WHY WATER PUTS OUT FIRE.
"You have never seen oxygen any more than you have seen air," continued
Monsieur Roger. "You have never seen it, and you never will see it with
your eyes,--for those organs are very imperfect. I need not therefore
say oxygen is a colorless gas; and yet I will say it to you by force of
habit. All books of chemistry begin in this way. Besides this, it is
without smell and without taste. Oxygen is extremely well fitted for
combustion. A half-extinguished candle--that is, one whose wick is still
burning but without flame--will relight instantly if placed in a globe
full of oxygen. Almost all the metals, except the precious metals, such
as gold, silver, and platinum, burn, or oxydize more or less rapidly,
when they are put in contact with oxygen; for, besides those lively
combustions, in which metals, or other materials, become hot and are
maintained in a state of incandescence, there are other kinds of burning
which may be called slow combustions. You have often had under your
eyes, without knowing it, examples of these slow combustions. For
example, you have seen bits of iron left in the air, or in the water,
and covered with a dark-red or light-red matter."
"That is rust," said Miette.
"Yes, that is what they call rust; and this rust is nothing less than
the product of the combustion of the iron. The oxygen which is found in
the air, or the water, has come in contact with the bit of iron and has
made it burn. It is a slow combustion, without flames, but it
nevertheless releases some heat. Verdigris, in some of its forms, is
nothing less than the product of the combustion----"
"Of copper," interrupted Miette again.
"Miette has said it. These metals burn when they come in contact with
the oxygen of the air,--or, in the language of science, they are
oxydized; and this oxydation is simple combustion. Therefore, oxygen is
the principal agent in combustion. The process which we call burning is
due to the oxygen uniting itself to some combustible body. There is no
doubt on that subject, for it has been found that the weight of the
products of combustion is equal to the sum of the weight of the body
which burns and that of the oxygen which combines with it. In the
experiment which we have made, if the oxygen has diminished in the
globe, if it seems to have disappeared, it is because it has united
itself and combined with the carbon of the candle to form the flame. In
the same way in Lavoisier's experiment it had combined itself with the
mercury to form the red pellicles. The candle had gone out when all the
oxygen in the globe had been absorbed; the red pellicles had ceased to
form when they found no more oxygen. In this way Lavoisier discovered
that the air was formed of a mixture of two gases: the first was oxygen,
of which we have just spoken; the second was nitrogen. The nitrogen,
which is also a colorless, odorless, and tasteless gas, possesses some
qualities that are precisely contrary to those of oxygen. Oxygen is the
agent of combustion. Nitrogen extinguishes bodies in combustion. Oxygen
is a gas indispensable to our existence, with which our lungs breathe,
and which revives our being. The nitrogen, on the contrary, contains no
properties that are directly useful to the body. Animals placed in a
globe full of nitrogen perish of asphyxia. In other words, they drown in
the gas, or are smothered by it. I suppose you will ask me what is the
use of this gas, and why it enters into the composition of the air? You
will ask it with all the more curiosity when you know that the air
contains four times as much nitrogen as oxygen; to be exact, a hundred
cubic feet of air contains seventy-nine cubic feet of nitrogen and
twenty-one cubic feet of oxygen. Now, the important part that nitrogen
plays is to moderate the action of the oxygen in respiration. You may
compare this nitrogen mixed with oxygen to the water which you put in a
glass of wine to temper it. Nitrogen possesses also another property
which is more general: it is one of the essential elements in a certain
number of mineral and vegetable substances and the larger portion of
animal substances. There are certain compounds containing nitrogen which
are indispensable to our food. An animal nourished entirely on food
which is destitute of nitrogen would become weak and would soon die."
"Excuse me, Monsieur Roger," said Albert Dalize: "how can nitrogen enter
into our food?"
"That is a very good question," added Miette, laughing; "surely you
cannot eat nitrogen and you cannot eat gas."
"The question is indeed a very sensible one," answered Monsieur Roger;
"but this is how nitrogen enters into our food. We are carnivorous, are
we not? we eat meat and flesh of animals. And what flesh do we chiefly
eat? The flesh of sheep and of cattle. Sheep and cattle are herbivorous:
they feed on herbs, on vegetables. Now, vegetables contain nitrogen.
They have taken this nitrogen, either directly or indirectly, from the
atmosphere and have fixed it in their tissues. Herbivorous animals, in
eating vegetables, eat nitrogen, and we, who are carnivorous, we also
eat nitrogen, since we eat the herbivorous animals. We also eat
vegetable food, many kinds of which contain more or less nitrogen. Do
you understand?"
"Yes, I understand," said Miette.
"There is nobody living who really understands this matter very well,
for it is an extremely obscure, though very important, subject," replied
Monsieur Roger. "But, to resume our explanation. Besides oxygen and
nitrogen, there is also in the air a little carbonic acid and vapor. The
carbonic acid will bring us back to the point from which we
started,--the phenomenon of breathing. Carbonic acid is a gas formed by
oxygen and carbon. The carbon is a body which is found under a large
variety of forms. It has two or more varieties,--it is either pure or
mixed with impurities. Its varieties can be united in two groups. The
first group comprises the diamond and graphite, or plumbago, which are
natural carbon. The second group comprises coal, charcoal, and the soot
of a chimney, which we may call, for convenience, artificial carbon.
When oxygen finds itself in contact with carbonaceous matter,--that is
to say, with matter that contains carbon,--and when the surrounding
temperature has reached the proper degree of heat, carbonic acid begins
to be formed. In the oven and the furnace, coal and charcoal mingle with
the oxygen of the air and give the necessary heat; but it is first
necessary that by the aid of a match, paper, and kindling-wood you
should have furnished the temperature at which oxygen can join with the
carbon in order to burn it. That is what we may call an active or a
live combustion; but there can also be a slow combustion of carbon,--a
combustion without flame, and still giving out heat. It is this
combustion which goes on in our body by means of respiration."
"Ah, now we have come around to it!" cried Miette. "That is the very
thing I was inquiring about."
"Well, now that we have come around to it," answered Monsieur Roger,
"tell me what I began to say to you on the subject of respiration."
"That is not very difficult," answered Miette, in her quiet manner. "You
told us that we swallowed oxygen and gave out carbonic acid; and you
also said, 'Whence comes this carbonic acid? From combustion.' That is
why I said, just now, 'We have come around to it.'"
"Very good,--very good, indeed, only we do not _swallow_ oxygen, but we
_inhale_ it," said Monsieur Dalize, charmed with the cleverness of his
little girl.
"What, then, is the cause of this production of carbonic acid?"
continued Monsieur Roger. "You don't know? Well, I am going to tell you.
The oxygen of the air which we breathe arrives into our lungs and finds
itself in contact with the carbon in the black or venous blood. The
carbon contained here joins with the oxygen, and forms the carbonic acid
which we breathe out. This is a real, a slow combustion which takes
place not only in our lungs,--as I said at first, in order not to make
the explanation too difficult,--but also in all the different portions
of our body. The air composed of oxygen and nitrogen--for the nitrogen
enters naturally with the oxygen--penetrates into the pulmonary cells,
spreads itself through the blood, and is borne through the numberless
little capillary vessels. It is in these little vessels that combustion
takes place,--that is to say, that the oxygen unites with the carbon and
that carbonic acid is formed. This carbonic acid circulates, dissolved
in the blood, until it can escape out of it. It is in the lungs that it
finds liberty. When it arrives there it escapes from the blood, is
exhaled, and is at once replaced by the new oxygen and the new nitrogen
which arrive from outside. The nitrogen absorbed in aspiration at the
same time as the oxygen is found to be of very much the same quantity
when it goes out. There has therefore been no appreciable absorption of
nitrogen. Now, this slow combustion causes the heat of our body; in
fact, what is called the animal-heat is due to the caloric set free at
the moment when the oxygen is converted into carbonic acid, in the same
way as in all combustion of carbon. In conclusion, I will remind you
that our digestion is exercised on two sorts of food,--nitrogenous food
and carbonaceous food. Nitrogenous food--like fibrin, which is the chief
substance in flesh; albumen, which is the principal substance of the
egg; caseine, the principal substance of milk; legumine, of peas and
beans--is assimilated in our organs, which they regenerate, which they
rebuild continually. Carbonaceous foods--like the starch of the potato,
of sugar, alcohol, oils, and the fat of animals--do not assimilate; they
do not increase at all the substance of our muscles or the solidity of
our bones. It is they which are burned and which aid in burning those
waste materials of the venous blood of which I have already spoken.
Still, many starchy foods do contain some nutritive principles, but in
very small quantity. You will understand how little when you know that
you would have to eat about fifteen pounds of potatoes to give your body
the force that would be given it by a single pound of beef."
"Oh," said Miette, "I don't like beef; but fifteen pounds of
potatoes,--I would care still less to eat so much at once."
"All the less that they would fatten you perceptibly," replied Monsieur
Roger; "in fact, it is the carbonaceous foods which fatten. If they are
introduced into the body in too great a quantity, they do not find
enough oxygen to burn them, and they are deposited in the adipose or
fatty tissue, where they will be useless and often harmful. You see how
indispensable oxygen is to human life, and you now understand that if
respiration does not go on with regularity, if the oxygen of your room
should become exhausted, if the lungs were filled with carbonic acid
produced by the combustion of fuel outside the body, there would follow
at first a great deal of difficulty in breathing, then fainting, torpor,
and, finally, asphyxia."
These last words, pronounced by Monsieur Roger with much emotion,
brought before them a remembrance so recent and so terrible that all
remained silent and thoughtful. It was Miss Miette who first broke the
spell by asking a new question of her friend Roger. Asphyxia had
recalled to her the fire. Then she had thought of the manner of
extinguishing fire, and she said, all of a sudden, her idea translating
itself upon her lips almost without consciousness,--
"Why does water extinguish fire?"
Monsieur Roger, drawn out of his thoughts by this question, raised his
head, looked at Miette, and said to her,--
"In the first place, do you know what water is?"
"No; but you were going to tell me."
"All right. The celebrated Lavoisier, after having shown that air is not
a simple body, but that it is composed of two gases, next turned his
attention to the study of water, which was also, up to that time,
considered to be an element; that is, a simple body. He studied it so
skilfully that he succeeded in showing that water was formed by the
combination of two gases."
"Of two gases!--water?" cried Miette.
"Certainly, of two gases. One of these gases is oxygen, which we have
already spoken of, and the other is hydrogen."
"Which we are going to speak of," added Miette.
"Of course," answered Monsieur Roger, "since you wish it. But it was not
Lavoisier, however, who first discovered hydrogen. This gas had been
discovered before his time by the chemists Paracelsus and Boyle, who had
found out that in placing iron or zinc in contact with an acid called
sulphuric acid, there was disengaged an air "like a breath." This air
"like a breath" is what we now call hydrogen. Lavoisier, with the
assistance of the chemist Meusnier, proved that it was this gas which in
combining with oxygen formed water. In order to do this he blew a
current of hydrogen into a retort filled with oxygen. As this hydrogen
penetrated into the retort, he set fire to it by means of electric
sparks. Two stop-cocks regulated the proper proportions of the oxygen
and the hydrogen in the retort. When the combustion took place, they saw
water form in drops upon the sides of the retort and unite at the
bottom. Water was therefore the product of the combination of hydrogen
with oxygen. The following anecdote is told in regard to this
combination. A chemist of the last century, who was fond of flattery,
was engaged to give some lessons to a young prince of the blood royal.
When he came to explain the composition of water, he prepared before his
scholar the necessary apparatus for making the combination of hydrogen
and oxygen, and, at the moment when he was about to send the electric
spark into the retort, he said, bowing his head,--
[Illustration]
"'If it please your Royal Highness, this hydrogen and oxygen are about
to have the honor of combining before you.'
"I don't know if the hydrogen and the oxygen were aware of the honor
which was being done them; but certainly they combined with no more
manners than if their spectator was an ordinary boy. Now, I may add, you
must not confound combinations with mixtures; thus, air is a mixture of
oxygen and nitrogen, while water is a combination of hydrogen and
oxygen. This combination is a union of the molecules of the two gases
which produces a composite body formed of new molecules. These new
molecules are water. Now, this last word recalls to me Miette's
question."
"Yes," said the latter: "why does water put out fire?"
"There are two reasons for this phenomenon," said Monsieur Roger: "the
first is that water thrown upon the fire forms around the matter in
combustion a thick cloud, or vapor, which prevents the air from reaching
it. The wood, which was burning--that is to say, which was mingling with
the oxygen of the air--finds its communication intercepted. The humid
vapor has interposed between the carbon of the wood and the oxygen of
the air; therefore, the combustion is forced to stop. Further, water
falling upon the fire is transformed, as you very well know, into vapor,
or steam. Now, this conversion into vapor necessitates the taking up of
a certain quantity of heat. This heat is taken away from the body which
is being burned, and that body is thus made much cooler; the combustion
therefore becomes less active, and the fire is at last extinguished."
"Very good," said Miette; "but still another question, and I will let
you alone."
"You promise?"
"Yes."
"Well, then, what is your last question?"
"Why is a candle put out by blowing on it, and why do they light a fire
by doing the same thing?"
"In these two cases there are two very different actions," replied
Monsieur Roger: "in the first there is a mechanical action, and in the
second a chemical action. In blowing upon a candle the violence of the
air which you send out of your mouth detaches a flame which holds on
only to the wick. The burning particles of this wick are blown away, and
consequently the combustion is stopped. But the case is very different
when you blow with a bellows or with your mouth upon the fire in the
stove. There the substance in combustion, whether wood or coal, is a
mass large enough to resist the violence of the current of air you throw
in, and it profits from the air which you send to it so abundantly, by
taking the oxygen which it contains and burning up still more briskly.
"Now, that is the answer to your last question; and I must beg you to
remember your promise, and ask me no more hard questions to-night."
"Yes, friend Roger," said Miette, "I will leave you alone; you may go to
sleep."
"And it will be a well-earned sleep," added Madame Dalize, with the
assent of every one.
[Illustration]
[Illustration]