|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 |
REPARATIONS had been made for some new experiments, which pupils
always like. On the table stood the tin box containing
the bottle with the phosphorous; also there was the
famous bell-glass resting on a plate, in the middle of
which was a saucer full of lime.
"What is Uncle going to show us with all these
fixings?" queried the boys.
"The air we breathe," he began, "is still very
imperfectly known to you. Of the two elements
composing it, only one, nitrogen, has been shown you;
the other, oxygen, less abundant but much more
important, is hardly known to you except by name. You
recall what the experiment with burning phosphorous
showed us,—that oxygen forms the fifth part of
our atmosphere,—and you also know, rather from my
telling you than from the evidence of actual facts,
that is the gas needed when anything is to be burned.
Without oxygen the flame goes out, and without it the
life of an animal also comes to an end. But what is
this gas? What will it do alone, by itself, and not
mixed with nitrogen as it is in the atmosphere? That,
my little friends,
 is the important question. I am going to try to
answer it for you.
"In five liters of atmospheric air there are four
liters of nitrogen and one of oxygen; so that is the
source we must go to when we wish to obtain in a pure
state either one of these two elements. Now, in the
atmosphere the two gases are not chemically combined,
but simply mixed, as I shall have occasion to prove to
you later. As they are only mixed, a simple separation
of the two is all that is needed, though even this is a
difficult matter; for how can we separate two
substances that cannot be handled or even seen? A
little while ago when we mixed powdered sulphur and
iron filings, Emile thought it not impossible to
separate the two, grain by grain, at a great cost of
time and patience. And he was right; the task is not
too much for nimble fingers and sharp eyes. With this
mixture called air, however, it is a very different
thing. The two substances forming the mixture can be
neither seen nor felt, and if they could be seen it
would still be hardly any easier to separate them, so
subtle is their nature. What, then, are we to do?"
"It was easy enough o separate iron and sulphur," said
Jules, after a moment's reflection, "by using a magnet,
even though the two substances were both of them
powdered very fine. Couldn't we use some means for
sorting the two gases that air is made of?"
"Yes," chimed in Emile, "I'd like to find something
that we could hold in the air and make it atract one of
the gases and leave the other behind,
 just as the magnet attracted the iron filings and left
"Do you know, my lads, that what you say shows more
understanding of the matter than I had expected?"
rejoined Uncle Paul. "Your answers delight me,
anticipating as they do what I was about to propose as
the only practicable means to be employed. What Emile
says he would like to find is already known to you; you
have seen it in operation, and no longer ago than day
"Phosphorous?" queried the boys.
"Yes, phosphorous. When it was burned under the
bell-glass, did it not take to itself the oxygen and
leave its companion, nitrogen, in the glass?"
"Yes, that's just what it did."
"Didn't it behave very much like the magnet you thrust
into the mixture of iron filings and sulphur so that it
drew the iron filings to itself and left the sulphur on
"To be sure it did!"
"The magnet attracts iron, but has no effect on
sulphur, which is thus by itself. In the same way
burning phosphorous attracts and retains the oxygen in
the air, but leaves the nitrogen, for which it has no
"Now we have it, I think," said Jules. "The magnet,
covered with iron filings, was drawn out of the
mixture, and then we rubbed off the filings on to
another piece of paper away from the sulphur. Let's
make the phosphorous take all the oxygen it wants, and
then we'll take it away again."
"A capital suggestion," applauded Uncle Paul;
 "but, unfortunately, it won't quite work. The magnet
readily gives up its load of filings, but not so with
phosphorus and its load of oxygen. I have told you of
its voracious appetite. Once having got its fill
oxygen, it is impossible to make it disgorge except by
forcible means not at our command in our humble
laboratory. What it gets it keeps a good hold of, so
that with our modest resources we should never succeed
in making it let go."
"Let it keep its oxygen, then!" cried Jules in vexation
at seeing his project fail just as he thought it about
to succeed. "I'll try another way. Isn't there
something that will work just the opposite of
phosphorous,—something that will take the
nitrogen from the air and leave the oxygen alone by
itself? That would be much simpler."
"No doubt that would be much simpler, but—"
"Is there a but?"
"Alas, yes, and a most serious one! You must know that
nitrogen is a most unsociable element, decidedly
hostile to the notion of alliances. No element will
have anything to do with it, as a rule, and it has no
use for a partner of any sort. Chemical combination it
abhors, and only when coaxed by the most skilful and
delicate devices will it consent to any such union.
Let us not, then, for a moment think of withdrawing
nitrogen from the air by combining it with another
substance; all attempts in that direction would be sure
to result in failure.
"Must we, then, give up in despair? Not at all. The
first method is excellent if we only use it with
discretion. Phosphorous, it is true, keeps a most
 obstinate hold on the gas it has united with in the act
of burning, and it is useless to expect it to let go of
the oxygen it has taken from the air. But,
fortunately, not all simple substances are like it. We
shall find some more accommodating, willing surrender
their plunder without too much coaxing. For to-day we
will content ourselves with learning how this gas is
accumulated and, as we might say, stored up in a burnt
substance; and for the purposes of this demonstration
phosphorous will serve.
"You have not forgotten how smoke formed in the
bell-glass when phosphorous was burned there, in our
experiment of day before yesterday. That thick cloud
of milk-white appearance made too deep an impression on
you to be soon forgotten. And you remember how, little
by little, it dispersed, being taken up the water in
the bowl. If I had not called your attention to this
point, perhaps that disappearance would have seemed to
you a real instance of annihilation, and you would have
retained the notion, so generally held, that fire
reduces to nothing the material that it burns. I
instructed you to the contrary, but that is not enough;
I wish to add to my mere assertion the more convincing
testimony of fact. Accordingly, I propose to show you
that fire does not annihilate, but only transforms;
that it changes the appearance and the properties of
the matter without affecting its existence.
Phosphorous will furnish us a fine example of this, and
at the same time give us some knowledge of the chief
topic of our to-day's lesson. The experiment I am
proposing will show us, on the other hand, the
indestructi-  bility of matter by fire, and, on the other, the
storing up of oxygen by combustion.
"The white fumes given out by burning phosphorous are
very easily dissolved in water, which accounts for
their prompt disappearance in our recent experiment.
To preserve them, to let them take on the state natural
to them when cold, and then to examine them at leisure,
it is absolutely necessary to examine them at leisure,
it is absolutely necessary to do the burning where
there is no water. Nor is even this precaution
sufficient, so great is the liking for water of this
compound formed by burning phosphorous. The atmosphere
is always moist, whence come rain and dew. However dry
it may seem to us, it is sure to contain more or less
of the invisible vapor of water, which the burned
phosphorous would greedily pounce upon, dissolving
itself therein just as sugar dissolves in water.
Consequently, we must have perfectly dry air in the
bell-glass where the burning is to take place.
"This dry air I obtain by means of
quicklime,—that is to say, lime before it is
slaked by the mason, or, in other words, lime just as
it comes from the lime-kiln where it has been prepared.
You don't need to be told what happens to a piece of
lime left in the air for some time."
"I know what you mean," said Jules. "The piece of lime
gradually cracks open, and then crumbles to dust, just
as it does when you sprinkle it with water; only then
it crumbles a good deal faster."
"That is it. Sprinkled with water, a piece of lime
cracks, splits, crumbles to dust. Exposed to the air
for some time, it acts in the same way, but
 more slowly. Why? Because it absorbs the moisture in
the surrounding air, until, little by little, this
moisture has had the same effect on it as a fine spray
of water would have had. Thus it is that lime has the
habit of attracting moisture, however little there may
be within its reach. It wrenches it by force, we might
say, from the surrounding atmosphere, and takes every
bit of it. Here we have, then, a very easy and
convenient way to obtain perfectly dry air.
"A few hours ago I took care to place a saucer of
quicklime in the middle of a large plate, and covered
it with the bell-glass, the latter resting on the plate
and being, of course, full of air. With this
precaution the fumes of the burning phosphorous cannot
escape, and not only the imprisoned air, but also the
surface of the plate under the glass and the inside of
the glass itself, will be rendered very dry. Now for
the burning of our phosphorous."
Uncle Paul cut off a piece of this substance under
water and dried it carefully with blotting-paper.
Then, placing it on a bit of broken crockery, he
withdrew the saucer full of lime and substituted the
phosphorous, which he set fire to, whereupon the
bell-glass with its contents of dry air was immediately
replaced on the plate. The burning did not at first
differ in any way from that already seen by the boys;
there was the same bright light, with the same eddies
of dense white smoke. But at this point there occurred
something new: on the inside of the cold glass the
smoke condensed and became a beautiful, white, flaky
substance that detached itself and
 fell here and there with all the appearance of falling
snow. Soon the plate was covered with a layer of this
strange snow-like substance that had come from the very
heart of the flames.
"Well, Emile," said his uncle, "what do you think of
"I think it is very wonderful. Who would ever have
expected fire to make a snow-storm? But I know well
enough it isn't real snow, though one might be fooled
by its looks. Those flakes, all so white and
beautiful, must come from the burning phosphorous, for
they couldn't come from anything else."
"Yes, that is quite clear. The substance we see
forming here has nothing of snow about it except its
appearance. In reality it is quite another thing, as
we shall soon see. But first let us make it snow a
little more. The fire is dying down; we will feed it."
Uncle Paul raised the bell-glass a little, and the
burning, which had begun to languish, started up again
with all its former vigor.
"Air was beginning to fail," said he; "the phosphorous
had nearly exhausted its supply of gas, and was about
to die down, but, by raising the bell-glass a little, I
have let in more air, and the fire revives. Let us
give it a little more air still, so that we may be sure
to have enough of this strange snow."
When, after three of four renewals of air, the layer of
snow on the plate was deemed thick enough, Uncle Paul
took a pair of pincers and drew out the piece of
crockery with the phosphorous on it and
 carried it into the garden, that the still unconsumed
material might there burn itself out with no
inconvenience to lungs or sense of smell.
"Now, my young friends," he resumed, "I invite you to
examine what there is in the plate. It is, as you see,
a white, flaky substance looking much like snow. That
is what phosphorous turns to when burned. Fire has not
destroyed it, but has changed it into something else,
and the change is so complete that if you did not know
where this false snow came from you could never guess
its nature. I repeat, fire does not destroy anything;
what it devours, what it consumes, is not reduced to
nothing, but changed into something else, which
sometimes vanishes from before our eyes as an invisible
gas, and at other times arrests even the least heedful
attention as a much grosser substance. What you see
here in the plate—this stuff that we can feel,
smell, and taste—is phosphorous consumed by fire,
phosphorous still in existence though it has been burnt
up. Thus is illustrated before your eyes the first
point I had in view in this experiment,—namely,
that nothing is ever annihilated even by the action of
"Suppose we had here a fine pair of scales for
weighing, exactly balanced as are scales used by
chemists, and capable of telling us with precision the
weight of a fly's wing. The weighing of even so
delicate operations of chemistry. With a pair of
scales of such sensitiveness, we could have ascertained
the weight of the piece of phosphorous in milligrams.
Nothing then would have stood in the way
 of our burning the whole of that piece under the
bell-glass by renewing the air as often as was
necessary; and at the end we could have taken a feather
and swept up the snowy deposit even to the last flake,
after which we could have weighed it on our scales.
Let us suppose these two weighings to have been carried
out, one before the phosphorous was burned and the
other after. Now, which will weigh the more, the
unburnt substance or the burnt?
"Misled by that false notion of fire as a destroyer,
the novice would answer at once that the burnt
substance would weigh less than the other, arguing that
if fire does not entirely destroy, it at leasts
destroys in part. Buy you, my boys, forewarned against
this error in our earlier talks, and having had your
eyes opened by a number of experiments, will not, I
believe, make this foolish answer."
"I should think not," was the confident reply of Jules.
"I should say the burnt phosphorous would be heavier
than the unburnt."
"Your reason, my lad. Let us make no assertions
without proof to support them."
"The reason is plain enough," said Jules. "You told us
and proved to us that when anything is burned it
combines with something in the air called oxygen.
Although an invisible gas, this oxygen is matter, and
consequently has some weight, if only a very little.
So that burnt phosphorous, having had oxygen added to
it, ought to weigh more than phosphorous alone."
"Golden Mouth could not have said it better," applauded
Uncle Paul. "Yes, my young friend, the
 burnt phosphorous must weigh as much as it did before
combustion, plus the weight of the gas combining with
it in burning. A delicate pair of scales would testify
to this in the most convincing fashion: it would show
us that this heap of what looks like snow-flakes weighs
more than the phosphorous that went to the making of
it. How account for the added weight except by
ascribing it to the air that played its part in the
combustion? So, then, in the substance on this plate,
in the burnt phosphorous, there is a small amount of
oxygen taken from the atmosphere and securely retained.
This oxygen has ceased to be an invisible gas occupying
a great space, and has become part of a solid substance
that can be seen and handled, and that occupies a
comparatively small space. It is stored up as in a
reservoir, where chemical combination has collected and
compressed it into the least possible bulk.
"A similar chemical action attends the burning of any
substance whatever. By being consumed it becomes a
storehouse of oxygen. Taken in its sum total, and with
no omission of any part, the resulting matter after
combustion is heavier than the substance before
combustion; and this excess of weight is due to the gas
that took part in the combustion. Most of these burnt
substances, veritable storehouses of oxygen, hold on to
the latter with a tenacity that can be overcome if
necessary, but that nevertheless offers very great
resistance, while the lesser number surrender this
oxygen easily. After a brief survey of this latter
class we will select the substance best suited to our
purpose of obtaining
 oxygen in its pure form. But we will first finish our
examination of burnt phosphorous while we have a
specimen before our eyes.
"Although derived chiefly from phosphorous, which is
highly inflammable, the snow-like powder in the plate
is absolutely incombustible, the hottest fire having no
effect on it, for the reason that what is once burned
cannot be burned again. This phosphorous, being
already combined with as much oxygen as its nature
permits, cannot take on any more; that is its
combustibility is at an end. Experiment will prove
this better than mere words."
On a pan of glowing coals from the kitchen fire was
sprinkled a little of the white powder, and the coals
were then blown to a more intense heat; but the powder
showed no signs of taking fire, its inflammability
having been quite exhausted.
"If you had not already," Uncle Paul resumed, "some
knowledge of the difference between compound substances
and the simple substances composing them, this
experiment would open your eyes, for the substance that
at first burned so freely now refuses to burn at all.
Let us proceed. You can convince yourselves that the
white powder in the plate has no smell whatever,
whereas the phosphorous had a very strong smell of
garlic. But I would not have you touch this powder,
its properties being such as to render handling
harmful; still less do I wish you to taste it, as it
would make you cry out with pain."
"Is it so terrible as all that?" asked Emile.
"So terrible that a drop of molten lead on your tongue
would be less painful."
 "But that snowy stuff looks harmless enough."
"Don't trust to looks, my little friend. Innocent
looks may disguise a very dangerous substance.
Forewarned is forearmed. From the kitchen of the
chemist there very rarely comes anything pleasant to
the taste. However, it is well for you to have some
idea of how phosphorous tastes, and to make it less
disagreeable to the tongue I will dissolve it in
So saying, Uncle Paul took up the feather again and
swept the contents of the plate into the glass of
water. As each particle fell into the liquid it gave a
hissing sound, like that produced when a black-smith
plunges red-hot iron into water.
"It must be awfully hot, musn't it," asked Emile, "to
hiss like that in the water?"
"Heat is not the cause of the hissing. The powder is
no hotter than anything else here, —no hotter
than the plate it lies on. I have already told you
that burnt phosphorous has an extraordinary liking for
water; and you know what extreme precautions I had to
take, with the aid of quicklime, to keep it from the
moisture in the atmosphere. Now I let this powder
drink as much water as it pleases, and it dissolves
immediately and even with violence, that sharp noise
testifying to a satisfied thirst.
"Behold, now, the snowy powder all dissolved in the
water. The liquid has not changed its appearance; it
is still water as far as looks are concerned; but dip
the tip of your finger in and taste it. You can do so
without the least fear."
As the children hesitated, remembering the
allu-  sion to the drop of molten lead, their uncle dipped the
tip of his little finger into the liquid and touched it
to his tongue; whereupon, emboldened by his example,
Emile and Jules did likewise.
"Oh, how sour it is!" they cried, surprised at the
disagreeable taste and making a wry face. "It is
sourer than any vinegar they use for making salad.
What would it have been like if Uncle hadn't weakened
it with a lot of water?"
"Your tongues would have suffered terrible tortures, my
little friends. The part touched would have been eaten
away at once by this violent chemical, and you would
have heard a hissing as of red-hot iron in contact with
"Then this strong vinegar isn't real vinegar?"
"It is not vinegar at all, though it tastes much like
it. Now let us go on. Phosphorous has still another
property that we must test. Here are some violets just
gathered from the garden. I dip one into the
sour-tasting liquid, and it immediately loses its blue
color and turns red. All flowers of the same color as
the violet—the iris and the bluebell, for
example—would likewise turn red in the sour
liquid. You shall at your leisure repeat this fine
experiment with all the flowers you can find in the
garden, and you will always see the blue flowers turn
red. Burnt phosphorous, then, always has this sour
taste and this quality of turning blue flowers red.
"I will add that most of the other metalloids, such as
sulphur, carbon, nitrogen, and many more, when they
combine with oxygen,—or when they burn, as we
say,—produce compounds of like sourness and
 of like ability to change the color of blue flowers to
red. All these compounds are called acids, from their
sour or acid taste, and they are distinguished one from
another by the addition of a second term indicating
their origin. Thus the snowy powder resulting from the
burning of phosphorous bears the name of phosphoric
acid, and in future that is what we will call burnt
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