consist of triple combinations. These are the vegetable acids, whose
common radical is a compound of hydrogen and carbon.
CAROLINE.
But if the basis of all the vegetable acids be the same, it should form
but one acid; it may indeed combine with different proportions of
oxygen, but the nature of the acid must be the same.
MRS. B.
The only difference that exists in the basis of vegetable acids, is the
various proportions of hydrogen and carbon from which they are severally
composed. But this is enough to produce a number of acids apparently
very dissimilar. That they do not, however, differ essentially, is
proved by their susceptibility of being converted into each other, by
the addition or subtraction of a portion of hydrogen or of carbon. The
names of these acids are,
The _Acetic_
_Oxalic_
_Tartarous_
_Citric_
_Malic_ Acids, of double bases, being of vegetable origin.
_Gallic_
_Mucous_
_Benzoic_
_Succinic_
_Camphoric_
_Suberic_
The 3d class of acids consists of those which have triple radicals, and
are therefore of a still more compound nature. This class comprehends
the animal acids, which are,
The _Lactic_
_Prussic_
_Formic_ Acids, of triple bases, or animal acids.
_Bombic_
_Sebacic_
_Zoonic_
_Lithic_
I have given you this summary account or enumeration of the acids, as
you may find it more satisfactory to have at once an outline or a
general notion of the extent of the subject; but we shall now confine
ourselves to the first class, which requires our more immediate
attention; and defer the few remarks which we shall have to make on the
others, till we treat of the chemistry of the animal and vegetable
kingdoms.
The acids of simple and known radicals are all capable of being
decomposed by combustible bodies, to which they yield their oxygen. If,
for instance, I pour a drop of sulphuric acid on this piece of iron, it
will produce a spot of rust, you know what that is?
CAROLINE.
Yes; it is an oxyd, formed by the oxygen of the acid combining with the
iron.
MRS. B.
In this case you see the sulphur deposits the oxygen by which it was
acidified on the metal. And again, if we pour some acid on a compound
combustible substance, (we shall try it on this piece of wood,) it will
combine with one or more of the constituents of that substance, and
occasion a decomposition.
EMILY.
It has changed the colour of the wood to black. How is that?
MRS. B.
The oxygen deposited by the acid has burnt it; you know that wood in
burning becomes black before it is reduced to ashes. Whether it derives
the oxygen which burns it from the atmosphere, or from any other source,
the chemical effect on the wood is the same. In the case of real
combustion, wood becomes black, because it is reduced to the state of
charcoal by the evaporation of its other constituents. But can you tell
me the reason why wood turns black when burnt by the application of an
acid?
CAROLINE.
First, tell me what are the ingredients of wood?
MRS. B.
Hydrogen and carbon are the chief constituents of wood, as of all other
vegetable substances.
CAROLINE.
Well, then, I suppose that the oxygen of the acid combines with the
hydrogen of the wood, to form water; and that the carbon of the wood,
remaining alone, appears of its usual black colour.
MRS. B.
Very well indeed, my dear; that is certainly the most plausible
explanation.
EMILY.
Would not this be a good method of making charcoal?
MRS. B.
It would be an extremely expensive, and, I believe, very imperfect
method; for the action of the acid on the wood, and the heat produced by
it, are far from sufficient to deprive the wood of all its evaporable
parts.
CAROLINE.
What is the reason that vinegar, lemon, and the acid of fruits, do not
produce this effect on wood?
MRS. B.
They are vegetable acids, whose bases are composed of hydrogen and
carbon; the oxygen, therefore, will not be disposed to quit this
radical, where it is already united with hydrogen. The strongest of
these may, perhaps, yield a little of their oxygen to the wood, and
produce a stain upon it; but the carbon will not be sufficiently
uncovered to assume its black colour. Indeed, the several mineral acids
themselves possess this power of charring wood in very different
degrees.
EMILY.
Cannot vegetable acids be decomposed, by any combustibles?
MRS. B.
No; because their radical is composed of two substances which have a
greater attraction for oxygen than any known body.
CAROLINE.
And are those strong acids, which burn and decompose wood, capable of
producing similar effects on the skin and flesh of animals?
MRS. B.
Yes; all the mineral acids, and one of them more especially, possess
powerful caustic qualities. They actually corrode and destroy the skin
and flesh; but they do not produce upon these exactly the same
alteration they do on wood, probably because there is a great proportion
of nitrogen and other substances in animal matter, which prevents the
separation of carbon from being so conspicuous.
CONVERSATION XVII.
OF THE SULPHURIC AND PHOSPHORIC ACIDS; OR THE COMBINATIONS OF OXYGEN
WITH SULPHUR AND PHOSPHORUS; AND OF THE SULPHATS AND PHOSPHATS.
MRS. B.
In addition to the general survey which we have taken of acids, I think
you will find it interesting to examine individually a few of the most
important of them, and likewise some of their principal combinations
with the alkalies, alkaline earths, and metals. The first of the acids,
in point of importance, is the SULPHURIC, formerly called _oil of
vitriol_.
CAROLINE.
I have known it a long time by that name, but had no idea that it was
the same fluid as sulphuric acid. What resemblance or connection can
there be between oil of vitriol and this acid?
MRS. B.
Vitriol is the common name for sulphat of iron, a salt which is formed
by the combination of sulphuric acid and iron; the sulphuric acid was
formerly obtained by distillation from this salt, and it very naturally
received its name from the substance which afforded it.
CAROLINE.
But it is still usually called oil of vitriol?
MRS. B.
Yes; a sufficient length of time has not yet elapsed, since the
invention of the new nomenclature, for it to be generally disseminated;
but, as it is adopted by all scientific chemists, there is every reason
to suppose that it will gradually become universal. When I received this
bottle from the chemists, _oil of vitriol_ was inscribed on the label;
but, as I knew you were very punctilious in regard to the nomenclature,
I changed it, and substituted the words _sulphuric acid_.
EMILY.
This acid has neither colour nor smell, but it appears much thicker than
water.
MRS. B.
It is nearly twice as heavy as water, and has, you see, an oily
consistence.
CAROLINE.
And it is probably from this circumstance that it has been called an
oil, for it can have no real claim to that name, as it does not contain
either hydrogen or carbon, which are the essential constituents of oil.
MRS. B.
Certainly; and therefore it would be the more absurd to retain a name
which owed its origin to such a mistaken analogy.
Sulphuric acid, in its purest state, would probably be a concrete
substance, but its attraction for water is such, that it is impossible
to obtain that acid perfectly free from it; it is, therefore, always
seen in a liquid form, such as you here find it. One of the most
striking properties of sulphuric acid is that of evolving a considerable
quantity of heat when mixed with water; this I have already shown you.
EMILY.
Yes, I recollect it; but what was the degree of heat produced by that
mixture?
MRS. B.
The thermometer may be raised by it to 300 degrees, which is
considerably above the temperature of boiling water.
CAROLINE.
Then water might be made to boil in that mixture?
MRS. B.
Nothing more easy, provided that you employ sufficient quantities of
acid and of water, and in the due proportions. The greatest heat is
produced by a mixture of one part of water to four of the acid: we shall
make a mixture of these proportions, and immerse in it this thin glass
tube, which is full of water.
CAROLINE.
The vessel feels extremely hot, but the water does not boil yet.
MRS. B.
You must allow some time for the heat to penetrate the tube, and raise
the temperature of the water to the boiling point--
CAROLINE.
Now it boils--and with increasing violence.
MRS. B.
But it will not continue boiling long; for the mixture gives out heat
only while the particles of the water and the acid are mutually
penetrating each other: as soon as the new arrangement of those
particles is effected, the mixture will gradually cool, and the water
return to its former temperature.
You have seen the manner in which sulphuric acid decomposes all
combustible substances, whether animal, vegetable, or mineral, and burns
them by means of its oxygen?
CAROLINE.
I have very unintentionally repeated the experiment on my gown, by
letting a drop of the acid fall upon it, and it has made a stain, which,
I suppose, will never wash out.
MRS. B.
No, certainly; for before you can put it into water, the spot will
become a hole, as the acid has literally burnt the muslin.
CAROLINE.
So it has, indeed! Well, I will fasten the stopper, and put the bottle
away, for it is a dangerous substance. --Oh, now I have done worse
still, for I have spilt some on my hand!
MRS. B.
It is then burned, as well as your gown, for you know that oxygen
destroys animal as well as vegetable matters; and, as far as the
decomposition of the skin of your finger is effected, there is no
remedy; but by washing it immediately in water, you will dilute the
acid, and prevent any further injury.
CAROLINE.
It feels extremely hot, I assure you.
MRS. B.
You have now learned, by experience, how cautiously this acid must be
used. You will soon become acquainted with another acid, the nitric,
which, though it produces less heat on the skin, destroys it still
quicker, and makes upon it an indelible stain. You should never handle
any substances of this kind, without previously dipping your fingers in
water, which will weaken their caustic effects. But, since you will not
repeat the experiment, I must put in the stopper, for the acid attracts
the moisture from the atmosphere, which would destroy its strength and
purity.
EMILY.
Pray, how can sulphuric acid be extracted from sulphat of iron by
distillation?
MRS. B.
The process of distillation, you know, consists in separating substances
from one another by means of their different degrees of volatility, and
by the introduction of a new chemical agent, caloric. Thus, if sulphat
of iron be exposed in a retort to a proper degree of heat, it will be
decomposed, and the sulphuric acid will be volatilised.
EMILY.
But now that the process of forming acids by the combustion of their
radicals is known, why should not this method be used for making
sulphuric acid?
MRS. B.
This is actually done in most manufactures; but the usual method of
preparing sulphuric acid does not consist in burning the sulphur in
oxygen gas (as we formerly did by the way of experiment), but in heating
it together with another substance, nitre, which yields oxygen in
sufficient abundance to render the combustion in common air rapid and
complete.
CAROLINE.
This substance, then, answers the same purpose as oxygen gas?
MRS. B.
Exactly. In manufactures the combustion is performed in a leaden
chamber, with water at the bottom, to receive the vapour and assist its
condensation. The combustion is, however, never so perfect but that a
quantity of _sulphureous_ acid is formed at the same time; for you
recollect that the sulphureous acid, according to the chemical
nomenclature, differs from the sulphuric only by containing less oxygen.
From its own powerful properties, and from the various combinations into
which it enters, sulphuric acid is of great importance in many of the
arts.
It is used also in medicine in a state of great dilution; for were it
taken internally, in a concentrated state, it would prove a most
dangerous poison.
CAROLINE.
I am sure it would burn the throat and stomach.
MRS. B.
Can you think of any thing that would prove an antidote to this poison?
CAROLINE.
A large draught of water to dilute it.
MRS. B.
That would certainly weaken the caustic power of the acid, but it would
increase the heat to an intolerable degree. Do you recollect nothing
that would destroy its deleterious properties more effectually?
EMILY.
An alkali might, by combining with it; but, then, a pure alkali is
itself a poison, on account of its causticity.
MRS. B.
There is no necessity that the alkali should be caustic. Soap, in which
it is combined with oil; or magnesia, either in the state of carbonat,
or mixed with water, would prove the best antidotes.
EMILY.
In those cases then, I suppose, the potash and the magnesia would quit
their combinations to form salts with the sulphuric acid?
MRS. B.
Precisely.
We may now make a few observations on the sulphure_ous_ acid, which we
have found to be the product of sulphur slowly and imperfectly burnt.
This acid is distinguished by its pungent smell, and its gaseous form.
CAROLINE.
Its aëriform state is, I suppose, owing to the smaller proportion of
oxygen, which renders it lighter than sulphur_ic_ acid?
MRS. B.
Probably; for by adding oxygen to the weaker acid, it may be converted
into the stronger kind. But this change of state may also be connected
with a change of affinity with regard to caloric.
EMILY.
And may sulphureous acid be obtained from sulphuric acid by a diminution
of oxygen?
MRS. B.
Yes; it can be done by bringing any combustible substance in contact
with the acid. This decomposition is most easily performed by some of
the metals; these absorb a portion of the oxygen from the sulphuric
acid, which is thus converted into the sulphureous, and flies off in its
gaseous form.
CAROLINE.
And cannot the sulphureous acid itself be decomposed and reduced to
sulphur?
MRS. B.
Yes; if this gas be heated in contact with charcoal, the oxygen of the
gas will combine with it, and the pure sulphur is regenerated.
Sulphureous acid is readily absorbed by water; and in this liquid state
it is found particularly useful in bleaching linen and woollen cloths,
and is much used in manufactures for those purposes. I can show you its
effect in destroying colours, by taking out vegetable stains--I think I
see a spot on your gown, Emily, on which we may try the experiment.
EMILY.
It is the stain of mulberries; but I shall be almost afraid of exposing
my gown to the experiment, after seeing the effect which the sulphuric
acid produced on that of Caroline--
MRS. B.
There is no such danger from the sulphureous; but the experiment must be
made with great caution, for, during the formation of sulphureous acid
by combustion, there is always some sulphuric produced.
CAROLINE.
But where is your sulphureous acid?
MRS. B.
We may easily prepare some ourselves, simply by burning a match; we must
first wet the stain with water, and now hold it in this way, at a little
distance, over the lighted match: the vapour that arises from it is
sulphureous acid, and the stain, you see, gradually disappears.
EMILY.
I have frequently taken out stains by this means, without understanding
the nature of the process. But why is it necessary to wet the stain
before it is exposed to the acid fumes?
MRS. B.
The moisture attracts and absorbs the sulphureous acid; and it serves
likewise to dilute any particles of sulphuric acid which might injure
the linen.
Sulphur is susceptible of a third combination with oxygen, in which the
proportion of the latter is too small to render the sulphur acid. It
acquires this slight oxygenation by mere exposure to the atmosphere,
without any elevation of temperature: in this case, the sulphur does not
change its natural form, but is only discoloured, being changed to red
or brown; and in this state it is an oxyd of sulphur.
Before we take leave of the sulphuric acid, we shall say a few words of
its principal combinations. It unites with all the alkalies, alkaline
earths and metals, to form compound salts.
CAROLINE.
Pray, give me leave to interrupt you for a moment: you have never
mentioned any other salts than the compound or neutral salts; is there
no other kind?
MRS. B.
The term _salt_ has been used, from time immemorial, as a kind of
general name for any substance that has savour, odour, is soluble in
water, and crystallisable, whether it be of an acid, an alkaline, or
compound nature; but the compound salts alone retain that appellation in
modern chemistry.
The most important of the salts, formed by the combinations of the
sulphuric acid, are, first, _sulphat of potash_, formerly called _sal
polychrest_: this is a very bitter salt, much used in medicine; it is
found in the ashes of most vegetables, but it may be prepared
artificially by the immediate combination of sulphuric acid and potash.
This salt is easily soluble in boiling water. Solubility is, indeed,
a property common to all salts; and they always produce cold in melting.
EMILY.
That must be owing to the caloric which they absorb in passing from a
solid to a fluid form.
MRS. B.
That is, certainly, the most probable explanation.
_Sulphat of soda_, commonly called Glauber’s salt, is another medicinal
salt, which is still more bitter than the preceding. We must prepare
some of these compounds, that you may observe the phenomena which take
place during their formation. We need only pour some sulphuric acid over
the soda which I have put into this glass.
CAROLINE.
What an amazing heat is disengaged! --I thought you said that cold was
produced by the melting of salts?
MRS. B.
But you must observe that we are now _making_, not _melting_ a salt.
Heat is disengaged during the formation of compound salts, and a faint
light is also emitted, which may sometimes be perceived in the dark.
EMILY.
And is this heat and light produced by the union of the opposite
electricities of the alkali and the acid?
MRS. B.
No doubt it is, if that theory be true.
CAROLINE.
The union of an acid and an alkali is then an actual combustion?
MRS. B.
Not precisely, though there is certainly much analogy in these
processes.
CAROLINE.
Will this sulphat of soda become solid?
MRS. B.
We have not, I suppose, mixed the acid and the alkali in the exact
proportions that are required for the formation of the salt, otherwise
the mixture would have been almost immediately changed to a solid mass;
but, in order to obtain it in crystals, as you see it in this bottle, it
would be necessary first to dilute it with water, and afterwards to
evaporate the water, during which operation the salt would gradually
crystallise.
CAROLINE.
But of what use is the addition of water, if it is afterwards to be
evaporated?
MRS. B.
When suspended in water, the acid and the alkali are more at liberty to
act on each other, their union is more complete, and the salt assumes
the regular form of crystals during the slow evaporation of its solvent.
Sulphat of soda liquefies by heat, and effloresces in the air.
EMILY.
Pray what is the meaning of the word _effloresces_? I do not recollect
your having mentioned it before.
MRS. B.
A salt is said to effloresce when it loses its water of crystallisation
on being exposed to the atmosphere, and is thus gradually converted into
a dry powder: you may observe that these crystals of sulphat of soda are
far from possessing the transparency which belongs to their crystalline
state; they are covered with a white powder, occasioned by their having
been exposed to the atmosphere, which has deprived their surface of its
lustre, by absorbing its water of crystallisation. Salts are, in
general, either _efflorescent_ or _deliquescent_: this latter property
is precisely the reverse of the former; that is to say, deliquescent
salts absorb water from the atmosphere, and are moistened and gradually
melted by it. Muriat of lime is an instance of great deliquescence.
EMILY.
But are there no salts that have the same degree of attraction for water
as the atmosphere, and that will consequently not be affected by it?
MRS. B.
Yes; there are many such salts, as, for instance, common salt, sulphat
of magnesia, and a variety of others.
_Sulphat of lime_ is very frequently met with in nature, and constitutes
the well-known substance called _gypsum_, or _plaster of Paris_.
_Sulphat of magnesia_, commonly called _Epsom salt_, is another very
bitter medicine, which is obtained from sea-water and from several
springs, or may be prepared by the direct combination of its
ingredients.
We have formerly mentioned _sulphat of alumine_ as constituting the
common _alum_; it is found in nature chiefly in the neighbourhood of
volcanos, and is particularly useful in the arts, from its strong
astringent qualities. It is chiefly employed by dyers and
calico-printers, to fix colours; and is used also in the manufacture of
some kinds of leather.
Sulphuric acid combines also with the metals.
CAROLINE.
One of these combinations, _sulphat of iron_, we are already well
acquainted with.
MRS. B.
That is the most important metallic salt formed by sulphuric acid, and
the only one that we shall here notice. It is of great use in the arts;
and, in medicine, it affords a very valuable tonic: it is of this salt
that most of those preparations called _steel medicines_ are composed.
CAROLINE.
But does any carbon enter into these compositions to form steel?
MRS. B.
Not an atom: they are, therefore, very improperly called steel: but it
is the vulgar appellation, and medical men themselves often comply with
the general custom.
Sulphat of iron may be prepared, as you have seen, by dissolving iron in
sulphuric acid; but it is generally obtained from the natural production
called _Pyrites_, which being a sulphuret of iron, requires only
exposure to the atmosphere to be oxydated, in order to form the salt;
this, therefore, is much the most easy way of procuring it on a large
scale.
EMILY.
I am surprised to find that both acids and compound salts are generally
obtained from their various combinations, rather than from the immediate
union of their ingredients.
MRS. B.
Were the simple bodies always at hand, their combinations would
naturally be the most convenient method of forming compounds; but you
must consider that, in most instances, there is great difficulty and
expense in obtaining the simple ingredients from their combinations; it
is, therefore, often more expedient to procure compounds from the
decomposition of other compounds. But, to return to the sulphat of iron.
--There is a certain vegetable acid called _Gallic acid_, which has the
remarkable property of precipitating this salt black--I shall pour a few
drops of the gallic acid into this solution of sulphat of iron--
CAROLINE.
It is become as black as ink!
MRS. B.
And it is ink in reality. Common writing ink is a precipitate of sulphat
of iron by gallic acid; the black colour is owing to the formation of
gallat of iron, which being insoluble, remains suspended in the fluid.
This acid has also the property of altering the colour of iron in its
metallic state. You may frequently see its effect on the blade of a
knife, that has been used to cut certain kinds of fruits.
CAROLINE.
True; and that is, perhaps, the reason that a silver knife is preferred
to cut fruits; the gallic acid, I suppose, does not act upon silver.
--Is this acid found in all fruits?
MRS. B.
It is contained, more or less, in the rind of most fruits and roots,
especially the radish, which, if scraped with a steel or iron knife, has
its bright red colour changed to a deep purple, the knife being at the
same time blackened. But the vegetable substance in which the gallic
acid most abounds is _nutgall_, a kind of excrescence that grows on
oaks, and from which the acid is commonly obtained for its various
purposes.
MRS. B.
We now come to the PHOSPHORIC and PHOSPHOROUS ACIDS. In treating of
phosphorus, you have seen how these acids may be obtained from it by
combustion?
EMILY.
Yes; but I should be much surprised if it was the usual method of
obtaining them, since it is so very difficult to procure phosphorus in
its pure state.
MRS. B.