carbonate of lime, chalk, the compound we obtained when
we shook up lime-water and carbonic acid obtained by
burning charcoal. So here we have fresh proof, to add
to the others, that limestone really contains the gas
produced by burning charcoal.
"The gas has disappeared, being shut up once more in
the stone,—or, rather, in a sort of mud that would
become stone if it were dried and pressed. Again I
bring out our apparatus and renew the supply of
carbonic-acid gas in the bottle. How do you suppose a
lighted candle would behave in such an atmosphere?"
"It would go out," was Emile's reply, "just as the
lighted paper did."
"Besides," added Jules, "nothing can burn except in
oxygen or in the air."
The candle did indeed go out, the very instant the
flame got as far as the neck of the bottle. The
results attained with nitrogen had not been quicker
 or more complete. Not only was the candle immediately
extinguished, but there was not the slightest glow left
for an instant in the wick.
"Without making any cruel experiments," Uncle Paul went
on, "we can feel sure that this gas, so manifestly
unfit for combustion, is equally unfit for maintaining
life. An animal would die in it, and very quickly,
just as you saw the sparrow die in an atmosphere of
nitrogen. Let us now proceed to prove that
carbonic-acid gas is heavier than air. I have turned
this comparative heaviness to account in collecting the
gas without the help of our bowl of water, so that
proof is in reality already before us. Nevertheless I
will give you a still more striking demonstration.
"We will take two bottles holding equal amounts and
having mouths of equal size. This one, at my right, is
full of carbonic-acid gas. I lower a lighted candle
into it, and the candle is immediately extinguished.
This other bottle, at my left, is full of atmospheric
air. I lower a lighted candle into it, and the flame
continues to burn. Now, removing the candle, I take
the right-hand bottle and gradually invert it, at the
same time fitting its mouth to that of the other
bottle. In fact, I do exactly as I should do if I were
pouring water from the one into the other. I decant
the carbonic-acid gas as if it were a liquid. Nothing
is seen to pass from the upper to the lower bottle, nor
is anything seen to rise from the lower to the upper;
and yet the exchange is made, as we shall soon prove.
The carbonic acid, the heavier gas, descends and fills
 lower bottle, while the atmospheric air the lighter
gas, rises and fills the upper bottle. After waiting a
few minutes for the exchange to become complete, I put
the two bottles back in their places and again try the
lighted candle. It burns in the right-hand bottle;
hence this one no longer holds carbonic acid, but air.
It goes out in the left-hand bottle, proof that the
latter has exchanged its original contents of
atmospheric air for the gas that will not maintain
combustion. Thus it is made clear that the bottled
carbonic acid has gone down and the bottled air has
gone up, the two changing places without getting mixed
in the process.
"Now listen to this. In various places there is a
constant escape of carbonic-acid gas from the ground,
especially in the neighborhood of volcanoes. There are
springs of carbonic-acid gas as well as springs of
water. The most celebrated spring of this sort is that
at Pozzuoli, near Naples. It is known as the Dog's
Grotto, from the distressing part a dog is made to play
for the amusement of the curious. The grotto is
hollowed out of the solid rock, and the air in this
cavity is earthy, damp, and warm. Bubbles of gas rise
here and there in the mud.
"The keeper of this grotto—for there is a keeper
who, for money, shows people the repulsive spectacle I
am going to tell you about—takes his dog, ties the
animal's legs together to prevent its running away, and
lays it on the ground in the middle of the grotto,
where he himself remains. There is nothing to arouse
the slightest suspicion of danger,
 no foul odor, no lack of clearness in the atmosphere.
Besides, is not the dog's master there, showing no sign
of fear, standing in the very middle of the cave?
Nevertheless there is the dog giving tokens of distress
by its groans; it writhes in frightful convulsions, its
eyes become dim, its head falls heavily, and it appears
to be at death's door. But at this point its master
carries it out of the grotto, unties its legs, and lets
it breathe the pure air. Little by little the animal
revives; it struggles to its feet; still giddy, it
looks about in a dull, stupid manner, and then runs off
as fast as its legs will carry it, evidently fearing a
"Does the dog act a part taught it by its master? Has
it been trained to play dead in the grotto, where its
master stands upright beside it without suffering the
least inconvenience? No, the dog really comes near
dying, as it knows well enough from passing its
wretched if in repeating the act several times a day.
It knows it so well that it submits to the experiment
with a very bad grace. The minute it sees a stranger
approaching in the distance it turns morose and surly,
growls and threatens to bite. Its master has to hold
it in leash when taking it to the grotto, dragging the
animal along while the poor beast shows its reluctance
by the drooping of its ears and tail. But when the
ordeal is over and the stranger gone, it is full of a
foolish joy that shows itself in unmistakable fashion.
The wretched animal is completely happy at being
allowed to come to life again.
"There is nothing about the famous grotto that
 cannot be easily explained. Carbonic-acid gas, as I
said, comes up out of the ground. This gas is not fit
for breathing; after a few breaths an animal dies.
Furthermore, it is heavier than air; and so, instead of
dispersing evenly throughout the grotto, it remains
close to the ground, where it forms a layer about half
a meter thick. It reaches only to the knees of the man
as he stands in the middle of the grotto, whereas the
dog, stretched on the ground, is entirely immersed in
it. The master is not compelled to breathe the harmful
gas, and so feels no inconvenience from it; the dog
breathes nothing else, and in consequence almost dies.
But were the dog's master to lie on the ground as does
the dog itself, he would share the poor animal's fate.
"The heavier gas is constantly added to and as
constantly escapes through the mouth of the grotto,
running out in a sort of stream when the air is calm.
No one sees this stream, and one passes through it
without suspecting it to be there. It makes no
mumurous sound and flows over no bed of pebbles, but
runs gently and quietly over the grass. Its presence
may, however, be detected with the help of a lighted
candle: beyond the limits of this invisible stream of
gas the candle burns, but as soon as its flame is
immersed in the current it goes out as if it were
plunged into water. In this way the stream of gas can
be traced for some distance from the grotto, while
beyond that it is dissipated by air-currents."
"If it weren't so far away," said Jules, when his uncle
had finished his story, "I should like to
 go and see that wonderful grotto; but I shouldn't ask
to have the dog suffer that terrible torture. I should
be satisfied with a test made with a lighted candle,
first high up in the grotto and then down near the
ground, to see whether it would go out when it was
"If such a test is all you wish for," replied his
uncle, "a trip to Pozzuoli is quite unnecessary, for we
can reproduce right here the essential conditions found
in the Dog's Grotto. A glass jar shall be our grotto,
and we will substitute for the carbonic acid rising
from the ground the gas our apparatus yields when
supplied with limestone and hydrochloric acid. Here is
just the jar we need, large enough and with a wide
mouth. I insert the long arm of our bent tube and let
it reach to the bottom of the jar. Carbonic-acid gas
flows through the tube, accumulates at the bottom of
the jar, and is retained there by its weight, forming a
layer of increasing depth and displacing an equal
volume of air. Nothing tells us how deep this layer is
at any moment, for the two gases, carbonic acid and
air, are both invisible. Nevertheless, from the
activity of the effervescence in our apparatus we may
give a shrewd guess as to when the jar is about half
full of carbonic-acid gas. Then I stop the flow of gas
by disconnecting the jar from the apparatus.
"If I am not much mistaken, now is the time to make
this disconnection. Having done this, we now see
before us an artificial Dog's Grotto,—that is to
say, a glass jar filled with carbonic-acid gas below
and atmospheric air above. Look through it. It
 appears to be all alike, the lower layer of
carbonic-acid gas being as colorless, as invisible, as
the upper layer of air. Our eyes cannot tell us just
where the fatal layer ends and the breathable one
begins. Though there is a sharp division between the
two, no indication of it is to be detected by the eye.
"Slowly I lower a lighted candle into the jar to test
its contents. At first it burns very well, and on
being lowered farther and farther it still burns; but
at last I reach a point where it begins to turn dim.
This is the dividing line between the two layers, and
if I lower the candle only a little more it immediately
goes out, being fully immersed in the carbonic-acid
gas. Here, then, we have what Jules was wishing to
see,—a reproduction of the Dog's Grotto and the
behavior of a lighted candle there. According to the
candle's position, high or low, it burns or goes out.
"Now imagine we have in the jar two very
different-sized animals, the smaller one completely
immersed in the bottom layer. The former will perish
in a short time, as it breathes a gas incapable of
maintaining life, whereas the latter will suffer no
discomfort whatever, having a plenty of pure air to
breathe. That is the relative position of the dog and
the man in the grotto with its lower layer of carbonic
acid gas and its upper one of pure air."