A DROP OF WATER
promised," Uncle Paul resumed, "to show you a balloon
with so strong a covering that, when inflated with
hydrogen, it could rise without danger of soon
breaking. The time has come to keep my promise. Emile
ought to have among his old playthings just what we
need for our purpose. He doubtless remembers the pretty
red balloons he used to buy at two sous apiece. To hold
one of those balloons by its long string was an
unfailing delight to him."
"Oh, yes, I remember them," Emile was quick to rejoin.
"They were more fun than my Noah's ark with all the
animals arranged in pairs, or my lead soldiers that I
used to set up in companies on the table. But they are
all crumpled up now in my old toy-box, and they've been
there a long time, ever since the day after I got them;
for they would n't go up at all after the first day,
but fell right down to the ground, though they were all
right for a little while and went up finely."
"And have you never wondered why those little red
balloons, so ready to go up the first day, were so very
slow about it the next day?"
"Oh, yes, I've often wondered, but I could n't tell why
it was so."
 "I will tell you the reason. Those balloons were
filled with hydrogen, the same gas that we used
yesterday for the soap-bubbles. Their covering is a
fine membrane of rubber, very elastic, so that it
stretches freely under the pressure of the hydrogen
inside. Despite its extreme thinness such a covering
is, so far as we can see, air-tight, being far superior
in this respect to even the most closely woven fabric
with its countless little open spaces between the
meshes; and yet so subtle a gas is hydrogen that it
manages to get through and make its escape. Then little
by little the balloon collapses; or else, remaining
full and round, it exchanges its hydrogen for air, the
two gases passing through the rubber envelop in
opposite directions. Consequently, whether by partial
deflation or by exchange of hydrogen for air, the
balloon loses much of its buoyancy before long, and
after twenty-four hours cannot rise at all. To give it
new life, we must again fill it with hydrogen."
"Oh, if I'd only known that before, you'd have had me
teasing you to put new life into my balloons."
"A little chemistry has its advantages, has it not, my
boy, were it only to put new life into toy balloons
that are dead? If yours are still in good condition—I
mean if there are no holes in the rubber—nothing is
easier than to make them as buoyant as ever. Go and get
Emile ran out of the room and was back in a trice with
two little red balloons, shapeless and shriveled, with
not a breath of hydrogen in them;
 nor had there
been for a good while. But the child took such care of
his things, was so particular about putting them away
properly, that all his old toys were still in excellent
condition. The rubber membranes that had once been
lively little balloons, but were now all lax and
lifeless, had not the slightest hole in them, as Uncle
Paul soon learned by blowing into them,
"Our little balloons are all right," said he, "and now
to work! I take a common bottle, the first to come to
hand, holding about a liter, and into it I put some
water and a good handful of zinc scraps. Then through a
perforated cork stopper, fitting the neck closely, I
pass a straight glass tube or, if that is lacking, a
pip-stem, or, better still, a goose-quill. The free end
of this I next insert in the opening of the balloon and
bind it with a string to make everything tight and
prevent the escape of gas. Now I pour sulphuric acid
into the bottle, and when the bubbling of the mixture
is well under way, and the hydrogen is being released
in ample quantity, I squeeze the balloon with my hand
to drive out any air through the goose-quill, and still
squeezing it I push the cork firmly into place. That
done, I let go of the balloon and allow things to take
their course. The crumpled membrane receives the gas,
swells up gradually, and is stretched taut by the
pressure of the hydrogen discharged from the bottle.
Now you see it all spherical, and threatening to burst
if I let much more gas go in. Finally, with a strong
thread, I tie tightly the mouth of the balloon a little
above the goose-quill to which it is
 attached. In
that way imprisoned hydrogen is retained, for a while
at least. I then free the balloon from the goose-quill,
so that the hydrogen forming in the bottle may escape
and not accumulate to the point of blowing out the
stopper with an awkward splash of the liquid itself."
"Now let's see if it will go way up high," proposed
Jules as the balloon swayed this way and that,
straining to get free, but held back by his uncle's
"I don't want to lose it," said Emile. "Let's tie a
string to it. Here's a long one that will do."
"Before talking about strings," was his uncle's advice,
"let us consider the matter a little. How much gas does
our little balloon hold? A liter at the most.
Conseqently, the weight of the hydrogen that fills it
is about a decigram. The same volume of air weights
fourteen times as much, making the difference between
the two about thirteen decigrams. We will say the
rubber covering weighs a grain, or ten decigrams. Three
decigrams, then, are left as the measure of our little
balloon's upward tug on its string. And with such
feeble power at its disposal you would have our balloon
drag the weight of a long string. Why not a rope or a
"You are right. The balloon is n't strong enough to
lift a long string up into the air. Then suppose we
take a fine thread."
Attached to the end of a thread of this sort, the
balloon rose, but, disappointing the children's
expectations, it refused to go very high.
"It has stopped going up," said they. "Why?"
 "Because the higher it rises the more thread it
has to pull up after it, and the weight of his is added
to that of the balloon itself. So the time soon comes
when the weight of the rubber covering, the hydrogen,
and the lifted thread all together equal the weight of
the air displaced by the hydrogen; and from that moment
further ascent becomes impossible. As Emile wishes to
keep this balloon we will inflate another and let it go
without a thread to weigh it down. Free from this, it
will go up as high as you wish."
And in fact, set free with no thread to shorten its
flight by increased weight, the little balloon rises
quickly and is very soon out of sight. How high will it
go? Who knows? But whatever height it attains, sooner
or later it will come down, because through the fine
rubber covering there is taking place a constant
exchange of gases, hydrogen going out and air coming
in. Thus becoming always heavier, the balloon gradually
falls back to the ground. But meanwhile who can say
whither the winds may have wafted it?
"If Emile had n't kept the red balloons that we used to
play with, could n't we have used a pig's bladder
instead?" asked Jules. "That would give us a balloon as
ready made, easy to get, and of a good size."
"I should have tried the pig's bladder only in case I
had been unable to get anything better. It would have
given us a large balloon, it is true, and made of a
strong membrane; but it is loaded here and there with a
layer of fat, which would be
ob-  jectionable. You
must not forget that the covering of our balloon should
be as light as possible, so that the hydrogen may not
lose its power of flight by having too great a load to
cary. A liter of his gas cannot raise much more than a
gram. Let us say the bladder will hold four liters,
making about four grams that the contained hydrogen can
raise. If, then, the bladder exceeds this weight, the
balloon cannot rise. Consequently, if we propose to
send up a balloon of this sort, the membrane must first
be rid of its layer of fat, scraped down, its thickness
reduced here and there, and all superfluous material
remove, so as to lessen the weight as much as possible,
while at the same time care must be taken not to make
any holes in the membrane. With theses precautions,
which call for the utmost patience and skill, success
"When our village festival is held, one of its features
is the ascent of a balloon, filled, not with hydrogen,
but with hot air. It goes up amid cheers from the crowd
and salvos from the village artillery, consisting of
mortars loaded with charges of powder well rammed home.
Why should not we, too, in our chemical pastime, have a
discharge of artillery to accompany the ascent of our
balloons? A bottle shall be the mortar, hydrogen the
powder. Every time we set fire to hydrogen, even to a
tiny bubble of it rising to the surface of the water,
you heard a slight explosion. It is only necessary to
find out what causes this explosion, and then to
produce a louder one. It is presumable that air is
necessary here, or more accurately the oxygen contained
 air. Let us see, then, how hydrogen and air
behave when mixed together.
"Into this narrow-necked bottle holding about a quarter
of a liter, I pour enough water to fill it one third
full, and then turn it upside down in the bowl so that
it may receive a supply of hydrogen, using the bent
tube once more and starting the release of the gas with
sulphuric acid as before. Air fills two thirds of the
bottle, water the other third. Hydrogen from our
apparatus expels this water and takes its place. Thus
the bottle contains a mixture of air and hydrogen, two
parts of the former to one of the latter. I cork it
tightly and wrap it up in several folds of a towel,
leaving the neck free. Our gun is loaded. Now to fire
With these words Uncle Paul grasped the towel-wrapped
bottle by its middle with one hand, uncorked it, and
held its mouth to the flame of a candle burning on the
table. An explosion followed, and it was loud enough to
make both the children start.
"Hurrah for the little hydrogen pistol!" cried Emile.
"Famous gunpowder that makes. Once more, Uncle,
The bottle-pistol was fired off again charged in the
same manner as before,—that is to say, filled with the
invisible powder of mixed hydrogen and air. Uncle Paul
repeated the process as many times as was desired, and
the explosions were more or less violent according to
the proportions of hydrogen and air used. Some were
full-toned and short, like the discharge of a firearm,
others were only a noisy blowing, and others still, to
 amusement, sounded like the
yelping of a dog that has had its tail stepped on.
"My artillery," resumed Uncle Paul, "shows you that
hydrogen and air form an explosive mixture that will
take fire immediately at the touch of a flame and will
explode violently. This mixture, invisible though it
is, has no lack of power, and could blow its container
to pieces if denied a sufficient outlet. That is why I
wrap the bottle in a towel to arrest the fragments if
the rupture should occur; and for a like reason it is
well in theses experiments to select a bottle of rather
small size, a quarter of a liter at most. With a larger
one there would be serious risk of injury: the gun
might burst and wound the gunner.
"Air, as you know, is a mixture of an active gas,
oxygen, and an inactive one, nitrogen. In exploding
hydrogen it is clear that nitrogen plays no part; or,
rather, by its inertia and considerable volume it
impedes chemical action and muffles the detonation. The
oxygen alone is active. Let us, then, do away with the
nitrogen and use pure oxygen, and the detonation will
be much louder. What we need for this purpose is all
ready here, as I took care this morning to prepare a
bottle of oxygen beforehand. It stands there in the
corner, upside down, with its mouth in a glass of
water. Before going farther I must tell you that to get
the loudest explosion, hydrogen and oxygen should be
mixed in the proportion of two parts of the former to
one of the latter.
"I fill with water a wide-mouthed glass jar to
 serve as receiver for holding our explosive mixture.
Turning it upside down with its mouth in the water, I
discharge into it one part of oxygen, using the first
bottle at hand as a measure. Then I add two parts or
bottlefuls of hydrogen. That done, our gunpowder is
ready. Look into the jar. What do you see there?
Nothing. Nevertheless it holds a dangerous explosive
which it would be unwise to set fire to without proper
precautions. The glass jar would fly into fragments
that might seriously disfigure us. But there is nothing
really to be feared as long as no match is applied. If
you ever undertake experiments of this sort by
yourselves, bear in mind that the mere fact of having
water at hand does not insure you against the serious
results of carelessness. This explosive does not in the
least mind getting wet; it could stay next to water
indefinitely without losing any of its terrible
explosive power. Dry and wet are all the same to it.
"With the aid of a funnel, I fill with the gaseous
mixture the bottle we have just been using, corking it
and wrapping it carefully in a cloth to guard against a
rupture, a thing more to be feared now than in our
previous experiment. Now I have only to uncork my piece
of artillery and hold its mouth near the candle-flame.
Attention! Say 'Fire!' "
"Fire!" cried the boys in unison.
Bang!> went the gun, and the room rang as if a
rifle had been discharged. Emile gave a jump; indeed,
he was almost frightened.
"To think that something you can't even see
 should make such a noise!" he exclaimed. "I can hardly
believe it. If I'd known beforehand what was coming I
should have stopped up my ears."
"Ah, indeed! That would look well, would n't it, Emile
stopping up his ears so as not to hear the chemical
pistol go off! No flinching, my boy, or I stop firing."
The candle having been relighted—for it was blown out
with each burst of ignited gas from the bottle—Uncle
Paul repeated the experiment. The explosion made the
window-panes rattle; but this time, to make amends for
his former show of fear, Emile stood as firm as a rock.
He even forced himself to watch the proceedings with
unwavering attention, and he saw a tongue of flame
almost a meter long dart out with furious impetuosity
from the mouth of the bottle. After a few more
repetitions of this performance he became so hardened
that he asked the great favor of being allowed to hold
the gun in his own hand and fire it off as his uncle
"I grant your request most willingly," was the reply.
"There is nothing further to fear now. The bottle has
been well tested, it has withstood the strain put upon
it, and it will continue to do so. However, as an extra
precaution keep it securely wrapped in the cloth, so
that if by any chance it should burst the pieces would
still stay in the wrapping. Grasp it boldly in your
hand; there is nothing to be afraid of."
Without flinching, Emile managed the battle when it had
been charged by his uncle. With sober face
standing rigidly erect, like an artilleryman
discharging his cannon, he fired off the glass gun.
Jules then took his turn, his delicate hand trembling a
little with excitement. And so they alternated until
the explosive gas was all used up. These repeated
discharges made the rest of the family wonder what in
the world Uncle Paul and his nephews could be up to
with their chemicals and their bottles and their other
pieces of apparatus.
"Now that our gun is silenced for want of ammunition,"
said Uncle Paul, "let us see what we have left from all
this burning of hydrogen in oxygen. The two gases
combine when the explosion takes place, and this
combination is attended by a streak of flame, not very
bright, which you saw shoot from the bottle. A new
substance, a compound of hydrogen and oxygen, is formed
as the explosion takes place, but it cannot be seen, as
it is an invisible vapor. This must be condensed if we
wish to examine it. But instead of igniting a great
volume of gas all at once, which would be somewhat
dangerous and, besides, would not serve the purpose of
careful study, we will arrange matters so that our
hydrogen and oxygen shall combine a little at a time;
that is, we will light a jet of hydrogen and let it
burn in the air so that we can watch it closely.
"Let us begin with the preparation of our apparatus. It
will be the same as we used for blowing our
soap-bubbles, except that the straight tube having a
straw in its upper end will be replaced by another with
a tapering tip. The channel as it is now would not do,
being too large; it must be made
 smaller until
the outlet does not much exceed the size of a pin. This
is how it is done. A piece of glass tubing, as easy to
melt as possible, is held in the flame of an alcohol
lamp and twirled between the fingers so as to be heated
equally all around. When it seems soft enough it is
pulled so that the dough-like heated part stretches
into a thread and two sections are obtained with a mere
string of glass between them. With one sharp tap in the
middle of this connection we get two tapering tubes,
either one of which will serve our purpose.
Accordingly, we select one and thrust its blunt end
into one of the two holes in the jar's stopper. The
second hole receives, as before, the straight tube for
the introduction of the acid. I will add that a clay
pipe-stem could be used if necessary instead of the
tapering glass tube. Of course, if you have a
two-necked jar, one neck would receive the tapering
tube and the other the tube for the acid.
"Water, zinc, and sulphuric acid having been put into
the jar, hydrogen is released and flows in a jet
through the tiny outlet of the tapering tube. I propose
to light this jet, but here we must exercise a little
prudence. We have just seen that hydrogen and air make
an explosive mixture. Now, when the release of hydrogen
began, the jar contained air, and hence an explosive
mixture is inevitable at the start. If we were so
careless as to hold a lighted match to the gas jet at
this stage of operations, the dangerous mixture would
explode in the jar, which might not withstand the
shock; it might be blown to bits, or at the very least
 would pop out and with it would come
splashes of the acid, attaining our clothes with red
spots and, much worse, perhaps getting into our eyes. I
warn you to beware of this explosive mixture and to be
constantly on your guard if you are preparing hydrogen
by yourselves. You must watch every moment to see that
no air mixes with the gas you are intending to light.
"In the experiment before us the presence of air is
inevitable at the start. What, then, is to be done? The
release of gas must be allowed to go on for a while. As
it comes out of the jar, the hydrogen brings air along
with it; and, as this is not renewed, a time comes when
there is none or next to none left. But as there is
nothing to indicate when this moment has arrived, we
must simply wait a while, and our waiting should be
rather too long than too short, so as to assure us of
safety. Here patience is prudence."
They waited while the release of hydrogen went on and
the liberated gas came whistling through the tiny
outlet of the tube. After a few minutes it was thought
safe to proceed.
"That ought to do," declared Uncle Paul. "There can
hardly be any more air left in the jar by thins time.
But we may be mistaken, and so, to prevent accident, I
wrap the jar in a towel to arrest, if necessary, any
flying fragments or splashes. With this final
precaution, I hold a lighted paper to the hydrogen jet.
Instantly the gas takes fire and burns quietly with a
very pale yellow flame. All danger is past. As there
was no explosion when I lighted
 the gas, there
will be none. All the air is driven out of the jar,
only hydrogen issues from the tapering tube. Our towel
was not needed, but it should nevertheless be used
whenever we suspect there may be an explosive mixture
in the jar. Not to hide from view what is going on
there, I now remove the protecting cloth, which is of
no further use. Instead of worrying about an explosion
that cannot occur, let us examine our hydrogen flame
"At the tip of the tapering tube we see a steadily
burning flame, fed by the gas constantly coming out;
and this flame is of a very pale yellow and gives
scarcely any light. Such is the appearance of burning
hydrogen, of which until now you have caught only
fleeting glimpses. It is not a bright flame, but an
exceedingly hot one. Try it"
The boys held each a finger-tip near the paltry little
flame, and each drew it back quickly. The heat was
"Oh, oh!" Emile cried out with pain. "That flame does
n't look like much, but it 's a good heater, all the
"It ought to be, coming as it does from burning
hydrogen, the best of fuels. Do you remember what the
blacksmith showed us?"
"Do you mean his wetting the coal in his forge so as to
heat his iron white-hot?"
"Yes. Water, decomposed by burning coal, gives
hydrogen; and this hydrogen burns and thus adds
considerably to the heat."
 "Then we could make iron red-hot in this flame,
though it is so small and pale?"
"You could make it not only red-hot, but white-hot.
Look here. I hold the end of an iron wire in the flame,
and immediately it becomes dazzling to the eye. It is
thus that the blacksmith's iron bar is heated in the
forge when he wets his coal.
"Another peculiarity, less important but more curious,
is that the hydrogen flame sings. Yes, my little
friends, it sings, and you shall hear its song as soon
as I have provided it with a suitable musical
instrument. This instrument is a glass tube about as
long and as slender as a walking-stick. But the tube
may be shorter and wider, with corresponding changes in
the tone, deeper if the tube is wide, on a higher note
if it is narrow. For want of such an instrument a lamp
chimney will serve, or a paste-board or even paper
cylinder. They are made in several lengths and
calibres. I have prepared a number of these tubes, one
of which is of glass. I begin with that."
Uncle Paul, holding the glass tube upright, lowered it
so as to enclose the flame, whereupon there was heard a
musical sound, continuous and full, not unlike that of
an organ-pipe. According as the tube was lowered or
raised, causing a greater or less extent of flame to be
within it, the note was on one pitch or another,
changing abruptly from octave to octave in a manner to
jar upon the least sensitive ear. Sometimes the note
was tremulous, then uniform, and then tremulous again.
Occasionally the effect was that of a solemn prayer
muttered in the
 hushed seclusion of some chapel,
after which would come a response in high falsetto. In
short, whatever the note, it always had a somewhat
strident tone, affecting the ear as a disagreeable
buzzing. By repeatedly changing the tube, selecting now
a long one and now a short, now of greater calibre and
now one of less, sometimes one of paper, then of glass
or metal or pasteboard, Uncle Paul ran through the
entire gamut in a riot of ear-piercing notes.
"Oh, what a crazy medley!" exclaimed the young hearers,
overcome with laughter and unable to stand the discord
any longer. They sought relief by stopping up their
ears. "Oh, if Bull were only here! He barks when any
one plays the flute or violin, and what fun it would be
to hear him join in with this hydrogen orchestra! Let
's go and find him."
The dog was soon found, and he came readily enough,
perhaps thinking he was going to get a bone. At the
first note of the mad music the animal became excited
and began to howl and moan, voicing his bewildered
surprise in the strangest kind of sounds, all to the
fit accompaniment of the hydrogen refrain. Emile and
Jules burst out laughing at this instrumental and vocal
concert, and even their sedate uncle failed to preserve
his usual gravity.
"Put that disorderly pupil out of the class room
immediately," he commanded, "or I shall be joining in
your foolish merriment and the lesson on hydrogen will
When the dog had been banished and the hilarity calmed,
Uncle Paul continued:
"You will of course understand that it was not
 for the sole purpose of treating you to such a
charivari that I took it into my head to enclose the
hydrogen flame in a tube; there was back of this mad
concert a serious motive, which I will explain after
answering a question that both of you must surely have
on your tongue's end. What make the hydrogen flame
sing? Enclosed in the tube, the gas jet meets with air,
and so there is continually being formed an explosive
mixture that causes a succession of tiny detonations,
one immediately after another, making the column of air
in the tube vibrate. The sound we hear is due to this
"But now let us drop this subject and see what is
produced when hydrogen is burned. What becomes of the
burned hydrogen? I take the glass tube again, and wipe
it thoroughly inside with a wad of blotting-paper on a
stick. So now we have it in a perfectly clean
condition, without a trace of moisture on the inside.
Again I pass it over the hydrogen flame. But don't
listen now to the singing of the gas; watch what is
going on in the tube. A fine dew soon forms on the
inner surface of the glass, increasing gradually until
it trickles down in colorless drops. This liquid is the
burned hydrogen, the compound resulting from the
combining of hydrogen with the oxygen of the
atmosphere. From its appearance one would call it
water, but before we can be sure we must taste it.
"With the tube I am using it is hardly possible to wet
the finger-tip in the drops running down the inside; so
let us make a slight change, substituting
for the tube a large jar with a wide mouth. I wipe
inside carefully and then introduce the hydrogen flame.
Dew again appears, and drops collect and run down. If
we wait long enough, some of them will reach the mouth
and you can wet the tip of a finger in them."
After the flame had burned some time in the jar, which
was held upside down and, to enable the condensed
liquid to gather in one place, a little slanting, some
drops did in fact collect> at one point on the
mouth of the jar. At their uncle's bidding the boys
hastened to wet a finger-tip in the liquid and taste
"It has n't any taste," declared Jules, "or any smell
or color. I should almost say it was water."
"You may say so without the 'almost,' for that is just
what it is. This is the wonderful thing I wished to
teach you when I made the flame sing. Water is burnt
hydrogen; it is composed of hydrogen and oxygen.
Commonly looked upon as the direct opposite of fire,
water in reality combines the elements essential to the
hottest kind of a fire, namely, hydrogen, the best of
fuels, and oxygen, in which metals, even including
iron, will burn. The two gases are present in unequal
parts, one of oxygen to two of hydrogen. That is why,
when I prepared the mixture that exploded so loudly, I
put into the bottle two measures of hydrogen and one of
oxygen. The explosion of this mixture produced a little
water, which, vaporized by the high temperature, rushed
with much violence and noise out of the bottle. From
the loudness of the explosion, causing
the window-panes to rattle at each occurrence, you
think a good deal of water had been produced. Undeceive
yourselves; great as was the noise, the quantity of
water produced was small, very small, perhaps a single
drop. You shall judge for yourselves from the figures
given by chemistry. It tells us that to make one liter
of water we must have one thousand, eight hundred, and
sixty liters of our explosive mixture, of which six
hundred and twenty are oxygen and one thousand, two
hundred, and forty, or twice that amount, hydrogen. How
much water, then, would come from out little bottle
holding a quarter of a liter? Hardly any at all. What a
noisy celebration over a chemical marriage from which
is to be born a single drop of water!
"Now we can see the reason for using sulphuric acid
with our metal, be it zinc or iron, in decomposing
water. We know that an acid unites with metal if the
latter is first converted into an oxid. A salt is
formed by this union of acid and oxid. Zinc, sulphuric
acid, and water are put into a jar together. The acid
has a natural liking for the metal if the metal first
becomes an oxid; and the metal, on its part, has a
liking for the oxygen. The combined effect of these two
likings is the decomposition of the water, which lets
its hydrogen escape and gives its oxygen to the zinc.
Thus the metal burns at the expense of the water, which
furnishes the element without which nothing can burn.
It burns, I say, or in other words it becomes an oxid,
which immediately combines with the sulphuric acid to
make a salt called sulphate of zinc. In short, what
zinc undergoes with the help of water is exactly what
 undergoes in a brazier when it burns with the
help of air. The metal burns there, or is turned into
an oxid. It is this burning of the metal that produces
the heat generated by our hydrogen apparatus when it is
in operation. You were surprised that a liquid should
get hot without fire. Now the mystery is explained.
"I said the zinc was converted into a salt. This salt,
this sulphate of zinc, readily dissolves in the water
used in the experiment and only slightly diminished in
volume by the decomposition of a very small part of it.
The metal, then, disappears from sight just as sugar in
melting disappears. Let us now take a look at out
hydrogen apparatus. For some time it has been producing
no gas. The zinc is all gone except a few blackish
flakes resulting from impurities in the metal. It is
dissolved in the liquid, having first been turned into
salt, and the liquid itself remains as colorless as at
first. We will let the jar stand in a corner, and
little by little the dissolved substance will
crystallize out, and we shall have a white deposit of
an unbearably sharp taste. This white substance will be
sulphate of zinc."