accidental, than as forming any of its component parts; and the
proportion they bear to the whole mass is quite inconsiderable.
ATMOSPHERICAL AIR is composed of two gasses, known by the names of
OXYGEN GAS and NITROGEN or AZOTIC GAS.
EMILY.
Pray what is a gas?
MRS. B.
The name of gas is given to any fluid capable of existing constantly in
an aeriform state, under the pressure and at the temperature of the
atmosphere.
CAROLINE.
Is not water, or any other substance, when evaporated by heat, called
gas?
MRS. B.
No, my dear; vapour is, indeed, an elastic fluid, and bears a strong
resemblance to a gas; there are, however, several points in which they
essentially differ, and by which you may always distinguish them. Steam,
or vapour, owes its elasticity merely to a high temperature, which is
equal to that of boiling water. And it differs from boiling water only
by being united with more caloric, which, as we before explained, is in
a latent state. When steam is cooled, it instantly returns to the form
of water; but air, or gas, has never yet been rendered liquid or solid
by any degree of cold.
EMILY.
But does not gas, as well as vapour, owe its elasticity to caloric?
MRS. B.
It was the prevailing opinion; and the difference of gas or vapour was
thought to depend on the different manner in which caloric was united
with the basis of these two kinds of elastic fluids. In vapour, it was
considered as in a latent state; in gas, it was said to be chemically
combined. But the late researches of Sir H. Davy have given rise to a
new theory respecting gasses; and there is now reason to believe that
these bodies owe their permanently elastic state, not solely to caloric,
but likewise to the prevalence of either the one or the other of the two
electricities.
EMILY.
When you speak, then, of the simple bodies oxygen and nitrogen, you mean
to express those substances which are the basis of the two gasses?
MRS. B.
Yes, in strict propriety, for they can properly be called gasses only
when brought to an aeriform state.
CAROLINE.
In what proportions are they combined in the atmosphere?
MRS. B.
The oxygen gas constitutes a little more than one-fifth, and the
nitrogen gas a little less than four-fifths. When separated, they are
found to possess qualities totally different from each other. For oxygen
gas is essential both to respiration and combustion, while neither of
these processes can be performed in nitrogen gas.
CAROLINE.
But if nitrogen gas is unfit for respiration, how does it happen that
the large proportion of it which enters into the composition of the
atmosphere is not a great impediment to breathing?
MRS. B.
We should breathe more freely than our lungs could bear, if we respired
oxygen gas alone. The nitrogen is no impediment to respiration, and
probably, on the contrary, answers some useful purpose, though we do not
know in what manner it acts in that process.
EMILY.
And by what means can the two gasses, which compose the atmospheric air,
be separated?
MRS. B.
There are many ways of analysing the atmosphere: the two gasses may be
separated first by combustion.
EMILY.
You surprise me! how is it possible that combustion should separate
them?
MRS. B.
I should previously remind you that oxygen is supposed to be the only
simple body naturally combined with negative electricity. In all the
other elements the positive electricity prevails, and they have
consequently, all of them, an attraction for oxygen.*
[Footnote *: If chlorine or oxymuriatic gas be a simple body,
according to Sir H. Davy’s view of the subject, it must be
considered as an exception to this statement; but this subject
cannot be discussed till the properties and nature of chlorine
come under examination.]
CAROLINE.
Oxygen the only negatively electrified body! that surprises me
extremely; how then are the combinations of the other bodies performed,
if, according to your explanation of chemical attraction, bodies are
supposed only to combine in virtue of their opposite states of
electricity?
MRS. B.
Observe that I said, that oxygen was the only _simple_ body, naturally
negative. Compound bodies, in which oxygen prevails over the other
component parts, are also negative, but their negative energy is greater
or less in proportion as the oxygen predominates. Those compounds into
which oxygen enters in less proportion than the other constituents, are
positive, but their positive energy is diminished in proportion to the
quantity of oxygen which enters into their composition.
All bodies, therefore, that are not already combined with oxygen, will
attract it, and, under certain circumstances, will absorb it from the
atmosphere, in which case the nitrogen gas will remain alone, and may
thus be obtained in its separate state.
CAROLINE.
I do not understand how a gas can be absorbed?
MRS. B.
It is only the oxygen, or basis of the gas, which is absorbed; and the
two electricities escaping, that is to say, the negative from the
oxygen, the positive from the burning body, unite and produce caloric.
EMILY.
And what becomes of this caloric?
MRS. B.
We shall make this piece of dry wood attract oxygen from the atmosphere,
and you will see what becomes of the caloric.
CAROLINE.
You are joking, Mrs. B--; you do not mean to decompose the atmosphere
with a piece of dry stick?
MRS. B.
Not the whole body of the atmosphere, certainly; but if we can make this
piece of wood attract any quantity of oxygen from it, a proportional
quantity of atmospherical air will be decomposed.
CAROLINE.
If wood has so strong an attraction for oxygen, why does it not
decompose the atmosphere spontaneously?
MRS. B.
It is found by experience, that an elevation of temperature is required
for the commencement of the union of the oxygen and the wood.
This elevation of temperature was formerly thought to be necessary, in
order to diminish the cohesive attraction of the wood, and enable the
oxygen to penetrate and combine with it more readily. But since the
introduction of the new theory of chemical combination, another cause
has been assigned, and it is now supposed that the high temperature, by
exalting the electrical energies of bodies, and consequently their force
of attraction, facilitates their combination.
EMILY.
If it is true, that caloric is composed of the two electricities, an
elevation of temperature must necessarily augment the electric energies
of bodies.
MRS. B.
I doubt whether that would be a necessary consequence; for, admitting
this composition of caloric, it is only by its being decomposed that
electricity can be produced. Sir H. Davy, however, in his numerous
experiments, has found it to be an almost invariable rule that the
electrical energies of bodies are increased by elevation of temperature.
What means then shall we employ to raise the temperature of the wood, so
as to enable it to attract oxygen from the atmosphere?
CAROLINE.
Holding it near the fire, I should think, would answer the purpose.
MRS. B.
It may, provided you hold it sufficiently close to the fire; for a very
considerable elevation of temperature is required.
CAROLINE.
It has actually taken fire, and yet I did not let it touch the coals,
but I held it so very close that I suppose it caught fire merely from
the intensity of the heat.
MRS. B.
Or you might say, in other words, that the caloric which the wood
imbibed, so much elevated its temperature, and exalted its electric
energy, as to enable it to attract oxygen very rapidly from the
atmosphere.
EMILY.
Does the wood absorb oxygen while it is burning?
MRS. B.
Yes, and the heat and light are produced by the union of the two
electricities which are set at liberty, in consequence of the oxygen
combining with the wood.
CAROLINE.
You astonish me! the heat of a burning body proceeds then as much from
the atmosphere as from the body itself?
MRS. B.
It was supposed that the caloric, given out during combustion, proceeded
entirely, or nearly so, from the decomposition of the oxygen gas; but,
according to Sir H. Davy’s new view of the subject, both the oxygen gas,
and the combustible body, concur in supplying the heat and light, by the
union of their opposite electricities.
EMILY.
I have not yet met with any thing in chemistry that has surprised or
delighted me so much as this explanation of combustion. I was at first
wondering what connection there could be between the affinity of a body
for oxygen and its combustibility; but I think I understand it now
perfectly.
MRS. B.
Combustion then, you see, is nothing more than the rapid combination of
a body with oxygen, attended by the disengagement of light and heat.
EMILY.
But are there no combustible bodies whose attraction for oxygen is so
strong, that they will combine with it, without the application of heat?
CAROLINE.
That cannot be; otherwise we should see bodies burning spontaneously.
MRS. B.
But there are some instances of this kind, such as phosphorus,
potassium, and some compound bodies, which I shall hereafter make you
acquainted with. These bodies, however, are prepared by art, for in
general, all the combustions that could occur spontaneously, at the
temperature of the atmosphere, have already taken place; therefore new
combustions cannot happen without the temperature of the body being
raised. Some bodies, however, will burn at a much lower temperature than
others.
CAROLINE.
But the common way of burning a body is not merely to approach it to one
already on fire, but rather to put the one in actual contact with the
other, as when I burn this piece of paper by holding it in the flame of
the fire.
MRS. B.
The closer it is in contact with the source of caloric, the sooner will
its temperature be raised to the degree necessary for it to burn. If you
hold it near the fire, the same effect will be produced; but more time
will be required, as you found to be the case with the piece of stick.
EMILY.
But why is it not necessary to continue applying caloric throughout the
process of combustion, in order to keep up the electric energy of the
wood, which is required to enable it to combine with the oxygen?
MRS. B.
The caloric which is gradually produced by the two electricities during
combustion, keeps up the temperature of the burning body; so that when
once combustion has begun, no further application of caloric is
required.
CAROLINE.
Since I have learnt this wonderful theory of combustion, I cannot take
my eyes from the fire; and I can scarcely conceive that the heat and
light, which I always supposed to proceed entirely from the coals, are
really produced as much by the atmosphere.
EMILY.
When you blow the fire, you increase the combustion, I suppose, by
supplying the coals with a greater quantity of oxygen gas?
MRS. B.
Certainly; but of course no blowing will produce combustion, unless the
temperature of the coals be first raised. A single spark, however, is
sometimes sufficient to produce that effect; for, as I said before, when
once combustion has commenced, the caloric disengaged is sufficient to
elevate the temperature of the rest of the body, provided that there be
a free access of oxygen. It however sometimes happens that if a fire be
ill made, it will be extinguished before all the fuel is consumed, from
the very circumstance of the combustion being so slow that the caloric
disengaged is insufficient to keep up the temperature of the fuel. You
must recollect that there are three things required in order to produce
combustion; a combustible body, oxygen, and a temperature at which the
one will combine with the other.
EMILY.
You said that combustion was one method of decomposing the atmosphere,
and obtaining the nitrogen gas in its simple state; but how do you
secure this gas, and prevent it from mixing with the rest of the
atmosphere?
MRS. B.
It is necessary for this purpose to burn the body within a close vessel,
which is easily done. --We shall introduce a small lighted taper (PLATE
VII. Fig. 1.) under this glass receiver, which stands in a bason over
water, to prevent all communication with the external air.
[Illustration: Plate VII. Vol. I. p. 181.
Fig. 1. Combustion of a taper under a receiver.
Fig. 2. A Retort on a stand.
Fig. 3. Preparation of oxygen gas.
A Furnace.
B Earthen Retort in the furnace.
C Water bath.
D Receiver.
E.E Tube conveying the gas from the Retort through the water into
the Receiver.
F.F.F Shelf perforated on which the Receiver stands.
Fig. 4. Combustion of iron wire in oxygen gas.]
CAROLINE.
How dim the light burns already! --It is now extinguished.
MRS. B.
Can you tell us why it is extinguished?
CAROLINE.
Let me consider. --The receiver was full of atmospherical air; the
taper, in burning within it, must have combined with the oxygen
contained in that air, and the caloric that was disengaged produced the
light of the taper. But when the whole of the oxygen was absorbed, the
whole of its electricity was disengaged; consequently no more caloric
could be produced, the taper ceased to burn, and the flame was
extinguished.
MRS. B.
Your explanation is perfectly correct.
EMILY.
The two constituents of the oxygen gas being thus disposed of, what
remains under the receiver must be pure nitrogen gas?
MRS. B.
There are some circumstances which prevent the nitrogen gas, thus
obtained, from being perfectly pure; but we may easily try whether the
oxygen has disappeared, by putting another lighted taper under it. --You
see how instantaneously the flame is extinguished, for want of oxygen to
supply the negative electricity required for the formation of caloric;
and were you to put an animal under the receiver, it would immediately
be suffocated. But that is an experiment which I do not think your
curiosity will tempt you to try.
EMILY.
Certainly not. --But look, Mrs. B., the receiver is full of a thick
white smoke. Is that nitrogen gas?
MRS. B.
No, my dear; nitrogen gas is perfectly transparent and invisible, like
common air. This cloudiness proceeds from a variety of exhalations,
which arise from the burning taper, and the nature of which you cannot
yet understand.
CAROLINE.
The water within the receiver has now risen a little above its level in
the bason. What is the reason of this?
MRS. B.
With a moment’s reflection, I dare say, you would have explained it
yourself. The water rises in consequence of the oxygen gas within it
having been destroyed, or rather decomposed, by the combustion of the
taper.
CAROLINE.
Then why did not the water rise immediately when the oxygen gas was
destroyed?
MRS. B.
Because the heat of the taper, whilst burning, produced a dilatation of
the air in the vessel, which at first counteracted this effect.
Another means of decomposing the atmosphere is the _oxygenation_ of
certain metals. This process is very analogous to combustion; it is,
indeed, only a more general term to express the combination of a body
with oxygen.
CAROLINE.
In what respect, then, does it differ from combustion?
MRS. B.
The combination of oxygen in combustion is always accompanied by a
disengagement of light and heat; whilst this circumstance is not a
necessary consequence of simple oxygenation.
CAROLINE.
But how can a body absorb oxygen without the combination of the two
electricities which produce caloric?
MRS. B.
Oxygen does not always present itself in a gaseous state; it is a
constituent part of a vast number of bodies, both solid and liquid, in
which it exists in a much denser state than in the atmosphere; and from
these bodies it may be obtained without much disengagement of caloric.
It may likewise, in some cases, be absorbed from the atmosphere without
any sensible production of light and heat; for, if the process be slow,
the caloric is disengaged in such small quantities, and so gradually,
that it is not capable of producing either light or heat. In this case
the absorption of oxygen is called _oxygenation_ or _oxydation_, instead
of _combustion_, as the production of sensible light and heat is
essential to the latter.
EMILY.
I wonder that metals can unite with oxygen; for, as they are so dense,
their attraction of aggregation must be very great; and I should have
thought that oxygen could never have penetrated such bodies.
MRS. B.
Their strong attraction for oxygen counterbalances this obstacle. Most
metals, however, require to be made red-hot before they are capable of
attracting oxygen in any considerable quantity. By this combination they
lose most of their metallic properties, and fall into a kind of powder,
formerly called _calx_, but now much more properly termed an _oxyd_;
thus we have _oxyd of lead_, _oxyd of iron_, &c.
EMILY.
And in the Voltaic battery, it is, I suppose, an oxyd of zinc, that is
formed by the union of the oxygen with that metal?
MRS. B.
Yes, it is.
CAROLINE.
The word oxyd, then, simply means a metal combined with oxygen?
MRS. B.
Yes; but the term is not confined to metals, though chiefly applied to
them. Any body whatever, that has combined with a certain quantity of
oxygen, either by means of oxydation or combustion, is called an _oxyd_,
and is said to be _oxydated_ or _oxygenated_.
EMILY.
Metals, when converted into oxyds, become, I suppose, negative?
MRS. B.
Not in general; because in most oxyds the positive energy of the metal
more than counterbalances the native energy of the oxygen with which it
combines.
This black powder is an oxyd of manganese, a metal which has so strong
an affinity for oxygen, that it attracts that substance from the
atmosphere at any known temperature: it is therefore never found in its
metallic form, but always in that of an oxyd, in which state, you see,
it has very little of the appearance of a metal. It is now heavier than
it was before oxydation, in consequence of the additional weight of the
oxygen with which it has combined.
CAROLINE.
I am very glad to hear that; for I confess I could not help having some
doubts whether oxygen was really a substance, as it is not to be
obtained in a simple and palpable state; but its weight is, I think,
a decisive proof of its being a real body.
MRS. B.
It is easy to estimate its weight, by separating it from the manganese,
and finding how much the latter has lost.
EMILY.
But if you can take the oxygen from the metal, shall we not then have it
in its palpable simple state?
MRS. B.
No; for I can only separate the oxygen from the manganese, by presenting
to it some other body, for which it has a greater affinity than for the
manganese. Caloric affording the two electricities is decomposed, and
one of them uniting with the oxygen, restores it to the aëriform state.
EMILY.
But you said just now, that manganese would attract oxygen from the
atmosphere in which it is combined with the negative electricity; how,
therefore, can the oxygen have a superior affinity for that electricity,
since it abandons it to combine with the manganese?
MRS. B.
I give you credit for this objection, Emily; and the only answer I can
make to it is, that the mutual affinities of metals for oxygen, and of
oxygen for electricity, vary at different temperatures; a certain degree
of heat will, therefore, dispose a metal to combine with oxygen, whilst,
on the contrary, the former will be compelled to part with the latter,
when the temperature is further increased. I have put some oxyd of
manganese into a retort, which is an earthen vessel with a bent neck,
such as you see here. (PLATE VII. Fig. 2.) --The retort containing the
manganese you cannot see, as I have enclosed it in this furnace, where
it is now red-hot. But, in order to make you sensible of the escape of
the gas, which is itself invisible, I have connected the neck of the
retort with this bent tube, the extremity of which is immersed in this
vessel of water. (PLATE VII. Fig. 3.) --Do you see the bubbles of air
rise through the water?
CAROLINE.
Perfectly. This, then, is pure oxygen gas; what a pity it should be
lost! Could you not preserve it?
MRS. B.
We shall collect it in this receiver. --For this purpose, you observe,
I first fill it with water, in order to exclude the atmospherical air;
and then place it over the bubbles that issue from the retort, so as to
make them rise through the water to the upper part of the receiver.
EMILY.
The bubbles of oxygen gas rise, I suppose, from their specific levity?
MRS. B.
Yes; for though oxygen forms rather a heavy gas, it is light compared to
water. You see how it gradually displaces the water from the receiver.
It is now full of gas, and I may leave it inverted in water on this
shelf, where I can keep the gas as long as I choose, for future
experiments. This apparatus (which is indispensable in all experiments
in which gases are concerned) is called a water-bath.
CAROLINE.
It is a very clever contrivance, indeed; equally simple and useful. How
convenient the shelf is for the receiver to rest upon under water, and
the holes in it for the gas to pass into the receiver! I long to make
some experiments with this apparatus.
MRS. B.
I shall try your skill that way, when you have a little more experience.
I am now going to show you an experiment, which proves, in a very
striking manner, how essential oxygen is to combustion. You will see
that iron itself will burn in this gas, in the most rapid and brilliant
manner.
CAROLINE.
Really! I did not know that it was possible to burn iron.
EMILY.
Iron is a simple body, and you know, Caroline, that all simple bodies
are naturally positive, and therefore must have an affinity for oxygen.
MRS. B.
Iron will, however, not burn in atmospherical air without a very great
elevation of temperature; but it is eminently combustible in pure oxygen
gas; and what will surprise you still more, it can be set on fire
without any considerable rise of temperature. You see this spiral iron
wire--I fasten it at one end to this cork, which is made to fit an
opening at the top of the glass-receiver. (PLATE VII. Fig. 4.)
EMILY.
I see the opening in the receiver; but it is carefully closed by a
ground glass-stopper.
MRS. B.
That is in order to prevent the gas from escaping; but I shall take out
the stopper, and put in the cork, to which the wire hangs. --Now I mean
to burn this wire in the oxygen gas, but I must fix a small piece of
lighted tinder to the extremity of it, in order to give the first
impulse to combustion; for, however powerful oxygen is in promoting
combustion, you must recollect that it cannot take place without some
elevation of temperature. I shall now introduce the wire into the
receiver, by quickly changing the stoppers.
CAROLINE.
Is there no danger of the gas escaping while you change the stoppers?
MRS. B.
Oxygen gas is a little heavier than atmospherical air, therefore it will
not mix with it very rapidly; and, if I do not leave the opening
uncovered, we shall not lose any----
CAROLINE.
Oh, what a brilliant and beautiful flame!
EMILY.
It is as white and dazzling as the sun! --Now a piece of the melted wire
drops to the bottom: I fear it is extinguished; but no, it burns again
as bright as ever.
MRS. B.
It will burn till the wire is entirely consumed, provided the oxygen is
not first expended: for you know it can burn only while there is oxygen
to combine with it.
CAROLINE.
I never saw a more beautiful light. My eyes can hardly bear it! How
astonishing to think that all this caloric was contained in the small
quantity of gas and iron that was enclosed in the receiver; and that,
without producing any sensible heat!
CAROLINE.
How wonderfully quick combustion goes on in pure oxygen gas! But pray,
are these drops of burnt iron as heavy as the wire was before?
MRS. B.
They are even heavier; for the iron, in burning, has acquired exactly
the weight of the oxygen which has disappeared, and is now combined with
it. It has become an oxyd of iron.
CAROLINE.
I do not know what you mean by saying that the oxygen has _disappeared_,
Mrs. B., for it was always invisible.
MRS. B.
True, my dear; the expression was incorrect. But though you could not
see the oxygen gas, I believe you had no doubt of its presence, as the
effect it produced on the wire was sufficiently evident.
CAROLINE.
Yes, indeed; yet you know it was the caloric, and not the oxygen gas
itself, that dazzled us so much.
MRS. B.