HE use of red-hot iron for obtaining hydrogen from water
is a slow and tiresome process, requiring many
repetitions of the same operation to secure even a
small quantity of the gas. With live coals instead of
hot iron, speedier results are obtained, but the
hydrogen is not pure; it is mixed with other gases
derived from the coals, and to these is due the bluish
tinge of the flames, a peculiarity detected by Jules.
Excellent, for practical reasons, as are these two
simple and easy methods when the sole object is to show
that water contains an inflammable gas, they must give
place to others when it is desired to obtain a
considerable quantity of hydrogen in a short time.
"Let us turn now," said Uncle Paul, "from this way of
getting hydrogen from water,—I mean by the use of live
coals. What we really get is a mixture of various gases
that have to be examined separately if we are to avoid
confusion. And let us turn, too, from the method in
which red-hot iron is used; though this time the
hydrogen is pure, there is very little of it. What we
are looking for is some simple process that will give
us all the hydrogen we desire, and that without
furnace, forge, or
 brazier, which are not always
conveniently at hand. I have to tell you here that iron
can decompose water without first being heated if it
only has the help of a little sulphuric acid. With
these two acting together, the hydrogen in water can be
set free with all the ease one could ask for. I must
also tell you that another common metal, zinc, is still
better than iron for decomposing water,—always, of
course, with the help of sulphuric acid. We can, then,
use whichever of the two metals we chance to have at
hand, though zinc is to be preferred. If that is
lacking, we will resort to iron filings, which, as they
consist of minute particles, readily yield to chemical
action when brought into contact with the other
substances used in this method.
"Into this tumbler I put water and some pieces of zinc
from the old watering-pot that supplied me the other
day with material for showing you that this metal will
burn. No results are apparent as yet, all remaining
quiet in the glass because cold zinc by itself has no
effect on water. But I add a little sulphuric acid and
stir it in well. Now things will go on unassisted. The
water begins to boil violently, sending up countless
bubbles of gas that burst on reaching the surface.
These bubbles come from the decamped water; they are
hydrogen, precisely the same inflammable gas that we
obtained by using red-hot iron in the blacksmith's
shop. Watch now. I hold a piece of lighted paper near
the surface of the water, and each bubble, as it
bursts, catches fire with a slight explosion, burning
with a flame so pale as to be visible only in
dark. As the bubbles follow one another thick and fast,
there is an almost continuous popping."
This minature artillery popping away on the surface of
the liquid, and these flames dancing on the water,
certainly offered a curious spectacle. But there was
something else that appeared to have even greater
interest for the young spectators: the water had
started to boil with no fire of any sort to heat it,
and the glass had become so hot as to make one almost
afraid to touch it. Uncle Paul anticipated the
surprised inquiries prompted by these remarkable
"Look into the glass," said he, "and you will see that
the hydrogen bubbles first make their appearance on the
zinc, for it is there that chemical action, resulting
in the decomposition of the water, takes place. These
bubbles of gas make their way up through the liquid and
in doing so cause considerable commotion, just as water
boiling over a fire is agitated by the bubbles of steam
that are being formed. In reality the water in this
glass is not in motion as a whole, but is merely
stirred by the uprushing bubbles in the same way it
would be stirred if you blew air into it through a
straw. The boiling of the water is only apparent, only
an agitation that deceives the eye."
"But the glass is awfully hot," remarked Emile; "I
can't bear my hand on it."
"Very true; but the heat is still far below that of
boiling water. If you should be ask me to prove it, I
should only have to take the tongs and lift out the
piece of zinc, whereupon the liquid would
immedi-  ately quiet down, there being not further
generation of hydrogen, which caused the commotion."
"All the same, there's lots of heat there. Where does
it come from, with no fire to make it?"
"I see Emile finds it hard to get used to the idea of
heat without fire. Did we need any fire to make the
mixture of powdered sulphur and iron filings raise the
temperature of the bottle to a burning heat? Does the
mason use fire when he pours cold water on lime and
makes a paste that is too hot for the hand to bear?
Without fire, without live coals, without any apparent
cause, great heat is produced in both cases, and
chemical combination explains it. In our tumbler, here,
we have an example of it. Water is being decomposed,
but at the same time the opposite process, combination,
is going on between the acid and the metal; and this
process generates heat. We will come back to this
interesting point later, and you will then see that the
heating of the liquid in our glass is only what might
have been expected, for zinc is really burning, or
"It is not enough to know how to get hydrogen with zinc
and sulphuric acid; you must also provide something to
receive and hold the gas. A little difficulty presents
itself at the start. We have to do with three
substances,—water to furnish the hydrogen, and
sulphuric acid and zinc for decomposing the water and
so releasing the hydrogen. All the water and all the
zinc to be used in the operation may be put into the
glass at once, but the sulphuric acid should be added
little by little as it is needed.
 If poured in
copiously and all at once, it would raise an
unmanageable commotion. Under these conditions the
operation would be too rapid, and the operator would
run the risk of being splashed with the boiling liquid.
Consequently, the sulphuric acid should be poured in
gradually, a fresh supply being added whenever the
release of gas slows up. Moreover, these successive
additions of acid should be made without opening or in
any way disarranging the vessel in which the hydrogen
is generated. In this manner we prevent any hydrogen
and form a dangerous mixture.
"The vessel commonly used in this operation is a sort
of bottle with two necks, one in the usual central
position, the other at one side. Into this bottle is
put a handful of zinc cut into small pieces, or,
better, a small sheet of zinc rolled up so as to pass
through the neck. Enough water is then poured in to
cover the metal completely. Through one of the necks,
no matter which, is passed a glass tube, which is held
in place by a tightly fitting cork stopper with a hole
in it to receive the tube, and which is bent over and
downward on the outside like the one we used in
producing oxygen. Finally, through the other neck and
into the liquid is passed a straight glass tube, which
is held in place in the same manner as its companion.
The apparatus is now ready for use, only sulphuric acid
having to be added. For this purposed the straight tube
is equipped at the top with a small glass funnel,
through which the acid required is gradually
introduced. As long as
 the release of gas
proceeds satisfactorily, no further attention is
necessary; but if it slackens, a fresh dose of acid is
poured in. This arrangement is very simple and very
ingenious. The straight tube, extending into the water
as it does, admits no air to mingle with the hydrogen,
a thing to be carefully avoided, as will be shown; but
it does allow the introduction of sulphuric acid
whenever needed. Furthermore, the hydrogen that is
being released cannot get out this way, as the water
keeps it back; hence its only issue is through the bent
tube, the nearer end of which is in the second neck of
the bottle and well above the water. In other words,
the gas-factory has two doors and only two,—the
straight tube, which allows entrance but not exit, and
the bent tube, which offers a way out, but no passage
inward, when the apparatus is in operation and
discharging its hydrogen.
"One thing more. Suppose the bent tube gets stopped up
in some way, or that it is too small for a sufficiently
rapid discharge of the gas set free by the
decomposition of the water; what will happen? The gas
collected in the bottle, and unable to get
will press downward on the liquid and drive it up
through the straight tube until it overflows the funnel
at the top. This rising of the water in the straight
tube warns us that something is wrong with our
apparatus, blocking the issue of the gas. But, unless
we pour in too much sulphuric acid at a time, we need
not trouble ourselves to watch the straight tube for
signs of danger.
"Such is the hydrogen apparatus used in laboratories,
and I regret that I cannot show you one in operation to
supplement my description; but a two-necked bottle is
not an easy thing to procure in our village."
"That's so," chimed in Emile. "I've never seen anything
of the sort around here. A bottle with two mouths, one
for pouring in and another for pouring out, is n't a
thing you 'd be likely to find In any rubbish heap. And
so we can't have our hydrogen factory, after all," he
concluded in a plaintive tone.
"But should I have aroused your expectations at the
blacksmith's if I had n't known beforehand that I could
gratify them? With an old pickle-jar and a little
ingenuity, need we despair of success? Your uncle
thinks not. What does our modest bottle lack to make it
a serviceable piece of apparatus? Two mouths; and we
will supply them without further delay."
So saying, he took a good-sized cork stopper that had
belonged to some demijohn, and shaped it carefully with
a file to make it fit the large neck of the pickle-jar.
Then he punched two holes in it, and in
 one of
these he fixed the bent glass tube, pushing it only
just through the cork, while through the other hole he
passed the straight tube, forcing it down much farther.
Bits of zinc and enough water to cover them well were
next put into the jar, after which the stopper was
inserted, with a little moist clay around the edge to
prevent any escape of gas. Emile was delighted with the
turn things were taking: he was going to see hydrogen
made in as large quantities as any one could wish.
Everything was arranged in accordance with his uncle's
"Here is the straight tube," the boy pointed out in
eager interest, "that you'll pour the acid into when
the time comes, and there is bent one for letting the
hydrogen out. The old pickle-jar is going to do very
well, now that it has two mounts made in the big
stopper. But there's one thing we have n't got yet,—the
little funnel for pouring in the acid."
"I have none," replied his uncle.
"What shall we do, then? The tube is so small we can't
pour anything into it without a funnel."
"Let us ask Jules and see whether he would allow such a
trifle to defeat our purpose."
"You will laugh at my idea," said Jules, on being thus
turned to for advice, "but why could n't we use a
little piece of paper rolled up into a cone open at the
"Your suggestion is unanimously adopted. Lacking a
regular chemist's funnel, we could hardly do better.
Your little paper cone shall take the place
the small glass funnel; but it will soon go to pieces,
I warn you, for sulphuric acid is exceedingly
destructive. However, that does n't matter in this
case, as we can renew our paper funnel as often as
necessary. Economy in this particular is not required."
So said, so done. The paper funnel inserted in the
upper end of the straight tube made it possible without
the slightest difficulty to pour in the sulphuric acid,
whereupon the water in the jar immediately began to
boil, as it appeared to the eye, and hydrogen came out
through the bent tube, the further end of which went
down into the water in the bowl. The boys hastened to
touch a lighted paper to the gas bubbles thus created.
Quick flashes of flame, a crackling sound, a pale white
light—all these duly followed as the gas came rushing
out of the pickle-jar and was ignited. It was really
and truly hydrogen; a regular laboratory outfit could
not have given better results.
"You are pretty well acquainted with this water
artillery now," said Uncle Paul. "Let us pass on to
something else and set fire to a large volume of
hydrogen. I dissolve a little soap in water, and into
this soapy water I lower the end of the tube through
which the gas is discharged. If we took a straw and
blew through it, we should get plenty of foam. The
bottle blows in its own peculiar way:
 it sends a
jet of gas into the midst of the soap-suds and makes a
mass of tiny bubbles, all filled with hydrogen. In this
manner we obtain a certain amount of the inflammable
gas stored up in little thin-walled cells. I apply a
piece of lighted paper, and the gas catches fire. The
explosion is louder, the flame larger than before,
though the light produced is still very pale."
At the request of the young pupils, who were quite
fascinated with this exhibition, the experiment was
repeated and a still greater volume of gas was
produced, which was then exploded with fine effect.
"We have nothing more to learn from this play-thing,"
concluded Uncle Paul. "It has shown us how readily
hydrogen catches fire: hardly do we touch the lighted
paper to the bubbles, when the imprisoned gas explodes.
Let us now proceed to another experiment, which will
show us that hydrogen, so highly inflammable in itself,
can yet be used for putting out fire. It burns as
nothing else will, and yet it stops the burning of
anything plunged, all afire, into the midst of this
gas. It will put out a lighted candle as quickly as
will nitrogen. Let us prove it. I will plunge the end
of the bent tube of our apparatus into the bowl of
water and fill with hydrogen either the gage or a tall
bottle with a wide neck, proceeding exactly as I did
Accordingly the gage was filled, after which Uncle Paul
"Here is our gage, now, full of hydrogen. I lift it out
of the water."
With these words he took the gage by its foot and
 withdrew it from the bowl, holding it upside down as
one would to empty it of a liquid. This procedure
seemed to the boys to betray absent-mindedness on their
"If you hold it that way," they exclaimed, "the gas
will all get out. The mount is pointing down, and it is
"No, my lads, the hydrogen will not get out. It is much
lighter than air, and so tends to rise and not to fall.
To keep it from escaping, we must block its way above,
not below, and this I do by holding the gage upside
down. There being no outlet upward, the gas is held
captive. As to the open mouth below, we need not give
it a thought; our hydrogen cannot go down and get out
that way. I put a lighted candle into the gage and push
it up almost to the inverted bottom. See what happens.
The lowest layer of hydrogen, being next to the outside
air, immediately catches fire with a slight explosion,
and the flame gradually works its way up to the top of
the column of gas. But as for the candle-flame, it went
out at the very first, being smothered by the hydrogen
as quickly and completely as it would have been by
This seemed very strange to the boys; they wondered how
a gas that burns so well itself could put out a fire
already burning. But the explanation soon given by
their uncle was found to be simple enough.
"All burning," said he, "let us repeat and continue to
repeat until the mind is quite familiar with this first
principle—all burning, I say, is nothing
 but the
chemical combination of some substance with oxygen,
which is always present in the air. Where there is no
oxygen, there nothing will burn. Well, then, the
candle, on being thrust into the gage of hydrogen, went
out because it did not find there the gas necessary for
feeding its flame; if found no oxygen, and the other
gas was unable to take its place, although very
inflammable in itself. This gas, the hydrogen, took
fire, but at first only in the bottom layer, because
there and only there, next to the outlet, was there any
air to feed the flame. Then this flame worked slowly
upward from bottom to top, as the consumed hydrogen
gave place to the air crowding in from below.
"Hydrogen is about fourteen times as light as air. This
has been ascertained by means of chemists' scales,
which are so delicately poised as to tip under the
weight of a hair. Although an extremely light gas,
hydrogen still weighs something, about one decigram to
the liter. No other substance even among the most
subtle, the gases, weighs so little. A liter of water
weighs a kilogram, or ten thousand times as much as
hydrogen. The heaviest of known substances is a metal
which weighs twenty-seven times as
much as hydrogen. Between these two extremes range all
the other substances known to us, some being heavy and
others light according to their
 position in this
scale. Accepting these statements as verified, we will
confine ourselves to showing by experiment that
hydrogen is indeed much lighter than air.
"You have just seen how the gage must be held,—that is,
with its mouth downward, to keep the hydrogen in. On
account of its extreme lightness this gas escapes
upward. Hence we may keep it confined by interposing
some obstacle to this upward flight. Now let us prove
that under the opposite conditions it will escape. We
hold the gage upright, its mouth at the top, as if we
had to do with nitrogen, oxygen, or atmospheric air,
all three of about the same weight. Nothing standing in
its way above, the hydrogen will quickly escape, you
may be sure."
Refilled with hydrogen, the gage was set upright on the
table, and they waited a few minutes. Nothing could be
seen to go our or to come in. The sharpest eye could
not have detected the departure of one gas and its
immediate replacement by another.
"We have waited long enough," announced Uncle Paul.
"There cannot be any hydrogen left there now. It is
gone, and air has taken its place."
"How do you know?" asked Emile. "For my part, I can't
see that anything has happened."
"Nor I, either; and if we had only our three pairs of
eyes to decide the matter, the gage would keep its
secret and never tell us what has taken place. But a
lighted candle will tell us what our eyes cannot. If it
keeps on burning in the gage, it will show that the
latter contains air; if, on the
con-  trary, it goes
out after setting fire to the contents, it will mean
that hydrogen is present."
A lighted candle was lowered into the gage and
continued to burn there the same as before, proving
that the hydrogen was gone and air, a heavier gas, had
taken its place.
"If we lowered an open can of oil into a barrel of
water," Uncle Paul went on, "what would happen? The
water, being heavier than the oil, would force the
latter out of the can and take its place, while the
oil, being lighter than water, would rise and float on
the surface. That is the way air and hydrogen act when
the gage is set upright. But I have a still better
experiment to show you in proof that hydrogen is
lighter than air. With a few straws and a little
soap-suds we can give a fine demonstration of the
lightness of hydrogen. This is the way of it. You know
better than I what will happen if we wet the end of a
straw in soapy water and then gently blow through the
straw. Emile used to play at the game not so very long
ago, and he found it great fun."
"You mean blowing soap-bubbles?" Emile was quick to
rejoin. "Oh, that's fine, Uncle! On the end of the
straw there comes a bubble, and it swells out bigger
and bigger, to the size of an apple or an orange if you
blow it the right way. And you can see all the colors
of the rainbow on it,—blue and green and red, and so
on,—more beautiful than the finest flowers in our
garden. You don't dare move the straw for fear the
magnificent bubble will burst. But before long it does
 anyway, all of a sudden, and you don't
know where it's gone to. How sorry I've felt, many a
time, because my soap-bubbles would n't fly up into the
air and soar about with all their splendid colors!"
"You won't have any reason to be sorry this time, my
child," his uncle assured him, "for you are going to
see you bubbles soar upward in fine style, and that
without any coaxing."
"My beautiful soap-bubbles?"
"Your beautiful soap-bubbles."
"Then I shall like them better than ever."
"Show us first some of the kind you know so well how to
Emile took a straw, thrust one end into the soapy water
that had been prepared, withdrew it, and blew gently
into the dry end, making a series of bubbles, the
largest of about the size of one's fist. All, as the
filmy envelop became thinner with continued blowing,
showed the brilliant hues of the rainbow; but they
also, as soon as detached from the straw, all few
softly to the floor. Not one would float aloft in the
"And why should they rise?" asked Uncle Paul. "They are
filled with air that is no different from the air all
about them, and consequently receive no impulse from
this gas either to rise or to fall. But their covering,
which is made of a thin film of soapy water, is heavier
than air, and so causes them to fall. Hence, if we wish
our bubbles to rise, we must fill them with a gas
lighter than air, a gas that will by its lightness not
only make up for the weight of
 the envelop, but
will also lift it up and bear it skyward. That gas is
"But how can I fill my bubbles with that?" asked Emile.
"I can't blow hydrogen into them with my mouth."
"We will make the bottle do the blowing, and it will
blow as well as any of us. First I take out the bent
tube and put in a straight one, running side by side
with that used for pouring in the acid and extending
upward a little farther. But, as the tube is rather
large, I contrive a smaller outlet at the top by
inserting a straw wrapped at the lower end in a bit of
wet paper. It is at the upper end of this straw, whence
issues a jet of hydrogen—or a breath of hydrogen, if
you prefer to call it so—that the soap-bubbles will
form. I place the bottle upright so that the little
balloons, borne upward by the lightness of the gas,
will find nothing in their way. Now all we need do is
to take a wisp of paper or something else and from time
to time put a drop of the soap-suds on the end of the
straw, whereupon we shall see bubbles form, filled with
No sooner said than done. At the end of the straw,
which was kept supplied with soapy water, there
appeared a succession of transparent globes, sometimes
larger, sometimes smaller, but always in an upright
position on the tip of the straw and straining to get
away from it. Many succeeded as soon as they were big
enough, and then away
 they soared, rising rapidly
and soon reaching the ceiling of the room, where they
burst on touching it. Others burst before they could
get clear of their moorings. Not for a good deal would
the boys have missed seeing this ravishing spectacle.
With wondering gaze they followed each balloon every
instant, from start to finish. First they beheld it as
a tiny bubble, then steadily swelling and all aglow
with brilliant colors. It would sway a little on the
end of the straw, then tear itself away, and off it
would go in its flight to the ceiling. Oh, how
gracefully it rose! But all too soon the ceiling was
reached and the magnificent sphere shattered. Another
soon followed it, however, and then another, and still
another, as many as one chose. Jules was thoughtful,
"I am going to make this chemical diversion of yours
still more entertaining," said Uncle Paul. "Ties a bit
of candle to a long stick, light the candle, and then
hold it up to a bubble while it is in the air."
Emile was not slow to carry out these instructions.
Fastening a candle to a reed, he gave chase after one
of the bubbles as it rose. Flack!> went the
little balloon, there was a sudden burst of flame in
mid-air, and then nothing more; the whole thing had
vanished. Emile gave a start; he had not expected so
sudden a flash or one of so short duration.
"Does that startle you?" asked his uncle. "Did n't you
know that hydrogen is exceedingly inflammable? Touch a
lighted candle to a bubble filled with this gas, and
there can't fail to be an
in-  stantaneous outburst
of flame. That is the whole secret of these little
a๋rial fireworks so astonishing to you."
"Yes, it's simple enough, but I was n't expecting it."
"Now that you know what to expect, let us try it
The experiment was repeated several times, Emile
allowing the bubbles to rise half-way to the ceiling
and then touching them off with the candle. Not one of
them, however quickly it rose, escaped the alert
incendiary's pursuit. Thus it was shown in a highly
diverting manner how readily hydrogen catches fire.
Jules, who never asked idle questions, finally broke
"Our soap-bubbles," said he, "hit against the ceiling
and that's the end of them. Would they go up very high
if they had plenty of room? Where would they go to?"
"In the open air and with nothing in the way they might
rise to a great height if they did n't burst too soon;
but the least agitation, the slightest breath or air,
is sufficient to destroy them, so delicate and fragile
a thing is a soap-bubble. Nevertheless if the
atmosphere is very calm, the bubbles may last long
enough to soar out of sight. We can try the thing out
of doors this minute, for luckily the air is perfectly
calm just now. Not a leaf is stirring on the tree in
The apparatus was carried outside and the
bubble-blowing resumed. Many of the bubbles burst when
no higher than the roof of the house, while others,
 though very few, rose out of sight. In a short
time even Emile's sharp eyes could no longer
distinguish them against the blue sky.
"Do they go very high?" asked Jules.
"I think not. A hundred meters, more or less, but at
that height, being so small and so transparent, they
become invisible. Their extremely thin and delicate
covering, too, bursts before long. The one you are
looking for now up there, hoping to catch another
glimpse of it, is probably no longer in existence."
"But if the covering never broke, how high would the
"On that point I can speak rather more definitely.
Learned men who wish to explore the upper regions of
the atmosphere and find out what is going on there,
make enormous balloons of some strong fabric, and then
varnish them outside and fill them with hydrogen, just
as we are now filling our soap-bubbles. With these
durable balloons they can go up to any height they
please. The most daring have gone up ten thousand
"Why not higher?" asked Jules. "I'd have gone a good
deal higher if I'd been in their place. I should have
wanted to see what there is at the very top of the blue
sky. How beautiful it must be up there above the
"In their place you would have done as they did, my
dear boy, or probably not so much, for it takes almost
superhuman courage to dare to visit those high regions.
When you get to where there is not air enough,
breathing becomes impossible and you
 have to come
down in a hurry, or you are dead in a few minutes. That
is why, up to the present time, the greatest height
attained by man is about ten thousand meters."
"But the hydrogen balloon could go still higher if
there was no danger for the balloonist?"
"Without a doubt, much higher."
"I can't tell you exactly, but let us say twice as
high. All that I can be sure of is that balloon-ascent
has some fixed limit, no matter how light and how
skilfully constructed the balloon may be. The layer of
atmosphere is thought to be only about fifteen leagues
in thinkness. Nothing that rises from the earth by
reason of its superior lightness to air can pass that
limit, since it no longer has air to buoy it up. At
about fifteen leagues above the earth's surface,
therefore, the ascent of any substance, even though it
be the lightest gas, ceases."
"I'd be satisfied with ten thousand meters, or even
much less, if I only had a strong enough covering to my
balloon not to break at a mere nothing, as our
"You shall have your strongly covered balloon no later
"And can I send it up as high as I want to?"
"Yes, as high as you want to."
At this prospect Emile clapped his hands with delight,
while a gentle smile of satisfaction showed that Jules,
too, was not indifferent to their uncle's promise. If
he could not, himself, explore that beautiful blue void
whose mystery so fascinated him,
 he could at
least send a hydrogen balloon up there.
"One more question," said he "before we stop blowing
soap-bubbles. When they are filled with our breath they
have all sorts of bright colors, and when they are
filled with hydrogen they look just the same, so it is
n't what's in the bubbles that makes those beautiful
"No, my boy; those colors, which are the same as you
see in the rainbow, do not come from air or hydrogen,
nor do they come from the soapy water that makes the
outside of the bubble. They are simply the play of
light on the extremely thin covering. Whenever any
transparent substance, whatever its nature, is in the
form of an exceedingly thin film, the light striking on
it causes this splendid coloring. Put a drop of oil,
for instance, on still water, and the drop will spread
out in the thinnest layer imaginable, whereupon the
rich colors you speak of will appear. A soap-bubble or
a thin layer of oil or a film of any transparent
substance is called iridescent because it shows the
colors of the rainbow, which the ancients called