|The Wonder Book of Chemistry|
|by Jean Henri Fabre|
|Starting with a mixture of iron filings and sulphur, Uncle Paul awakens in his young nephews an eagerness to learn more about the properties of the elements. Through a series of carefully-devised experiments and conversations about the experiments, he leads the boys to an understanding of some of the basic principles of chemistry. Excellent as a follow-on to 'The Story Book of Science' and 'The Secret of Everyday Things' by the same author. Ages 11-15 |
EXPERIMENTS WITH AIR
NCLE PAUL took a rather deep dish and in the center he
fixed a candle with a few drops of melted wax. Then he
lighted the candle and covered it with a large,
wide-mouthed bottle of clear glass. After this he
poured water into the dish until it was quite full.
Meanwhile the children looked on, chattering between
themselves and wondering what their uncle was up to
with his lighted candle all surrounded with water and
burning in a bottle upside down in the middle of the
dish. What curious experiment was he about to perform?
They were not left long in doubt. Everything being
ready, Uncle Paul began thus:
"What is there in the bottle?"
"A lighted candle," Emile hastened to answer.
"Is there nothing else there?"
"No, nothing. I don't see anything but the candle."
"You forget that there are some things we cannot
 see. You must here the eyes of the mind, not those of
Emile scratched behind his ear with the tip of his
finger and winked hard, as was his wont when puzzled.
He was trying to think what the invisible thing could
be that his uncle referred to. Jules came to his aid.
"It is air," declared the older boy. "There is air in
the bottle where the candle is burning."
"But Uncle Paul didn't put any there," Emile rejoined.
"It is necessary to put any there, little giddypate?"
his uncle demanded. "Is not the bottle full of it
without any help from us? All the vessels we use, all
our flasks, jars, bottles, glasses, containers of
whatever kind, being immersed in the atmosphere, in the
very depths of this ocean air, are filled with this gas
as they would be with water if they were plunged
uncorked into that liquid. If it does not contain
anything else, every bottle, whether right side up or
wrong side up, is full of air, which gets in of
itself, usually without our paying any heed to it.
When a bottle of wine is drained to the last drop, we
say it is empty. Should we really call it empty if we
chose our words with extreme care? Certainly not, for
the so-called empty bottle is as full as ever, full to
the top; it is filled with the air that has taken the
wine's place. And so it is with all vessels when we
empty them of their contents: if they were full
before, they remain full afterward, though the nature
of their contents has changed. Nothing is
abso-  lutely empty, nothing can be empty, when air is free to
enter it. We can, it is true empty a vessel in the
literal sense; we can create a void or vacuum, as the
learned men say; but that is an elaborate operation,
and its success demands proper appliances."
"You mean an air-pump?" asked Jules.
"Yes, my boy, an air-pump, a special kind of pump that
sucks the air from a tightly closed vessel and
discharges it into the outer atmosphere. But as
nothing of the sort has been used here, my bottle is
full of air exactly like the air around us.
Consequently the candle is burning in the midst of the
air contained in the bottle. Now, why did I fill the
dish with water? For this reason: The air in the
bottle is the substance I propose to study with you by
subjecting it to certain experiments, particularly
this experiment with the lighted candle. And so we
must separate this air from the rest of the atmosphere,
isolating it in a closed container, as otherwise our
experiment could not be carried through; and,
furthermore, we should be unable to tell what portion
of the atmosphere we had been experimenting with. The
bottle alone does not furnish complete isolation, for,
between its mouth and the bottom of the dish, air could
easily slip in and mix with that already in the bottle.
A barrier must be interposed to separate the inner from
the outer air, and this barrier is furnished by the bed
of water in the dish. Thus we obtain perfect
isolation; and you will also see that the water serves
a second purpose, being an indicator of what takes
place in the bottle.
 But I must not distract your attention by too much
explanation. Now watch closely what is going on in the
Behold, in a few minutes the candle-flame, which at
first was full and bright like that of a candle burning
in the open air instead of in a bottle, began to get
dimmer little by little, becoming shorter, shrinking in
width, and looking dull and smoky. Soon it had
diminished to a mere point, and finally to nothing at
all. The flame was quite extinguished.
"Look!" cried Emile. "The candle has gone out without
any one's blowing it out."
"Wait a moment, Emile, and we will presently talk the
thing over. Just now keep your eyes open and watch
what is happening in the water, the indicator I spoke
Emile and Jules watched attentively and saw the water
gradually rise in the neck of the bottle, completely
fill it, and go still farter, so that an appreciable
part of the bottle , at first filled with air, was
occupied by the liquid coming form the dish and
slipping in form beneath. Its ascent was slow, and
when at last it was finished Uncle Paul broke the
"Now," said he, "you may ask all the questions you
"I should like to have one thing explained," said
Emile. "When a candle is burning and you want to put
it out, you have to blow on it. But here no one blew
out the candle, and if we had wanted to we couldn't
have done it on account of the bottle
 over it. Not a breath of air came, that is certain; no
puff of wind. Under cover of the bottle no breath of
air or puff of wind could get at it. The flame was
perfectly still; it stood up straight and calm; and
yet, without any reason that can see, it got dim and
dwindled to nothing but a point, and then at last went
"I should like to ask something, too," put in the older
boy. "The bottle was at first filled full of air; now,
besides the air that is left, it has several fingers of
water from the dish. I saw this water rise little by
little as the flame dwindled. So something has been
taken from what was in the bottle at first, for the
water rose and took its place. But how did that
something disappear, and where has it gone to? If you
had not told us and made us understand that nothing is
ever annihilated, I should say that a part of the air
was annihilated while the candle burned."
"Jules's point shall have our attention first, as it
will furnish us the explanation of Emile's difficulty.
A part of the bottle's gascous contents has
disappeared, apparently; the water's rising above its
level in the dish to fill the vacancy would seem to
prove it in a way to convince the most incredulous.
Something, I repeat, has disappeared, so far as we can
see; but we must not for a moment suppose it has been
annihilated. Let us investigate further, and we shall
discover what has become of this something that seems
to be lacking.
"I have already told you that heat and light are
 nearly always signs of the mingling of substances of
different natures,—of chemical combination, in short."
"I remember," said Jules. "You called it the fireworks
that celebrate chemical weddings. Can such weddings
take place in a bottle?"
"Yes. The flame was hot and it gave out light; hence
a chemical combination was going on, producing this
heat and light; hence a chemical combination was going
on, producing this heat and light. And what substances
were they that were thus combining? One of them,
beyond a doubt, was supplied by the fatty material of
the candle as it melted under the heat from the lighted
wick; the other could have come only from the air,
since the bottle contained nothing else. From this
combination sprang something new, something that was no
longer either candle grease or air, something with
properties that neither air nor candle possesses. The
compound thus formed is an invisible substance, a gas
just as air is a gas, and for that reason invisible."
"But if a gas is made out of air and candle grease,"
Jules objected, "this gas takes the place of the air
that disappeared, and the bottle ought to be just as
full as it was before. I don't see why the water
"Wait; we are coming to that. The compound we are
speaking of is readily soluble in water, somewhat as
sugar and salt easily dissolve in that liquid. Once
dissolved in this way, sugar and salt disappear, become
invisible, and the only evidence we have of their
presence is the sweet or the salt
 taste of the water. In like manner, what is produced
by the flame disappears: it enters the water and
becomes incorporated with it. Similar but much richer
gaseous solutions are familiar to you. I hardly need
to remind you of beer, cider, sparkling wine, and
soda-water, drinks that make the cork pop and that foam
when poured into the glass. All these liquids hold
quantities of gas in solution,—so much of it, in fact,
that they cannot retain it when escape is possible; and
therefore it thrusts out the cork and covers the drink
with foam. Now, curiously enough, the gas that makes
these beverages foam is precisely the gas that the
candle-flame manufactures. Some day I will take this
interesting subject up again. I mention it here in
passing, but have not time to dwell on it.
"Since the compound produced by the candle grease and
the air disappears from sight, being dissolved in the
water, a vacant space must be left; and this it is that
the water in the dish, under pressure of the
atmosphere, rises in the bottle to occupy, showing us
by the height to which it rises the amount of air that
"It hasn't risen far," said Emile. "See! It's only
just above the neck."
"That shows that the candle burned up only a small part
of the air in the bottle,—a tenth part, let us say, if
the water that has made its way in fills a tenth part
of the bottle's total capacity."
"But as there is still so much air in the bottle, why
didn't the candle use that up just as it did the
 other part? I don't see that it is any different form
what that was. It is still transparent, invisible, and
hasn't a particle of smoke."
"Here is where we come to your question, my boy, the
question why the candle went out without being blown
out. The candle-flame is caused by the combination of
the material of the candle with certain other material
contained in the air. Air and candle are equally
necessary in feeding the flame. If either be lacking,
the flame is extinguished. As to the candle's being
necessary, that is plain enough: no fuel, no flame.
But in regard to the air you don't feel quite so sure.
And yet what you have just witnessed ought to give you
food for reflection. If the candle went out of its own
accord, there was certainly something wanting."
"I'm ready enough to admit that. Yes, that must have
been the trouble; something was wanting, as no one blew
out the candle and there wasn't a breath of wind. Now,
what was it?"
"What was lacking must have been air, as the bottle
contained nothing else to begin with. Air is
indispensable for keeping a flame burning."
"But there is still air in the bottle,—lots of it,
almost the whole bottleful."
"I do not deny it; but listen a moment. It is not
possible that air, instead of being made of only one
substance, is made of two of different kinds, equally
colorless, equally subtle and invisible, and very
thoroughly mixed together? Is it not possible, too,
that one of these gaseous substances can keep a
 flame going, whereas the other cannot, and that the
first-named forms but a small part of the atmosphere as
compared with the second? If so, when this first part
is used up in the bottle, the candle goes out of
itself, as we say, the candle grease no longer finding
what it needs for burning, and there will be left in
the bottle, colorless and invisible, as it was in the
first place, the other gaseous element, of no use for
keeping a flame alive."
"How clear that makes everything, Uncle," said Jules.
"Now I see it all perfectly. The candle went out of
itself when there was no more of this gas material to
keep it burning. By combining with a little burnt
grease, this matter turned into something else, a gas
that is dissolving disappeared in the water while the
water in the dish rose and took its place. Now the
bottle holds only the kind of gas of no use to a flame,
and that is why the candle stopped burning."
"Yes, that's the way of it, with one slight correction.
A candle-flame is not by any means a fire strong enough
to use up all the gaseous element needed for the act of
burning; there is still some of it left over, but too
little to keep the candle alight. The air has become
impoverished, but not deprived of all it contained of
the element in question. Another day we will try to
find a way to remove the last trace of this
flame-supporting gas. At present let us rest content
with our partial result. The contents of the bottle
will no longer keep a candle burning, and a lighted
candle put into the bottle would immediately be
 "A candle would go out if put into that bottle?" asked
Emile, still rather inclined to doubt.
"Certainly it would, and almost as quickly as if you
plunged it into water. How can you expect it to burn?
If the one that was there could not burn, why should
another do any better? They are all made alike."
"All the same, I'd like to see it tried."
"Your curiosity shall be gratified."
With this Uncle Paul took a short candle-end and tied
it to a wire bent up at the bottom. Then, raising the
bottle a little with one hand, he slipped the palm of
the other over the mouth, which was still under water,
thus stopping it up. After this he set the bottle
upright on the table without letting it lose any of its
contents, liquid or gaseous, and withdrew the hand he
had used as a stopper.
"But the air in the bottle will get out," objected
Emile, "if you leave the bottle open."
"There is no danger of that," his uncle reassured him.
"The invisible gas there, being as heavy as air will
not escape. However, to make sure of it, here is a
stopper we can use."
It was a little piece of glass from a broken
window-pane. Uncle Paul placed it over the bottle's
"Now," said he, "let us proceed to our experiment."
The candle fastened the wire was lighted and allowed to
burn until the flame was bright and full, after which
Uncle Paul removed the piece of glass
 and gently let down into the bottle the lighted candle,
which almost immediately became dim and went out. A
second attempt met with the same speedy result. If
violently blown upon or plunged into water, the candle
could not have gone out more promptly.
"Well, are you convinced now, Master Unbeliever? Here,
try it for yourself, so as to be quite satisfied."
Emile took the candle and began the experiment over
again, lowering the flame little by little, very
gently, very cautiously, so as to keep it away from the
sides of the bottle, thinking that careful management
might accustom the flame to this unsuitable atmosphere.
But it was of no use. Though several times repeated,
the attempt invariably failed: a little higher or a
little lower in the bottle, the flame always went out.
"It's no good, the candle won't burn there," declared
the boy, tired of attempting the impossible. "I should
be perfectly sure of it if I were certain the bottle
had nothing to do with it. Couldn't the nearness of
the glass and the want of space make the candle go
"That is a very natural question, but it is soon
answered. Here is a bottle like the first, of the same
size and with the same wide neck. It is filled with
air that has not been impoverished by anything
 burning in it, the same kind of air as that all around
us. Repeat your experiment with that."
Emile lowered the candle into the bottle, and it burned
very well, exactly as if in the open air. Whether put
in abruptly or gently, near to or away from the
enclosing glass, it remained alight and burned as it
had outside. His constant failure with the first
bottle and his repeated success with the second
dispelled Emile's last lingering doubt.
"I have nothing more to say," he declared. "The stuff
in the bottle where the first candle burned doesn't
suit it now at all."
"You are convinced, then?"
"Then I will continue. What we conclude from our
experiment is that air is composed of two different
gases, equally invisible but of so unlike a nature,
each to the other, as to prevent our confusing them.
One, the less abundant, suits the candle-flame and
combustion in general; the other, more plentiful, does
not. The first is called oxygen, the second nitrogen.
They are two simple substances, two metalloids. As to
air, which is a mixture of these two gases, we can no
longer properly call it an element as did the ancients.
It is a compound of two substances of very different
natures. This has been known only a comparatively
"I wonder why it took so long to find out about air,"
said Jules. "It would have been such a simple
 thing to burn a candle in a bottle turned upside down
in a dish of water."
"Very simple, undoubtedly; but it had to be thought of,
and there was the difficulty."
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