extracted from bones, in which it is contained in the state of phosphat
of lime; from this salt the phosphoric acid is separated by means of the
sulphuric, which combines with the lime. In its pure state, phosphoric
acid is either liquid or solid, according to its degree of
concentration.
Among the salts formed by this acid, _phosphat of lime_ is the only one
that affords much interest; and this, we have already observed,
constitutes the basis of all bones. It is also found in very small
quantities in some vegetables.
CONVERSATION XVIII.
OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATIONS OF OXYGEN WITH
NITROGEN AND CARBON; AND OF THE NITRATS AND CARBONATS.
MRS. B.
I am almost afraid of introducing the subject of the NITRIC ACID, as I
am sure that I shall be blamed by Caroline for not having made her
acquainted with it before.
CAROLINE.
Why so, Mrs. B.?
MRS. B.
Because you have long known its radical, which is nitrogen or azote; and
in treating of that element, I did not even hint that it was the basis
of an acid.
CAROLINE.
And what could be your reason for not mentioning this acid sooner?
MRS. B.
I do not know whether you will think the reason sufficiently good to
acquit me; but the omission, I assure you, did not proceed from
negligence. You may recollect that nitrogen was one of the first simple
bodies which we examined; you were then ignorant of the theory of
combustion, which I believe was, for the first time, mentioned in that
lesson; and therefore it would have been in vain, at that time, to have
attempted to explain the nature and formation of acids.
CAROLINE.
I wonder, however, that it never occurred to us to enquire whether
nitrogen could be acidified; for, as we knew it was classed among the
combustible bodies, it was natural to suppose that it might produce an
acid.
MRS. B.
That is not a necessary consequence; for it might combine with oxygen
only in the degree requisite to form an oxyd. But you will find that
nitrogen is susceptible of various degrees of oxygenation, some of which
convert it merely into an oxyd, and others give it all the acid
properties.
The acids, resulting from the combination of oxygen and nitrogen, are
called the NITROUS and NITRIC acids. We will begin with the NITRIC, in
which nitrogen is in the highest state of oxygenation. This acid
naturally exists in the form of gas; but is so very soluble in water,
and has so great an affinity for it, that one grain of water will absorb
and condense ten grains of acid gas, and form the limpid fluid which you
see in this bottle.
CAROLINE.
What a strong offensive smell it has!
MRS. B.
This acid contains a greater abundance of oxygen than any other, but it
retains it with very little force.
EMILY.
Then it must be a powerful caustic, both from the facility with which it
parts with its oxygen, and the quantity which it affords?
MRS. B.
Very well, Emily; both cause and effect are exactly such as you
describe: nitric acid burns and destroys all kinds of organised matter.
It even sets fire to some of the most combustible substances. --We shall
pour a little of it over this piece of dry warm charcoal--you see it
inflames it immediately; it would do the same with oil of turpentine,
phosphorus, and several other very combustible bodies. This shows you
how easily this acid is decomposed by combustible bodies, since these
effects must depend upon the absorption of its oxygen.
Nitric acid has been used in the arts from time immemorial, but it is
only within these twenty-five years that its chemical nature has been
ascertained. The celebrated Mr. Cavendish discovered that it consisted
of about 10 parts of nitrogen and 25 of oxygen.* These principles, in
their gaseous state, combine at a high temperature; and this may be
effected by repeatedly passing the electrical spark through a mixture of
the two gases.
[Footnote *: The proportion stated by Sir H. Davy, in his Chemical
Researches, is as 1 to 2.389.]
EMILY.
The nitrogen and oxygen gases, of which the atmosphere is composed, do
not combine, I suppose, because their temperature is not sufficiently
elevated?
CAROLINE.
But in a thunder-storm, when the lightning repeatedly passes through
them, may it not produce nitric acid? We should be in a strange
situation, if a violent storm should at once convert the atmosphere into
nitric acid.
MRS. B.
There is no danger of it, my dear; the lightning can affect but a very
small portion of the atmosphere, and though it were occasionally to
produce a little nitric acid, yet this never could happen to such an
extent as to be perceivable.
EMILY.
But how could the nitric acid be known, and used, before the method of
combining its constituents was discovered?
MRS. B.
Before that period the nitric acid was obtained, and it is indeed still
extracted, for the common purposes of art, from the compound salt which
it forms with potash, commonly called _nitre_.
CAROLINE.
Why is it so called? Pray, Mrs. B., let these old unmeaning names be
entirely given up, by us at least; and let us call this salt _nitrat of
potash_.
MRS. B.
With all my heart; but it is necessary that I should, at least, mention
the old names, and more especially those which are yet in common use;
otherwise, when you meet with them, you would not be able to understand
their meaning.
EMILY.
And how is the acid obtained from this salt?
MRS. B.
By the intervention of sulphuric acid, which combines with the potash,
and sets the nitric acid at liberty. This I can easily show you, by
mixing some nitrat of potash and sulphuric acid in this retort, and
heating it over a lamp; the nitric acid will come over in the form of
vapour, which we shall collect in a glass bell. This acid, diluted in
water, is commonly called _aqua fortis_, if Caroline will allow me to
mention that name.
CAROLINE.
I have often heard that aqua fortis will dissolve almost all metals; it
is no doubt because it yields its oxygen so easily.
MRS. B.
Yes; and from this powerful solvent property, it derived the name of
aqua fortis, or strong water. Do you not recollect that we oxydated, and
afterwards dissolved, some copper in this acid?
EMILY.
If I remember right, the nitrat of copper was the first instance you
gave us of a compound salt.
CAROLINE.
Can the nitric acid be completely decomposed and converted into nitrogen
and oxygen?
EMILY.
That cannot be the case, Caroline; since the acid can be decomposed only
by the combination of its constituents with other bodies.
MRS. B.
True; but caloric is sufficient for this purpose. By making the acid
pass through a red hot porcelain tube, it is decomposed; the nitrogen
and oxygen regain the caloric which they had lost in combining, and are
thus both restored to their gaseous state.
The nitric acid may also be partly decomposed, and is by this means
converted into NITROUS ACID.
CAROLINE.
This conversion must be easily effected, as the oxygen is so slightly
combined with the nitrogen.
MRS. B.
The partial decomposition of nitric acid is readily effected by most
metals; but it is sufficient to expose the nitric acid to a very strong
light to make it give out oxygen gas, and thus be converted into nitrous
acid. Of this acid there are various degrees, according to the
proportions of oxygen which it contains; the strongest, and that into
which the nitric is first converted, is of a yellow colour, as you see
in this bottle.
CAROLINE.
How it fumes when the stopper is taken out!
MRS. B.
The acid exists naturally in a gaseous state, and is here so strongly
concentrated in water, that it is constantly escaping.
Here is another bottle of nitrous acid, which, you see, is of an orange
red; this acid is weaker, the nitrogen being combined with a smaller
quantity of oxygen; and with a still less proportion of oxygen it is an
olive-green colour, as it appears in this third bottle. In short, the
weaker the acid, the deeper is its colour.
Nitrous acid acts still more powerfully on some inflammable substances
than the nitric.
EMILY.
I am surprised at that, as it contains less oxygen.
MRS. B.
But, on the other hand, it parts with its oxygen much more readily: you
may recollect that we once inflamed oil with this acid.
The next combinations of nitrogen and oxygen form only oxyds of
nitrogen, the first of which is commonly called _nitrous air_; or more
properly _nitric oxyd gas_. This may be obtained from nitric acid, by
exposing the latter to the action of metals, as in dissolving them it
does not yield the whole of its oxygen, but retains a portion of this
principle sufficient to convert it into this peculiar gas, a specimen of
which I have prepared, and preserved within this inverted glass bell.
EMILY.
It is a perfectly invisible elastic fluid.
MRS. B.
Yes; and it may be kept any length of time in this manner over water, as
it is not, like the nitric and nitrous acids, absorbable by it. It is
rather heavier than atmospherical air, and is incapable of supporting
either combustion or respiration. I am going to incline the glass gently
on one side, so as to let some of the gas escape--
EMILY.
How very curious! --It produces orange fumes like the nitrous acid! that
is the more extraordinary, as the gas within the glass is perfectly
invisible.
MRS. B.
It would give me much pleasure if you could make out the reason of this
curious change without requiring any further explanation.
CAROLINE.
It seems, by the colour and smell, as if it were converted into nitrous
acid gas: yet that cannot be, unless it combines with more oxygen; and
how can it obtain oxygen the very instant it escapes from the glass?
EMILY.
From the atmosphere, no doubt. Is it not so, Mrs. B.?
MRS. B.
You have guessed it; as soon as it comes in contact with the atmosphere,
it absorbs from it the additional quantity of oxygen necessary to
convert it into nitrous acid gas. And, if I now remove the bottle
entirely from the water, so as to bring at once the whole of the gas
into contact with the atmosphere, this conversion will appear still more
striking--
EMILY.
Look, Caroline, the whole capacity of the bottle is instantly tinged of
an orange colour!
MRS. B.
Thus, you see, it is the most easy process imaginable to convert
_nitrous oxyd gas_ into _nitrous acid gas_. The property of attracting
oxygen from the atmosphere, without any elevation of temperature, has
occasioned this gaseous oxyd being used as a test for ascertaining the
degree of purity of the atmosphere. I am going to show you how it is
applied to this purpose. --You see this graduated glass tube, which is
closed at one end, (PLATE X. Fig. 2.) --I first fill it with water, and
then introduce a certain measure of nitrous gas, which, not being
absorbable by water, passes through it, and occupies the upper part of
the tube. I must now add rather above two-thirds of oxygen gas, which
will just be sufficient to convert the nitrous oxyd gas into nitrous
acid gas.
CAROLINE.
So it has! --I saw it turn of an orange colour; but it immediately
afterwards disappeared entirely, and the water, you see, has risen, and
almost filled the tube.
MRS. B.
That is because the acid gas is absorbable by water, and in proportion
as the gas impregnates the water, the latter rises in the tube. When the
oxygen gas is very pure, and the required proportion of nitrous oxyd gas
very exact, the whole is absorbed by the water; but if any other gas be
mixed with the oxygen, instead of combining with the nitrous oxygen, it
will remain and occupy the upper part of the tube; or, if the gases be
not in the due proportion, there will be a residue of that which
predominates. --Before we leave this subject, I must not forget to
remark that nitrous acid may be formed by dissolving nitrous oxyd gas in
nitric acid. This solution may be effected simply by making bubbles of
nitrous oxyd gas pass through nitric acid.
EMILY.
That is to say, that nitrogen at its highest degree of oxygenation,
being mixed with nitrogen at its lowest degree of oxygenation, will
produce a kind of intermediate substance, which is nitrous acid.
MRS. B.
You have stated the fact with great precision. --There are various other
methods of preparing nitrous oxyd, and of obtaining it from compound
bodies; but it is not necessary to enter into these particulars. It
remains for me only to mention another curious modification of
oxygenated nitrogen, which has been distinguished by the name of
_gaseous oxyd of nitrogen_. It is but lately that this gas has been
accurately examined, and its properties have been investigated chiefly
by Sir H. Davy. It has obtained also the name of _exhilarating_ gas,
from the very singular property which that gentleman has discovered in
it, of elevating the animal spirits, when inhaled into the lungs, to a
degree sometimes resembling delirium or intoxication.
CAROLINE.
Is it respirable, then?
MRS. B.
It can scarcely be called respirable, as it would not support life for
any length of time; but it may be breathed for a few moments without any
other effects, than the singular exhilaration of spirits I have just
mentioned. It affects different people, however, in a very different
manner. Some become violent, even outrageous: others experience a
languor, attended with faintness; but most agree in opinion, that the
sensations it excites are extremely pleasant.
CAROLINE.
I think I should like to try it--how do you breathe it?
MRS. B.
By collecting the gas in a bladder, to which a short tube with a
stop-cock is adapted; this is applied to the mouth with one hand, whilst
the nostrils are kept closed with the other, that the common air may
have no access. You then alternately inspire, and expire the gas, till
you perceive its effects. But I cannot consent to your making the
experiment; for the nerves are sometimes unpleasantly affected by it,
and I would not run any risk of that kind.
EMILY.
I should like, at least, to see somebody breathe it; but pray by what
means is this curious gas obtained?
MRS. B.
It is procured from _nitrat of ammonia_, an artificial salt which yields
this gas on the application of a gentle heat. I have put some of the
salt into a retort, and by the aid of a lamp the gas will be
extricated.--
CAROLINE.
Bubbles of air begin to escape through the neck of the retort into the
water apparatus; will you not collect them?
MRS. B.
The gas that first comes over need not be preserved, as it consists of
little more than the common air that was in the retort; besides, there
is always in this experiment a quantity of watery vapour which must come
away before the nitrous oxyd appears.
EMILY.
Watery vapour! Whence does that proceed? There is no water in nitrat of
ammonia?
MRS. B.
You must recollect that there is in every salt a quantity of water of
crystallisation, which may be evaporated by heat alone. But, besides
this, water is actually generated in this experiment, as you will see
presently. First tell me, what are the constituent parts of nitrat of
ammonia?
EMILY.
Ammonia, and nitric acid: this salt, therefore, contains three different
elements, nitrogen and hydrogen, which produce the ammonia; and oxygen,
which, with nitrogen, forms the acid.
MRS. B.
Well then, in this process the ammonia is decomposed; the hydrogen quits
the nitrogen to combine with some of the oxygen of the nitric acid, and
forms with it the watery vapour which is now coming over. When that is
effected, what will you expect to find?
EMILY.
Nitrous acid instead of nitric acid, and nitrogen instead of ammonia.
MRS. B.
Exactly so; and the nitrous acid and nitrogen combine, and form the
gaseous oxyd of nitrogen, in which the proportion of oxygen is 37 parts
to 63 of nitrogen.
You may have observed, that for a little while no bubbles of air have
come over, and we have perceived only a stream of vapour condensing as
it issued into the water. --Now bubbles of air again make their
appearance, and I imagine that by this time all the watery vapour is
come away, and that we may begin to collect the gas. We may try whether
it is pure, by filling a phial with it, and plunging a taper into
it--yes, it will do now, for the taper burns brighter than in the common
air, and with a greenish flame.
CAROLINE.
But how is that? I thought no gas would support combustion but oxygen or
chlorine.
MRS. B.
Or any gas that contains oxygen, and is ready to yield it, which is the
case with this in a considerable degree; it is not, therefore,
surprising that it should accelerate the combustion of the taper.
You see that the gas is now produced in great abundance; we shall
collect a large quantity of it, and I dare say that we shall find some
of the family who will be curious to make the experiment of respiring
it. Whilst this process is going on, we may take a general survey of the
most important combinations of the nitric and nitrous acids with the
alkalies.
The first of these is _nitrat of potash_, commonly called _nitre_ or
_saltpetre_.
CAROLINE.
Is not that the salt with which gunpowder is made?
MRS. B.
Yes. Gunpowder is a mixture of five parts of nitre to one of sulphur,
and one of charcoal. --Nitre from its great proportion of oxygen, and
from the facility with which it yields it, is the basis of most
detonating compositions.
EMILY.
But what is the cause of the violent detonation of gunpowder when set
fire to?
MRS. B.
Detonation may proceed from two causes; the sudden formation or
destruction of an elastic fluid. In the first case, when either a solid
or liquid is instantaneously converted into an elastic fluid, the
prodigious and sudden expansion of the body strikes the air with great
violence, and this concussion produces the sound called detonation.
CAROLINE.
That I comprehend very well; but how can a similar effect be produced by
the destruction of a gas?
MRS. B.
A gas can be destroyed only by condensing it to a liquid or solid state;
when this takes place suddenly, the gas, in assuming a new and more
compact form, produces a vacuum, into which the surrounding air rushes
with great impetuosity; and it is by that rapid and violent motion that
the sound is produced. In all detonations, therefore, gases are either
suddenly formed, or destroyed. In that of gunpowder, can you tell me
which of these two circumstances takes place?
EMILY.
As gunpowder is a solid, it must, of course, produce the gases in its
detonation; but how, I cannot tell.
MRS. B.
The constituents of gunpowder, when heated to a certain degree, enter
into a number of new combinations, and are instantaneously converted
into a variety of gases, the sudden expansion of which gives rise to the
detonation.
CAROLINE.
And in what instance does the destruction or condensation of gases
produce detonation?
MRS. B.
I can give you one with which you are well acquainted; the sudden
combination of the oxygen and hydrogen gases.
CAROLINE.
True; I recollect perfectly that hydrogen detonates with oxygen when the
two gases are converted into water.
MRS. B.
But let us return to the nitrat of potash. --This salt is decomposed
when exposed to heat, and mixed with any combustible body, such as
carbon, sulphur, or metals, these substances oxydating rapidly at the
expense of the nitrat. I must show you an instance of this. --I expose
to the fire some of the salt in a small iron ladle, and, when it is
sufficiently heated, add to it some powdered charcoal; this will attract
the oxygen from the salt, and be converted into carbonic acid.--
EMILY.
But what occasions that crackling noise, and those vivid flashes that
accompany it?
MRS. B.
The rapidity with which the carbonic acid gas is formed occasions a
succession of small detonations, which, together with the emission of
flame, is called _deflagration_.
_Nitrat of ammonia_ we have already noticed, on account of the gaseous
oxyd of nitrogen which is obtained from it.
_Nitrat of silver_ is the lunar caustic, so remarkable for its property
of destroying animal fibre, for which purpose it is often used by
surgeons. --We have said so much on a former occasion, on the mode in
which caustics act on animal matter, that I shall not detain you any
longer on this subject.
We now come to the CARBONIC ACID, which we have already had many
opportunities of noticing. You recollect that this acid may be formed by
the combustion of carbon, whether in its imperfect state of charcoal, or
in its purest form of diamond. And it is not necessary, for this
purpose, to burn the carbon in oxygen gas, as we did in the preceding
lecture; for you need only light a piece of charcoal and suspend it
under a receiver on the water bath. The charcoal will soon be
extinguished, and the air in the receiver will be found mixed with
carbonic acid. The process, however, is much more expeditious if the
combustion be performed in pure oxygen gas.
CAROLINE.
But how can you separate the carbonic acid, obtained in this manner,
from the air with which it is mixed?
MRS. B.
The readiest mode is to introduce under the receiver a quantity of
caustic lime, or caustic alkali, which soon attracts the whole of the
carbonic acid to form a carbonat. --The alkali is found increased in
weight, and the volume of the air is diminished by a quantity equal to
that of the carbonic acid which was mixed with it.
EMILY.
Pray is there no method of obtaining pure carbon from carbonic acid?
MRS. B.
For a long time it was supposed that carbonic acid was not
decompoundable; but Mr. Tennant discovered, a few years ago, that this
acid may be decomposed by burning phosphorus in a closed vessel with
carbonat of soda or carbonat of lime: the phosphorus absorbs the oxygen
from the carbonat, whilst the carbon is separated in the form of a black
powder. This decomposition, however, is not effected simply by the
attraction of the phosphorus for oxygen, since it is weaker than that of
charcoal; but the attraction of the alkali of lime for the phosphoric
acid, unites its power at the same time.
CAROLINE.
Cannot we make that experiment?
MRS. B.
Not easily; it requires being performed with extreme nicety, in order to
obtain any sensible quantity of carbon, and the experiment is much too
delicate for me to attempt it. But there can be no doubt of the accuracy
of Mr. Tennant’s results; and all chemists now agree, that one hundred
parts of carbonic acid gas consists of about twenty-eight parts of
carbon to seventy-two of oxygen gas. But if you recollect, we decomposed
carbonic acid gas the other day by burning potassium in it.
CAROLINE.
True, so we did; and found the carbon precipitated on the regenerated
potash.
MRS. B.
Carbonic acid gas is found very abundantly in nature; it is supposed to
form about one thousandth part of the atmosphere, and is constantly
produced by the respiration of animals; it exists in a great variety of
combinations, and is exhaled from many natural decompositions. It is
contained in a state of great purity in certain caves, such as the
_Grotto del Cane_, near Naples.
EMILY.
I recollect having read an account of that grotto, and of the cruel
experiments made on the poor dogs, to gratify the curiosity of
strangers. But I understood that the vapour exhaled by this cave was
called _fixed air_.
MRS. B.
That is the name by which carbonic acid was known before its chemical
composition was discovered. --This gas is more destructive of life than
any other; and if the poor animals that are submitted to its effects are
not plunged into cold water as soon as they become senseless, they do
not recover. It extinguishes flame instantaneously. I have collected
some in this glass, which I will pour over the candle.
CAROLINE.
This is extremely singular--it seems to extinguish it as it were by
enchantment, as the gas is invisible. I never should have imagined that
gas could have been poured like a liquid.
MRS. B.
It can be done with carbonic acid only, as no other gas is sufficiently
heavy to be susceptible of being poured out in the atmospherical air
without mixing with it.
EMILY.
Pray by what means did you obtain this gas?
MRS. B.