A crystallised body held in solution only by water is scarcely ever so
volatile as the fluid itself, and care must be taken to manage the heat
so that it may be sufficient to evaporate the water only.

I should not omit also to mention that bodies, in crystallising from
their watery solution, always retain a small portion of water, which
remains confined in the crystal in a solid form, and does not reappear
unless the body loses its crystalline state. This is called the _water
of crystallisation_. But you must observe, that whilst a body may be
separated from its solution in water or caloric simply by cooling or by
evaporation, an acid can be taken from a metal with which it is combined
only by stronger affinities, which produce a decomposition.

EMILY.

Are the perfect metals susceptible of being dissolved and converted into
compound salts by acids?

MRS. B.

Gold is acted upon by only one acid, the _oxygenated muriatic_, a very
remarkable acid, which, when in its most concentrated state, dissolves
gold or any other metal, by burning them rapidly.

Gold can, it is true, be dissolved likewise by a mixture of two acids,
commonly called _aqua regia_; but this mixed solvent derives that
property from containing the peculiar acid which I have just mentioned.
Platina is also acted upon by this acid only; silver is dissolved by
nitric acid.

CAROLINE.

I think you said that some of the metals might be so strongly oxydated
as to become acid?

MRS. B.

There are five metals, arsenic, molybdena, chrome, tungsten, and
columbium, which are susceptible of combining with a sufficient quantity
of oxygen to be converted into acids.

CAROLINE.

Acids are connected with metals in such a variety of ways, that I am
afraid of some confusion in remembering them. --In the first place,
acids will yield their oxygen to metals. Secondly, they will combine
with them in their state of oxyds, to form compound salts; and lastly,
several of the metals are themselves susceptible of acidification.

MRS. B.

Very well; but though metals have so great an affinity for acids, it is
not with that class of bodies alone that they will combine. They are
most of them, in their simple state, capable of uniting with sulphur,
with phosphorus, with carbon, and with each other; these combinations,
according to the nomenclature which was explained to you on a former
occasion, are called _sulphurets_, _phosphorets_, _carburets_, &c.

The metallic phosphorets offer nothing very remarkable. The sulphurets
form the peculiar kind of mineral called _pyrites_, from which certain
kinds of mineral waters, as those of Harrogate, derive their chief
chemical properties. In this combination, the sulphur, together with the
iron, have so strong an attraction for oxygen, that they obtain it both
from the air and from water, and by condensing it in a solid form,
produce the heat which raises the temperature of the water in such a
remarkable degree.

EMILY.

But if pyrites obtain oxygen from water, that water must suffer a
decomposition, and hydrogen gas be evolved.

MRS. B.

That is actually the case in the hot springs alluded to, which give out
an extremely fetid gas, composed of hydrogen impregnated with sulphur.

CAROLINE.

If I recollect right, steel and plumbago, which you mentioned in the
last lesson, are both carburets of iron?

MRS. B.

Yes; and they are the only carburets of much consequence.

A curious combination of metals has lately very much attracted the
attention of the scientific world: I mean the meteoric stones that fall
from the atmosphere. They consist principally of native or pure iron,
which is never found in that state in the bowels of the earth; and
contain also a small quantity of nickel and chrome, a combination
likewise new in the mineral kingdom.

These circumstances have led many scientific persons to believe that
those substances have fallen from the moon, or some other planet, while
others are of opinion either that they are formed in the atmosphere, or
are projected into it by some unknown volcano on the surface of our
globe.

CAROLINE.

I have heard much of these stones, but I believe many people are of
opinion that they are formed on the surface of the earth, and laugh at
their pretended celestial origin.

MRS. B.

The fact of their falling is so well ascertained, that I think no person
who has at all investigated the subject, can now entertain any doubt of
it. Specimens of these stones have been discovered in all parts of the
world, and to each of them some tradition or story of its fall has been
found connected. And as the analysis of all those specimens affords
precisely the same results, there is strong reason to conjecture that
they all proceed from the same source. It is to Mr. Howard that
philosophers are indebted for having first analysed these stones, and
directed their attention to this interesting subject.

CAROLINE.

But pray, Mrs. B., how can solid masses of iron and nickel be formed
from the atmosphere, which consists of the two airs, nitrogen and
oxygen?

MRS. B.

I really do not see how they could, and think it much more probable that
they fall from the moon. --But we must not suffer this digression to
take up too much of our time.

The combinations of metals with each other are called alloys; thus brass
is an alloy of copper and zinc; bronze, of copper and tin, &c.

EMILY.

And is not pewter also a combination of metal?

MRS. B.

It is. The pewter made in this country is mostly composed of tin, with a
very small proportion of zinc and lead.

CAROLINE.

Block-tin is a kind of pewter, I believe?

MRS. B.

Properly speaking, block-tin means tin in blocks, or square massive
ingots; but in the sense in which it is used by ignorant workmen, it is
iron plated with tin, which renders it more durable, as tin will not so
easily rust. Tin alone, however, would be too soft a metal to be worked
for common use, and all tin-vessels and utensils are in fact made of
plates of iron, thinly coated with tin, which prevents the iron from
rusting.

CAROLINE.

Say rather _oxydating_, Mrs. B. --Rust is a word that should be exploded
in chemistry.

MRS. B.

Take care, however, not to introduce the word oxydate, instead of rust,
in general conversation; for you would probably not be understood, and
you might be suspected of affectation.

Metals differ very much in their affinity for each other; some will not
unite at all, others readily combine together, and on this property of
metals the art of _soldering_ depends.

EMILY.

What is soldering?

MRS. B.

It is joining two pieces of metal together, by a more fusible metal
interposed between them. Thus tin is a solder for lead; brass, gold, or
silver, are solder for iron, &c.

CAROLINE.

And is not _plating_ metals something of the same nature?

MRS. B.

In the operation of plating, two metals are united, one being covered
with the other, but without the intervention of a third; iron or copper
may thus be covered with gold or silver.

EMILY.

Mercury appears to me of a very different nature from the other metals.

MRS. B.

One of its greatest peculiarities is, that it retains a fluid state at
the temperature of the atmosphere. All metals are fusible at different
degrees of heat, and they have likewise each the property of freezing or
becoming solid at a certain fixed temperature. Mercury congeals only at
seventy-two degrees below the freezing point.

EMILY.

That is to say, that in order to freeze, it requires a temperature of
seventy-two degrees colder than that at which water freezes.

MRS. B.

Exactly so.

CAROLINE.

But is the temperature of the atmosphere ever so low as that?

MRS. B.

Yes, often in Siberia; but happily never in this part of the globe.
Here, however, mercury may be congealed by artificial cold; I mean such
intense cold as can be produced by some chemical mixtures, or by the
rapid evaporation of ether under the air-pump.*

    [Footnote *: By a process analogous to that described, page 155.
    of this volume.]

CAROLINE.

And can mercury be made to boil and evaporate?

MRS. B.

Yes, like any other liquid; only it requires a much greater degree of
heat. At the temperature of six hundred degrees, it begins to boil and
evaporate like water.

Mercury combines with gold, silver, tin, and with several other metals;
and, if mixed with any of them in a sufficient proportion, it penetrates
the solid metal, softens it, loses its own fluidity, and forms an
_amalgam_, which is the name given to the combination of any metal with
mercury, forming a substance more or less solid, according as the
mercury or the other metal predominates.

EMILY.

In the list of metals there are some whose names I have never before
heard mentioned.

MRS. B.

Besides those which Sir H. Davy has obtained, there are several that
have been recently discovered, whose properties are yet but little
known, as for instance, titanium, which was discovered by the Rev. Mr.
Gregor, in the tin-mines of Cornwall; columbium or tantalium, which has
lately been discovered by Mr. Hatchett; and osmium, iridium, palladium,
and rhodium, all of which Dr. Wollaston and Mr. Tennant found mixed in
minute quantities with crude platina, and the distinct existence of
which they proved by curious and delicate experiments.

CAROLINE.

Arsenic has been mentioned amongst the metals. I had no notion that it
belonged to that class of bodies, for I had never seen it but as a
powder, and never thought of it but as a most deadly poison.

MRS. B.

In its pure metallic state, I believe, it is not so poisonous; but it
has such a great affinity for oxygen, that it absorbs it from the
atmosphere at its natural temperature: you have seen it, therefore, only
in its state of oxyd, when, from its combination with oxygen, it has
acquired its very poisonous properties.

CAROLINE.

Is it possible that oxygen can impart poisonous qualities? That valuable
substance which produces light and fire, and which all bodies in nature
are so eager to obtain?

MRS. B.

Most of the metallic oxyds are poisonous, and derive this property from
their union with oxygen. The white lead, so much used in paint, owes its
pernicious effects to oxygen. In general, oxygen, in a concrete state,
appears to be particularly destructive in its effects on flesh or any
animal matter; and those oxyds are most caustic that have an acrid
burning taste, which proceeds from the metal having but a slight
affinity for oxygen, and therefore easily yielding it to the flesh,
which it corrodes and destroys.

EMILY.

What is the meaning of the word _caustic_, which you have just used?

MRS. B.

It expresses that property which some bodies possess, of disorganizing
and destroying animal matter, by operating a kind of combustion, or at
least a chemical decomposition. You must often have heard of caustic
used to burn warts, or other animal excrescences; most of these bodies
owe their destructive power to the oxygen with which they are combined.
The common caustic, called _lunar caustic_, is a compound formed by the
union of nitric acid and silver; and it is supposed to owe its caustic
qualities to the oxygen contained in the nitric acid.

CAROLINE.

But, pray, are not acids still more caustic than oxyds, as they contain
a greater proportion of oxygen?

MRS. B.

Some of the acids are; but the caustic property of a body depends not
only upon the quantity of oxygen which it contains, but also upon its
slight affinity for that principle, and the consequent facility with
which it yields it.

EMILY.

Is not this destructive property of oxygen accounted for?

MRS. B.

It proceeds probably from the strong attraction of oxygen for hydrogen;
for if the one rapidly absorb the other from the animal fibre,
a disorganisation of the substance must ensue.

EMILY.

Caustics are, then, very properly said to burn the flesh, since the
combination of oxygen and hydrogen is an actual combustion.

CAROLINE.

Now, I think, this effect would be more properly termed an oxydation, as
there is no disengagement of light and heat.

MRS. B.

But there really is a sensation of heat produced by the action of
caustics.

EMILY.

If oxygen is so caustic, why does not that which is contained in the
atmosphere burn us?

MRS. B.

Because it is in a gaseous state, and has a greater attraction for its
electricity than for the hydrogen of our bodies. Besides, should the air
be slightly caustic, we are in a great measure sheltered from its
effects by the skin; you know how much a wound, however trifling, smarts
on being exposed to it.

CAROLINE.

It is a curious idea, however, that we should live in a slow fire. But,
if the air was caustic, would it not have an acrid taste?

MRS. B.

It possibly may have such a taste; though in so slight a degree, that
custom has rendered it insensible.

CAROLINE.

And why is not water caustic? When I dip my hand into water, though
cold, it ought to burn me from the caustic nature of its oxygen.

MRS. B.

Your hand does not decompose the water; the oxygen in that state is much
better supplied with hydrogen than it would be by animal matter, and if
its causticity depend on its affinity for that principle, it will be
very far from quitting its state of water to act upon your hand. You
must not forget that oxyds are caustic in proportion as the oxygen
adheres slightly to them.

EMILY.

Since the oxyd of arsenic is poisonous, its acid, I suppose, is fully as
much so?

MRS. B.

Yes; it is one of the strongest poisons in nature.

EMILY.

There is a poison called _verdigris_, which forms on brass and copper
when not kept very clean; and this, I have heard, is an objection to
these metals being made into kitchen utensils. Is this poison likewise
occasioned by oxygen?

MRS. B.

It is produced by the intervention of oxygen; for verdigris is a
compound salt formed by the union of vinegar and copper; it is of a
beautiful green colour, and much used in painting.

EMILY.

But, I believe, verdigris is often formed on copper when no vinegar has
been in contact with it.

MRS. B.

Not real verdigris, but compound salts, somewhat resembling it, may be
produced by the action of any acid on copper.

The solution of copper in nitric acid, if evaporated, affords a salt
which produces an effect on tin that will surprise you, and I have
prepared some from the solution we made before, that I might show it to
you. I shall first sprinkle some water on this piece of tin-foil, and
then some of the salt. --Now observe that I fold it up suddenly, and
press it into one lump.

CAROLINE.

What a prodigious vapour issues from it--and sparks of fire I declare!

MRS. B.

I thought it would surprise you. The effect, however, I dare say you
could account for, since it is merely the consequence of the oxygen of
the salt rapidly entering into a closer combination with the tin.

There is also a beautiful green salt too curious to be omitted; it is
produced by the combination of cobalt with muriatic acid, which has the
singular property of forming what is called _sympathetic ink_.
Characters written with this solution are invisible when cold, but when
a gentle heat is applied, they assume a fine bluish green colour.

CAROLINE.

I think one might draw very curious landscapes with the assistance of
this ink; I would first make a water-colour drawing of a winter-scene,
in which the trees should be leafless, and the grass scarcely green:
I would then trace all the verdure with the invisible ink, and whenever
I chose to create spring, I should hold it before the fire, and its
warmth would cover the landscape with a rich verdure.

MRS. B.

That will be a very amusing experiment, and I advise you by all means to
try it.

  [Transcriber's Note:
  Several cobalt compounds, including the cobalt chloride described
  here, are still in use as invisible ("sympathetic") inks. They are
  safe if used appropriately.]

Before we part, I must introduce to your acquaintance the curious metals
which Sir H. Davy has recently discovered. The history of these
extraordinary bodies is yet so much in its infancy, that I shall confine
myself to a very short account of them; it is more important to point
out to you the vast, and apparently inexhaustible, field of research
which has been thrown open to our view by Sir H. Davy's memorable
discoveries, than to enter into a minute account of particular bodies or
experiments.

CAROLINE.

But I have heard that these discoveries, however splendid and
extraordinary, are not very likely to prove of any great benefit to the
world, as they are rather objects of curiosity than of use.

MRS. B.

Such may be the illiberal conclusions of the ignorant and narrow-minded;
but those who can duly estimate the advantages of enlarging the sphere
of science, must be convinced that the acquisition of every new fact,
however unconnected it may at first appear with practical utility, must
ultimately prove beneficial to mankind. But these remarks are scarcely
applicable to the present subject; for some of the new metals have
already proved eminently useful as chemical agents, and are likely soon
to be employed in the arts. For the enumeration of these metals, I must
refer you to our list of simple bodies; they are derived from the
alkalies, the earths, and three of the acids, all of which had been
hitherto considered as undecompoundable or simple bodies.

When Sir H. Davy first turned his attention to the effects of the
Voltaic battery, he tried its power on a variety of compound bodies, and
gradually brought to light a number of new and interesting facts, which
led the way to more important discoveries. It would be highly
interesting to trace his steps in this new department of science, but it
would lead us too far from our principal object. A general view of his
most remarkable discoveries is all that I can aim at, or that you could,
at present, understand.

The facility with which compound bodies yielded to the Voltaic
electricity, induced him to make trial of its effects on substances
hitherto considered as simple, but which he suspected of being compound,
and his researches were soon crowned with the most complete success.

The body which he first submitted to the Voltaic battery, and which had
never yet been decomposed, was one of the fixed alkalies, called potash.
This substance gave out an elastic fluid at the positive wire, which was
ascertained to be oxygen, and at the negative wire, small globules of a
very high metallic lustre, very similar in appearance to mercury; thus
proving that potash, which had hitherto been considered as a simple
incombustible body, was in fact a metallic oxyd; and that its
incombustibility proceeded from its being already combined with oxygen.

EMILY.

I suppose the wires used in this experiment were of platina, as they
were when you decomposed water; for if of iron, the oxygen would have
combined with the wire, instead of appearing in the form of gas.

MRS. B.

Certainly: the metal, however, would equally have been disengaged. Sir
H. Davy has distinguished this new substance by the name of POTASSIUM,
which is derived from that of the alkali, from which it is procured.
I have some small pieces of it in this phial, but you have already seen
it, as it is the metal which we burnt in contact with sulphur.

EMILY.

What is the liquid in which you keep it?

MRS. B.

It is naptha, a bituminous liquid, with which I shall hereafter make you
acquainted. It is almost the only fluid in which potassium can be
preserved, as it contains no oxygen, and this metal has so powerful an
attraction for oxygen, that it will not only absorb it from the air, but
likewise from water, or any body whatever that contains it.

EMILY.

This, then, is one of the bodies that oxydates spontaneously without the
application of heat?

MRS. B.

Yes; and it has this remarkable peculiarity that it attracts oxygen much
more rapidly from water than from air; so that when thrown into water,
however cold, it actually bursts into flame. I shall now throw a small
piece, about the size of a pin's head, on this drop of water.

CAROLINE.

It instantaneously exploded, producing a little flash of light! this is,
indeed, a most curious substance!

MRS. B.

By its combustion it is reconverted into potash; and as potash is now
decidedly a compound body, I shall not enter into any of its properties
till we have completed our review of the simple bodies; but we may here
make a few observations on its basis, potassium. If this substance is
left in contact with air, it rapidly returns to the state of potash,
with a disengagement of heat, but without any flash of light.

EMILY.

But is it not very singular that it should burn better in water than in
air?

CAROLINE.

I do not think so: for if the attraction of potassium for oxygen is so
strong that it finds no more difficulty in separating it from the
hydrogen in water, than in absorbing it from the air, it will no doubt
be more amply and rapidly supplied by water than by air.

MRS. B.

That cannot, however, be precisely the reason, for when potassium is
introduced under water, without contact of air, the combustion is not so
rapid, and indeed, in that case, there is no luminous appearance; but a
violent action takes place, much heat is excited, the potash is
regenerated, and hydrogen gas is evolved.

Potassium is so eminently combustible, that instead of requiring, like
other metals, an elevation of temperature, it will burn rapidly in
contact with water, even below the freezing point. This you may witness
by throwing a piece on this lump of ice.

CAROLINE.

It again exploded with flame, and has made a deep hole in the ice.

MRS. B.

This hole contains a solution of potash; for the alkali being extremely
soluble, disappears in the water at the instant it is produced. Its
presence, however, may be easily ascertained, alkalies having the
property of changing paper, stained with turmeric, to a red colour; if
you dip one end of this slip of paper into the hole in the ice you will
see it change colour, and the same, if you wet it with the drop of water
in which the first piece of potassium was burnt.

CAROLINE.

It has indeed changed the paper from yellow to red.

MRS. B.

This metal will burn likewise in carbonic acid gas, a gas that had
always been supposed incapable of supporting combustion, as we were
unacquainted with any substance that had a greater attraction for oxygen
than carbon. Potassium, however, readily decomposes this gas, by
absorbing its oxygen, as I shall show you. This retort is filled with
carbonic acid gas. --I will put a small piece of potassium in it; but
for this combustion a slight elevation of temperature is required, for
which purpose I shall hold the retort over the lamp.

CAROLINE.

Now it has taken fire, and burns with violence! It has burst the retort.

MRS. B.

Here is the piece of regenerated potash; can you tell me why it is
become so black?

EMILY.

No doubt it is blackened by the carbon, which, when its oxygen entered
into combination with the potassium, was deposited on its surface.

MRS. B.

You are right. This metal is perfectly fluid at the temperature of one
hundred degrees; at fifty degrees it is solid, but soft and malleable;
at thirty-two degrees it is hard and brittle, and its fracture exhibits
an appearance of confused crystallization. It is scarcely more than half
as heavy as water; its specific gravity being about six when water is
reckoned at ten; so that this metal is actually lighter than any known
fluid, even than ether.

Potassium combines with sulphur and phosphorus, forming sulphurets and
phosphurets; it likewise forms alloys with several metals, and
amalgamates with mercury.

EMILY.

But can a sufficient quantity of potassium be obtained, by means of the
Voltaic battery, to admit of all its properties and relations to other
bodies being satisfactorily ascertained?

MRS. B.

Not easily; but I must not neglect to inform you that a method of
obtaining this metal in considerable quantities has since been
discovered. Two eminent French chemists, Thenard and Gay Lussac,
stimulated by the triumph which Sir H. Davy had obtained, attempted to
separate potassium from its combination with oxygen, by common chemical
means, and without the aid of electricity. They caused red hot potash in
a state of fusion to filter through iron turnings in an iron tube,
heated to whiteness. Their experiment was crowned with the most complete
success; more potassium was obtained by this single operation, that
could have been collected in many weeks by the most diligent use of the
Voltaic battery.

EMILY.

In this experiment, I suppose, the oxygen quitted its combination with
the potassium to unite with the iron turnings?

MRS. B.

Exactly so; and the potassium was thus obtained in its simple state.
From that time it has become a most convenient and powerful instrument
of deoxygenation in chemical experiments. This important improvement,
engrafted on Sir H. Davy's previous discoveries, served but to add to
his glory, since the facts which he had established, when possessed of
only a few atoms of this curious substance, and the accuracy of his
analytical statements, were all confirmed when an opportunity occurred
of repeating his experiments upon this substance, which can now be
obtained in unlimited quantities.

CAROLINE.

What a satisfaction Sir H. Davy must have felt, when by an effort of
genius he succeeded in bringing to light and actually giving existence,
to these curious bodies, which without him might perhaps have ever
remained concealed from our view!

MRS. B.

The next substance which Sir H. Davy submitted to the influence of the
Voltaic battery was _Soda_, the other fixed alkali, which yielded to the
same powers of decomposition; from this alkali too, a metallic substance
was obtained, very analogous in its properties to that which had been
discovered in potash; Sir H. Davy has called it SODIUM. It is rather
heavier than potassium, though considerably lighter than water; it is
not so easily fusible as potassium.

Encouraged by these extraordinary results, Sir H. Davy next performed a
series of beautiful experiments on _Ammonia_, or the volatile alkali,
which, from analogy, he was led to suspect might also contain oxygen.
This he soon ascertained to be the fact, but he has not yet succeeded in
obtaining the basis of ammonia in a separate state; it is from analogy,
and from the power which the volatile alkali has, in its gaseous form,
to oxydate iron, and also from the amalgams which can be obtained from
ammonia by various processes, that the proofs of that alkali being also
a metallic oxyd are deduced.

Thus, then, the three alkalies, two of which had always been considered
as simple bodies, have now lost all claim to that title, and I have
accordingly classed the alkalies amongst the compounds, whose properties
we shall treat of in a future conversation.

EMILY.

What are the other newly discovered metals which you have alluded to in
your list of simple bodies?

MRS. B.

They are the metals of the earths which became next the object of Sir H.
Davy's researches; these bodies had never yet been decomposed, though
they were strongly suspected not only of being compounds, but of being
metallic oxyds. From the circumstance of their incombustibility it was
conjectured, with some plausibility, that they might possibly be bodies
that had been already burnt.

CAROLINE.

And metals, when oxydated, become, to all appearance, a kind of earthy
substance.

MRS. B.

They have, besides, several features of resemblance with metallic oxyds;
Sir H. Davy had therefore great reason to be sanguine in his
expectations of decomposing them, and he was not disappointed. He could
not, however, succeed in obtaining the basis of the earths in a pure
separate state; but metallic alloys were formed with other metals, which
sufficiently proved the existence of the metallic basis of the earths.

The last class of new metallic bodies which Sir H. Davy discovered was
obtained from the three undecompounded acids, the boracic, the fluoric,
and the muriatic acids; but as you are entirely unacquainted with these
bodies, I shall reserve the account of their decomposition till we come
to treat of their properties as acids.

Thus in the course of two years, by the unparalleled exertions of a
single individual, chemical science has assumed a new aspect. Bodies
have been brought to light which the human eye never before beheld, and
which might have remained eternally concealed under their impenetrable
disguise.

It is impossible at the present period to appreciate to their full
extent the consequences which science or the arts may derive from these
discoveries; we may, however, anticipate the most important results.

In chemical analysis we are now in possession of more energetic agents
of decomposition than were ever before known.

In geology new views are opened, which will probably operate a
revolution in that obscure and difficult science. It is already proved
that all the earths, and, in fact, the solid surface of this globe, are
metallic bodies mineralized by oxygen, and as our planet has been
calculated to be considerably more dense upon the whole than on the
surface, it is reasonable to suppose that the interior part is composed
of a metallic mass, the surface of which only has been mineralized by
the atmosphere.

The eruptions of volcanos, those stupendous problems of nature, admit
now of an easy explanation. For if the bowels of the earth are the grand
recess of these newly discovered inflammable bodies, whenever water
penetrates into them, combustions and explosions must take place; and it
is remarkable that the lava which is thrown out, is the very kind of
substance which might be expected to result from these combustions.

I must now take my leave of you; we have had a very long conversation
to-day, and I hope you will be able to recollect what you have learnt.
At our next interview we shall enter on a new subject.

END OF THE FIRST VOLUME.

  Printed by A. Strahan,
  Printers-Street, London.

                 CONVERSATIONS
                       ON
                   CHEMISTRY;

                    In Which
          The Elements Of That Science
                      Are
             _Familiarly Explained_
                      And
          Illustrated By Experiments.

                IN TWO VOLUMES.

    _The Fifth Edition, revised, corrected,_
          _and considerably enlarged._

                    VOL. II.
              ON COMPOUND BODIES.

  _London:_
  Printed For Longman, Hurst, Rees, Orme, and Brown,
  Paternoster-Row.
  1817.

CONVERSATION XIII.

ON THE ATTRACTION OF COMPOSITION.

MRS. B.

Having completed our examination of the simple or elementary bodies, we
are now to proceed to those of a compound nature; but before we enter on
this extensive subject, it will be necessary to make you acquainted with
the principal laws by which chemical combinations are governed.

You recollect, I hope, what we formerly said of the nature of the
attraction of composition, or chemical attraction, or affinity, as it is
also called?

EMILY.

Yes, I think perfectly; it is the attraction that subsists between
bodies of a different nature, which occasions them to combine and form a
compound, when they come in contact, and, according to Sir H. Davy's
opinion, this effect is produced by the attraction of the opposite
electricities, which prevail in bodies of different kinds.

MRS. B.

Very well; your definition comprehends the first law of chemical
attraction, which is, that _it takes place only between bodies of a
different nature_; as, for instance, between an acid and an alkali;
between oxygen and a metal, &c.

CAROLINE.

That we understand of course; for the attraction between particles of a
similar nature is that of aggregation, or cohesion, which is independent
of any chemical power.

MRS. B.

The 2d law of chemical attraction is, that _it takes place only between
the most minute particles of bodies_; therefore, the more you divide the
particles of the bodies to be combined, the more readily they act upon
each other.

CAROLINE.

That is again a circumstance which we might have supposed, for the finer
the particles of the two substances are, the more easily and perfectly
they will come in contact with each other, which must greatly facilitate
their union. It was for this purpose, you said, that you used iron
filings, in preference to wires or pieces of iron, for the decomposition
of water.

MRS. B.

It was once supposed that no mechanical power could divide bodies into
particles sufficiently minute for them to act on each other; and that,
in order to produce the extreme division requisite for a chemical
action, one, if not both of the bodies, should be in a fluid state.
There are, however, a few instances in which two solid bodies, very
finely pulverized, exert a chemical action on one another; but such
exceptions to the general rule are very rare indeed.

EMILY.

In all the combinations that we have hitherto seen, one of the
constituents has, I believe, been either liquid or aeriform. In
combustions, for instance, the oxygen is taken from the atmosphere, in
which it existed in the state of gas; and whenever we have seen acids
combine with metals or with alkalies, they were either in a liquid or an
aeriform state.

MRS. B.

The 3d law of chemical attraction is, that _it can take place between
two, three, four, or even a greater number of bodies_.

CAROLINE.

Oxyds and acids are bodies composed of two constituents; but I recollect
no instance of the combination of a greater number of principles.

MRS. B.

The compound salts, formed by the union of the metals with acids, are
composed of three principles. And there are salts formed by the
combination of the alkalies with the earths which are of a similar
description.

CAROLINE.

Are they of the same kind as the metallic salts?

MRS. B.

Yes; they are very analogous in their nature, although different in many
of their properties.

A methodical nomenclature, similar to that of the acids, has been
adopted for the compound salts. Each individual salt derives its name
from its constituent parts, so that every name implies a knowledge of
the composition of the salt.

The three alkalies, the alkaline earths, and the metals, are called
_salifiable bases_ or _radicals_; and the acids, _salifying principles_.
The name of each salt is composed both of that of the acid and the
salifiable base; and it terminates in _at_ or _it_, according to the
degree of the oxygenation of the acid. Thus, for instance, all those
salts which are formed by the combination of the sulphuric acid with any
of the salifiable bases are called _sulphats_, and the name of the
radical is added for the specific distinction of the salt; if it be
potash, it will compose a _sulphat of potash_; if ammonia, _sulphat of
ammonia_, &c.

EMILY.

The crystals which we obtained from the combination of iron and
sulphuric acid were therefore _sulphat of iron_?

MRS. B.

Precisely; and those which we prepared by dissolving copper in nitric
acid, _nitrat of copper_, and so on. --But this is not all; if the salt
be formed by that class of acids which ends in _ous_, (which you know
indicates a less degree of oxygenation,) the termination of the name of
the salt will be in _it_, as _sulphit of potash_, _sulphit of
ammonia_, &c.

EMILY.

There must be an immense number of compound salts, since there is so
great a variety of salifiable radicals, as well as of salifying
principles.

MRS. B.

Their real number cannot be ascertained, since it increases every day.
But we must not proceed further in the investigation of the compound
salts, until we have completed the examination of the nature of the
ingredients of which they are composed.

The 4th law of chemical attraction is, that _a change of temperature
always takes place at the moment of combination_. This arises from the
extrication of the two electricities in the form of caloric, which takes
place when bodies unite; and also sometimes in part from a change of
capacity of the bodies for heat, which always takes place when the
combination is attended with an increase of density, but more especially
when the compound passes from the liquid to the solid form. I shall now
show you a striking instance of a change of temperature from chemical
union, merely by pouring some nitrous acid on this small quantity of oil
of turpentine--the oil will instantly combine with the oxygen of the
acid, and produce a considerable change of temperature.

CAROLINE.

What a blaze! The temperature of the oil and the acid must be greatly
raised, indeed, to produce such a violent combustion.

MRS. B.

There is, however, a peculiarity in this combustion, which is, that the
oxygen, instead of being derived from the atmosphere alone, is
principally supplied by the acid itself.

EMILY.

And are not all combustions instances of the change of temperature
produced by the chemical combination of two bodies?

MRS. B.

Undoubtedly; when oxygen loses its gaseous form, in order to combine
with a solid body, it becomes condensed, and the caloric evolved
produces the elevation of temperature. The specific gravity of bodies is
at the same time altered by chemical combination; for in consequence of
a change of capacity for heat, a change of density must be produced.

CAROLINE.

That was the case with the sulphuric acid and water, which, by being
mixed together, gave out a great deal of heat, and increased in density.

MRS. B.

The 5th law of chemical attraction is, that _the properties which
characterise bodies, when separate, are altered or destroyed by their
combination_.

CAROLINE.

Certainly; what, for instance, can be so different from water as the
hydrogen and oxygen gases?

EMILY.

Or what more unlike sulphat of iron than iron or sulphuric acid?

MRS. B.

Every chemical combination is an illustration of this rule. But let us
proceed--

The 6th law is, that _the force of chemical affinity between the
constituents of a body is estimated by that which is required for their
separation_. This force is not always proportional to the facility with
which bodies unite; for manganese, for instance, which, you know, is so
much disposed to unite with oxygen that it is never found in a metallic
state, yields it more easily than any other metal.

EMILY.

But, Mrs. B., you speak of estimating the force of attraction between
bodies, by the force required to separate them; how can you measure
these forces?

MRS. B.

They cannot be precisely measured, but they are comparatively
ascertained by experiment, and can be represented by numbers which
express the relative degrees of attraction.

The 7th law is, that _bodies have amongst themselves different degrees
of attraction_. Upon this law, (which you may have discovered yourselves
long since,) the whole science of chemistry depends; for it is by means
of the various degrees of affinity which bodies have for each other,
that all the chemical compositions and decompositions are effected.
Every chemical fact or experiment is an instance of the same kind; and
whenever the decomposition of a body is performed by the addition of any
single new substance, it is said to be effected by _simple elective
attractions_. But it often happens that no simple substance will
decompose a body, and that, in order to effect this, you must offer to
the compound a body which is itself composed of two, or sometimes three
principles, which would not, each separately, perform the decomposition.
In this case there are two new compounds formed in consequence of a
reciprocal decomposition and recomposition. All instances of this kind
are called _double elective attractions_.

CAROLINE.

I confess I do not understand this clearly.

MRS. B.

You will easily comprehend it by the assistance of this diagram, in
which the reciprocal forces of attraction are represented by numbers:

                   _Original Compound_
                      Sulphat of Soda.

                      Soda    8 Sulphuric Acid

                              | _Quies-_
                              | _cent_
  _Result_                                        _Result_
  Nitrat        7 _Divellent Attractions_ 6} 13   Sulphat
  of Soda                                         of Lime
                              | _Attrac-_
                              | _tions_

                 Nitric Acid  4  Lime
                             --
                             12

                   _Original Compound_
                      Nitrat of Lime.

We here suppose that we are to decompose sulphat of soda; that is, to
separate the acid from the alkali; if, for this purpose, we add some
lime, in order to make it combine with the acid, we shall fail in our
attempt, because the soda and the sulphuric acid attract each other by a
force which is superior, and (by way of supposition) is represented by
the number 8; while the lime tends to unite with this acid by an
affinity equal only to the number 6. It is plain, therefore, that the
sulphat of soda will not be decomposed, since a force equal to 8 cannot
be overcome by a force equal only to 6.

CAROLINE.

So far, this appears very clear.

MRS. B.

If, on the other hand, we endeavour to decompose this salt by nitric
acid, which tends to combine with soda, we shall be equally
unsuccessful, as nitric acid tends to unite with the alkali by a force
equal only to 7.

In neither of these cases of simple elective attraction, therefore, can
we accomplish our purpose. But let us previously combine together the
lime and nitric acid, so as to form a nitrat of lime, a compound salt,
the constituents of which are united by a power equal to 4. If then we
present this compound to the sulphat of soda, a decomposition will
ensue, because the sum of the forces which tend to preserve the two
salts in their actual state is not equal to that of the forces which
tend to decompose them, and to form new combinations. The nitric acid,
therefore, will combine with the soda, and the sulphuric acid with the
lime.

CAROLINE.

I understand you now very well. This double effect takes place because
the numbers 8 and 4, which represent the degrees of attraction of the
constituents of the two original salts, make a sum less than the numbers
7 and 6, which represent the degrees of attraction of the two new
compounds that will in consequence be formed.

MRS. B.

Precisely so.

CAROLINE.

But what is the meaning of _quiescent_ and _divellent_ forces, which are
written in the diagram?

MRS. B.

Quiescent forces are those which tend to preserve compounds in a state
of rest, or such as they actually are: divellent forces, those which
tend to destroy that state of combination, and to form new compounds.

These are the principal circumstances relative to the doctrine of
chemical attractions, which have been laid down as rules by modern
chemists; a few others might be mentioned respecting the same theory,
but of less importance, and such as would take us too far from our plan.
I should, however, not omit to mention that Mr. Berthollet, a celebrated
French chemist, has questioned the uniform operation of elective
attraction, and has advanced the opinion, that, in chemical
combinations, the changes which take place depend not only upon the
affinities, but also, in some degree, on the respective quantities of
the substances concerned, on the heat applied during the process, and
some other circumstances.

CAROLINE.

In that case, I suppose, there would hardly be two compounds exactly
similar, though composed of the same materials?

MRS. B.

On the contrary, it is found that a remarkable uniformity prevails, as
to proportions, between the ingredients of bodies of similar
composition. Thus water, as you may recollect to have seen in a former
conversation, is composed of two volumes of hydrogen gas to one of
oxygen, and this is always found to be precisely the proportion of its
constituents, from whatever source the water be derived. The same
uniformity prevails with regard to the various salts; the acid and
alkali, in each kind of salt, being always found to combine in the same
proportions. Sometimes, it is true, the same acid, and the same alkali,
are capable of making two distinct kinds of salts; but in all these
cases it is found that one of the salts contains just twice, or in some
instances, thrice as much acid, or alkali, as the other.

EMILY.

If the proportions in which bodies combine are so constant and so well
defined, how can Mr. Berthollet's remark be reconciled with this uniform
system of combination?

MRS. B.

Great as that philosopher's authority is in chemistry, it is now
generally supposed that his doubts on this subject were in a great
degree groundless, and that the exceptions he has observed in the laws
of definite proportions, have been only apparent, and may be accounted
for consistently with those laws.

CAROLINE.

Pray, Mrs. B., can you decompose a salt by means of electricity, in the
same way as we decompose water?

MRS. B.

Undoubtedly; and I am glad this question occurred to you, because it
gives me an opportunity of showing you some very interesting experiments
on the subject.

If we dissolve a quantity, however small, of any salt in a glass of
water, and if we plunge into it the extremities of the wires which
proceed from the two ends of the Voltaic battery, the salt will be
gradually decomposed, the acid being attracted by the positive, and the
alkali by the negative wire.

EMILY.

But how can you render that decomposition perceptible?

MRS. B.

By placing in contact with the extremities of each wire, in the
solution, pieces of paper stained with certain vegetable colours, which
are altered by the contact of an acid or an alkali. Thus this blue
vegetable preparation called litmus becomes red when touched by an acid;
and the juice of violets becomes green by the contact of an alkali.

But the experiment can be made in a much more distinct manner, by
receiving the extremities of the wires into two different vessels, so
that the alkali shall appear in one vessel and the acid in the other.

CAROLINE.

But then the Voltaic circle will not be completed; how can any effect be
produced?

MRS. B.

You are right; I ought to have added that the two vessels must be
connected together by some interposed substance capable of conducting
electricity. A piece of moistened cotton-wick answers this purpose very
well. You see that the cotton (PLATE XIII. fig. 2. c.) has one end
immersed in one glass and the other end in the other, so as to establish
a communication between any fluids contained in them. We shall now put
into each of the glasses a little glauber salt, or sulphat of soda,
(which consists of an acid and an alkali,) and then we shall fill the
glasses with water, which will dissolve the salt. Let us now connect the
glasses by means of the wires (e, d,) with the two ends of the battery,
thus . . . .

  [Illustration: Plate XIII. Vol. II. page 16.

  Fig. 1. Voltaic Battery of improved construction with the Plates
    out of the Cells.

  Fig. 2. 3 & 4. Instances of Chemical decomposition by the Voltaic
    Battery.]

CAROLINE.

The wires are already giving out small bubbles; is this owing to the
decomposition of the salt?

MRS. B.

No; these are bubbles produced by the decomposition of the water, as you
saw in a former experiment. In order to render the separation of the
acid from the alkali visible, I pour into the glass (a), which is
connected with the positive wire, a few drops of a solution of litmus,
which the least quantity of acid turns red; and into the other
glass (b), which is connected with the negative wire, I pour a few drops
of the juice of violets . . . .

EMILY.

The blue solution is already turning red all round the wire.

CAROLINE.

And the violet solution is beginning to turn green. This is indeed very
singular!

MRS. B.

You will be still more astonished when we vary the experiment in this
manner:-- These three glasses (fig. 3. f, g, h,) are, as in the former
instance, connected together by wetted cotton, but the middle one alone
contains a saline solution, the two others containing only distilled
water, coloured as before by vegetable infusions. Yet, on making the
connection with the battery, the alkali will appear in the negative
glass (h), and the acid in the positive glass (f), though neither of
them contained any saline matter.

EMILY.

So that the acid and alkali must be conveyed right and left from the
central glass, into the other glasses, by means of the connecting
moistened cotton?

MRS. B.

Exactly so; and you may render the experiment still more striking, by
putting into the central glass (k, fig. 3.) an alkaline solution, the
glauber salt being placed into the negative glass (l), and the positive
glass (i) containing only water. The acid will be attracted by the
positive wire (m), and will actually appear in the vessel (i), after
passing through the alkaline solution (k), without combining with it,
although, you know, acids and alkalies are so much disposed to combine.
--But this conversation has already much exceeded our usual limits, and
we cannot enlarge more upon this interesting subject at present.

CONVERSATION XIV.

ON ALKALIES.

MRS. B.

Having now given you some idea of the laws by which chemical attractions
are governed, we may proceed to the examination of bodies which are
formed in consequence of these attractions.

The first class of compounds that present themselves to our notice, in
our gradual ascent to the most complicated combinations, are bodies
composed of only two principles. The sulphurets, phosphurets, carburets,
&c. are of this description; but the most numerous and important of
these compounds are the combinations of oxygen with the various simple
substances with which it has a tendency to unite. Of these you have
already acquired some knowledge, but it will be necessary to enter into
further particulars respecting the nature and properties of those most
deserving our notice. Of this class are the ALKALIES and the EARTHS,
which we shall successively examine.

We shall first take a view of the alkalies, of which there are three,
viz. POTASH, SODA, and AMMONIA. The two first are called _fixed
alkalies_, because they exist in a solid form at the temperature of the
atmosphere, and require a great heat to be volatilised. They consist, as
you already know, of metallic bases combined with oxygen. In potash, the
proportions are about eighty-six parts of potassium to fourteen of
oxygen; and in soda, seventy-seven parts of sodium to twenty-three of
oxygen. The third alkali, ammonia, has been distinguished by the name of
_volatile alkali_, because its natural form is that of gas. Its
composition is of a more complicated nature, of which we shall speak
hereafter.

Some of the earths bear so strong a resemblance in their properties to
the alkalies, that it is difficult to know under which head to place
them. The celebrated French chemist, Fourcroy, has classed two of them
(barytes and strontites) with the alkalies; but as lime and magnesia
have almost an equal title to that rank, I think it better not to
separate them, and therefore have adopted the common method of classing
them with the earths, and of distinguishing them by the name of
_alkaline earths_.

The general properties of alkalies are, an acrid burning taste,
a pungent smell, and a caustic action on the skin and flesh.

CAROLINE.

I wonder they should be caustic, Mrs. B., since they contain so little
oxygen.

MRS. B.

Whatever substance has an affinity for any one of the constituents of
animal matter, sufficiently powerful to decompose it, is entitled to the
appellation of caustic. The alkalies, in their pure state, have a very
strong attraction for water, for hydrogen, and for carbon, which, you
know, are the constituent principles of oil, and it is chiefly by
absorbing these substances from animal matter that they effect its
decomposition; for, when diluted with a sufficient quantity of water, or
combined with any oily substance, they lose their causticity.

But, to return to the general properties of alkalies--they change, as we
have already seen, the colour of syrup of violets, and other blue
vegetable infusions, to green; and have, in general, a very great
tendency to unite with acids, although the respective qualities of these
two classes of bodies form a remarkable contrast.

We shall examine the result of the combination of acids and alkalies
more particularly hereafter. It will be sufficient at present to inform
you, that whenever acids are brought in contact with alkalies, or
alkaline earths, they unite with a remarkable eagerness, and form
compounds perfectly different from either of their constituents; these
bodies are called _neutral_ or _compound salts_.

The dry white powder which you see in this phial is pure caustic POTASH;
it is very difficult to preserve it in this state, as it attracts, with
extreme avidity, the moisture from the atmosphere, and if the air were
not perfectly excluded, it would, in a very short time, be actually
melted.

EMILY.

It is then, I suppose, always found in a liquid state?

MRS. B.

No; it exists in nature in a great variety of forms and combinations,
but is never found in its pure separate state; it is combined with
carbonic acid, with which it exists in every part of the vegetable
kingdom, and is most commonly obtained from the ashes of vegetables,
which are the residue that remains after all the other parts have been
volatilised by combustion.

CAROLINE.

But you once said, that after all the volatile parts of a vegetable were
evaporated, the substance that remained was charcoal?

MRS. B.

I am surprised that you should still confound the processes of
volatilisation and combustion. In order to procure charcoal, we
evaporate such parts as can be reduced to vapour by the operation of
heat alone; but when we _burn_ the vegetable, we burn the carbon also,
and convert it into carbonic acid gas.

CAROLINE.

That is true; I hope I shall make no more mistakes in my favourite
theory of combustion.

MRS. B.

Potash derives its name from the _pots_ in which the vegetables, from
which it was obtained, used formerly to be burnt; the alkali remained
mixed with the ashes at the bottom, and was thence called potash.

EMILY.

The ashes of a wood-fire, then, are potash, since they are vegetable
ashes?

MRS. B.

They always contain more or less potash, but are very far from
consisting of that substance alone, as they are a mixture of various
earths and salts which remain after the combustion of vegetables, and
from which it is not easy to separate the alkali in its pure form. The
process by which potash is obtained, even in the imperfect state in
which it is used in the arts, is much more complicated than simple
combustion. It was once deemed impossible to separate it entirely from
all foreign substances, and it is only in chemical laboratories that it
is to be met with in the state of purity in which you find it in this
phial. Wood-ashes are, however, valuable for the alkali which they
contain, and are used for some purposes without any further preparation.
Purified in a certain degree, they make what is commonly called
_pearlash_, which is of great efficacy in taking out grease, in washing
linen, &c.; for potash combines readily with oil or fat, with which it
forms a compound well known to you under the name of _soap_.

CAROLINE.

Really! Then I should think it would be better to wash all linen with
pearlash than with soap, as, in the latter case, the alkali being
already combined with oil, must be less efficacious in extracting
grease.

MRS. B.

Its effect would be too powerful on fine linen, and would injure its
texture; pearlash is therefore only used for that which is of a strong
coarse kind. For the same reason you cannot wash your hands with plain
potash; but, when mixed with oil in the form of soap, it is soft as well
as cleansing, and is therefore much better adapted to the purpose.

Caustic potash, as we already observed, acts on the skin, and animal
fibre, in virtue of its attraction for water and oil, and converts all
animal matter into a kind of saponaceous jelly.

EMILY.

Are vegetables the only source from which potash can be derived?

MRS. B.

No: for though far most abundant in vegetables, it is by no means
confined to that class of bodies, being found also on the surface of the
earth, mixed with various minerals, especially with earths and stones,
whence it is supposed to be conveyed into vegetables by the roots of the
plant. It is also met with, though in very small quantities, in some
animal substances. The most common state of potash is that of
_carbonat_; I suppose you understand what that is?

EMILY.

I believe so; though I do not recollect that you ever mentioned the word
before. If I am not mistaken, it must be a compound salt, formed by the
union of carbonic acid with potash.

MRS. B.

Very true; you see how admirably the nomenclature of modern chemistry is
adapted to assist the memory; when you hear the name of a compound, you
necessarily learn what are its constituent parts; and when you are
acquainted with these constituents, you can immediately name the
compound which they form.

CAROLINE.

Pray, how were bodies arranged and distinguished before this
nomenclature was introduced?

MRS. B.

Chemistry was then a much more difficult study; for every substance had
an arbitrary name, which it derived either from the person who
discovered it, as _Glauber's salts_ for instance; or from some other
circumstance relative to it, though quite unconnected with its real
nature, as potash.

These names have been retained for some of the simple bodies; for as
this class is not numerous, and therefore can easily be remembered, it
has not been thought necessary to change them.

EMILY.

Yet I think it would have rendered the new nomenclature more complete to
have methodised the names of the elementary, as well as of the compound
bodies, though it could not have been done in the same manner. But the
names of the simple substances might have indicated their nature, or, at
least, some of their principal properties; and if, like the acids and
compound salts, all the simple bodies had a similar termination, they
would have been immediately known as such. So complete and regular a
nomenclature would, I think, have given a clearer and more comprehensive
view of chemistry than the present, which is a medley of the old and new
terms.

MRS. B.

But you are not aware of the difficulty of introducing into science an
entire set of new terms; it obliges all the teachers and professors to
go to school again, and if some of the old names, that are least
exceptionable, were not left as an introduction to the new ones, few
people would have had industry and perseverance enough to submit to the
study of a completely new language; and the inferior classes of artists,
who can only act from habit and routine, would, at least for a time,
have felt material inconvenience from a total change of their habitual
terms. From these considerations, Lavoisier and his colleagues, who
invented the new nomenclature, thought it most prudent to leave a few
links of the old chain, in order to connect it with the new one.
Besides, you may easily conceive the inconvenience which might arise
from giving a regular nomenclature to substances, the simple nature of
which is always uncertain; for the new names might, perhaps, have proved
to have been founded in error. And, indeed, cautious as the inventors of
the modern chemical language have been, it has already been found
necessary to modify it in many respects. In those few cases, however, in
which new terms have been adopted to designate simple bodies, these
names have been so contrived as to indicate one of the chief properties
of the body in question; this is the case with oxygen, which, as I
explained to you, signifies generator of acids; and hydrogen generator
of water. If all the elementary bodies had a similar termination, as you
propose, it would be necessary to change the name of any that might
hereafter be found of a compound nature, which would be very
inconvenient in this age of discovery.

But to return to the alkalies. --We shall now try to melt some of this
caustic potash in a little water, as a circumstance occurs during its
solution very worthy of observation. --Do you feel the heat that is
produced?

CAROLINE.

Yes, I do; but is not this directly contrary to our theory of latent
heat, according to which heat is disengaged when fluids become solid,
and cold produced when solids are melted?

MRS. B.

The latter is really the case in all solutions; and if the solution of
caustic alkalies seems to make an exception to the rule, it does not,
I believe, form any solid objection to the theory. The matter may be
explained thus: When water first comes in contact with the potash, it
produces an effect similar to the slaking of lime, that is, the water is
solidified in combining with the potash, and thus loses its latent heat;
this is the heat that you now feel, and which is, therefore, produced
not by the melting of the solid, but by the solidification of the fluid.
But when there is more water than the potash can absorb and solidify,
the latter then yields to the solvent power of the water; and if we do
not perceive the cold produced by its melting, it is because it is
counterbalanced by the heat previously disengaged.*

A very remarkable property of potash is the formation of glass by its
fusion with siliceous earth. You are not yet acquainted with this last
substance, further than its being in the list of simple bodies. It is
sufficient, for the present, that you should know that sand and flint
are chiefly composed of it; alone, it is infusible, but mixed with
potash, it melts when exposed to the heat of a furnace, combines with
the alkali, and runs into glass.

    [Footnote *: This defence of the general theory, however
    plausible, is liable to some obvious objections. The phenomenon
    might perhaps be better accounted for by supposing that a solution
    of alkali in water has less capacity for heat than either water or
    alkali in their separate state.]

CAROLINE.

Who would ever have supposed that the same substance which converts
transparent oil into such an opake body as soap, should transform that
opake substance, sand, into transparent glass!

MRS. B.

The transparency, or opacity of bodies, does not, I conceive, depend so
much upon their intimate nature, as upon the arrangement of their
particles: we cannot have a more striking instance of this, than is
afforded by the different states of carbon, which, though it commonly
appears in the form of a black opake body, sometimes assumes the most
dazzling transparent form in nature, that of diamond, which, you
recollect, is carbon, and which, in all probability, derives its
beautiful transparency from the peculiar arrangement of its particles
during their crystallisation.

EMILY.

I never should have supposed that the formation of glass was so simple a
process as you describe it.

MRS. B.

It is by no means an easy operation to make perfect glass; for if the
sand, or flint, from which the siliceous earth is obtained, be mixed
with any metallic particles, or other substance, which cannot be
vitrified, the glass will be discoloured, or defaced, by opake specks.

CAROLINE.

That, I suppose, is the reason why objects so often appear irregular and
shapeless through a common glass-window.

MRS. B.

This species of imperfection proceeds, I believe, from another cause. It
is extremely difficult to prevent the lower part of the vessels, in
which the materials of glass are fused, from containing a more dense
vitreous matter than the upper, on account of the heavier ingredients
falling to the bottom. When this happens, it occasions the appearance of
veins or waves in the glass, from the difference of density in its
several parts, which produces an irregular refraction of the rays of
light that pass through it.

Another species of imperfection sometimes arises from the fusion not
being continued for a length of time sufficient to combine the two
ingredients completely, or from the due proportion of potash and silex
(which are as two to one) not being carefully observed; the glass, in
those cases, will be liable to alteration from the action of the air, of
salts, and especially of acids, which will effect its decomposition by
combining with the potash, and forming compound salts.

EMILY.

What an extremely useful substance potash is!

MRS. B.

Besides the great importance of potash in the manufactures of glass and
soap, it is of very considerable utility in many of the other arts, and
in its combinations with several acids, particularly the nitric, with
which it forms saltpetre.

CAROLINE.

Then saltpetre must be a _nitrat of potash_? But we are not yet
acquainted with the nitric acid?

MRS. B.

We shall therefore defer entering into the particulars of these
combinations till we come to a general review of the compound salts. In
order to avoid confusion, it will be better at present to confine
ourselves to the alkalies.

EMILY.

Cannot you show us the change of colour which you said the alkalies
produced on blue vegetable infusions?

MRS. B.

Yes; very easily. I shall dip a piece of white paper into this syrup of
violets, which, you see, is of a deep blue, and dyes the paper of the
same colour. --As soon as it is dry, we shall dip it into a solution of
potash, which, though itself colourless, will turn the paper green--

CAROLINE.

So it has, indeed! And do the other alkalies produce a similar effect?

MRS. B.

Exactly the same. --We may now proceed to SODA, which, however
important, will detain us but a very short time; as in all its general
properties it very strongly resembles potash; indeed, so great is their
similitude, that they have been long confounded, and they can now
scarcely be distinguished, except by the difference of the salts which
they form with acids.

The great source of this alkali is the sea, where, combined with a
peculiar acid, it forms the salt with which the waters of the ocean are
so strongly impregnated.

EMILY.

Is not that the common table salt?

MRS. B.

The very same; but again we must postpone entering into the particulars
of this interesting combination, till we treat of the neutral salts.
Soda may be obtained from common salt; but the easiest and most usual
method of procuring it is by the combustion of marine plants, an
operation perfectly analogous to that by which potash is obtained from
vegetables.

EMILY.

From what does soda derive its name?

MRS. B.

From a plant called by us _soda_, and by the Arabs _kali_, which affords
it in great abundance. Kali has, indeed, given its name to the alkalies
in general.

CAROLINE.

Does soda form glass and soap in the same manner as potash?

MRS. B.

Yes, it does; it is of equal importance in the arts, and is even
preferred to potash for some purposes; but you will not be able to
distinguish their properties till we examine the compound salts which
they form with acids; we must therefore leave soda for the present, and
proceed to AMMONIA, or the VOLATILE ALKALI.

EMILY.

I long to hear something of this alkali; is it not of the same nature as
hartshorn?

MRS. B.

Yes, it is, as you will see by-and-bye. This alkali is seldom found in
nature in its pure state; it is most commonly extracted from a compound
salt, called _sal ammoniac_, which was formerly imported from _Ammonia_,
a region of Libya, from which both these salts and the alkali derive
their names. The crystals contained in this bottle are specimens of this
salt, which consists of a combination of ammonia and muriatic acid.

CAROLINE.

Then it should be called _muriat of ammonia_; for though I am ignorant
what muriatic acid is, yet I know that its combination with ammonia
cannot but be so called; and I am surprised to see sal ammoniac
inscribed on the label.

MRS. B.

That is the name by which it has been so long known, that the modern
chemists have not yet succeeded in banishing it altogether; and it is
still sold under that name by druggists, though by scientific chemists
it is more properly called muriat of ammonia.

CAROLINE.

Both the popular and the common name should be inscribed on labels--this
would soon introduce the new nomenclature.

EMILY.

By what means can the ammonia be separated from the muriatic acid?

MRS. B.

By chemical attractions; but this operation is too complicated for you
to understand, till you are better acquainted with the agency of
affinities.

EMILY.

And when extracted from the salt, what kind of substance is ammonia?

MRS. B.

Its natural form, at the temperature of the atmosphere, when free from
combination, is that of gas; and in this state it is called _ammoniacal
gas_. But it mixes very readily with water, and can be thus obtained in
a liquid form.

CAROLINE.

You said that ammonia was more complicated in its composition than the
other alkalies; pray of what principles does it consist?

MRS. B.

It was discovered a few years since, by Berthollet, a celebrated French
chemist, that it consisted of about one part of hydrogen to four parts
of nitrogen. Having heated ammoniacal gas under a receiver, by causing
the electrical spark to pass repeatedly through it, he found that it
increased considerably in bulk, lost all its alkaline properties, and
was actually converted into hydrogen and nitrogen gases; and from the
latest and most accurate experiments, the proportions appear to be, one
volume of nitrogen gas to three of hydrogen gas.

CAROLINE.

Ammonia, therefore, has not, like the two other alkalies, a metallic
basis?

MRS. B.

It is believed it has, though it is extremely difficult to reconcile
that idea with what I have just stated of its chemical nature. But the
fact is, that although this supposed metallic basis of ammonia has never
been obtained distinct and separate, yet both Professor Berzelius, of
Stockholm, and Sir H. Davy, have succeeded in forming a combination of
mercury with the basis of ammonia, which has so much the appearance of
an amalgam, that it strongly corroborates the idea of ammonia having a
metallic basis.* But these theoretical points are full of difficulties
and doubts, and it would be useless to dwell any longer upon them.

Let us therefore return to the properties of volatile alkali. Ammoniacal
gas is considerably lighter than oxygen gas, and only about half the
weight of atmospherical air. It possesses most of the properties of the
fixed alkalies; but cannot be of so much use in the arts on account of
its volatile nature. It is, therefore, never employed in the manufacture
of glass, but it forms soap with oils equally as well as potash and
soda; it resembles them likewise in its strong attraction for water; for
which reason it can be collected in a receiver over mercury only.

    [Footnote *: This amalgam is easily obtained, by placing a globule
    of mercury upon a piece of muriat, or carbonat of ammonia, and
    electrifying this globule by the Voltaic battery. The globule
    instantly begins to expand to three or four times its former size,
    and becomes much less fluid, though without losing its metallic
    lustre, a change which is ascribed to the metallic basis of
    ammonia uniting with the mercury. This is an extremely curious
    experiment.]

CAROLINE.

I do not understand this?

MRS. B.

Do you recollect the method which we used to collect gases in a
glass-receiver over water?

CAROLINE.

Perfectly.

MRS. B.

Ammoniacal gas has so strong a tendency to unite with water, that,
instead of passing through that fluid, it would be instantaneously
absorbed by it. We can therefore neither use water for that purpose, nor
any other liquid of which water is a component part; so that, in order
to collect this gas, we are obliged to have recourse to mercury,
(a liquid which has no action upon it,) and a mercurial bath is used
instead of a water bath, such as we employed on former occasions. Water
impregnated with this gas is nothing more than the fluid which you
mentioned at the beginning of the conversation--hartshorn; it is the
ammoniacal gas escaping from the water which gives it so powerful a
smell.

EMILY.

But there is no appearance of effervescence in hartshorn.

MRS. B.

Because the particles of gas that rise from the water are too subtle and
minute for their effect to be visible.

Water diminishes in density, by being impregnated with ammoniacal gas;
and this augmentation of bulk increases its capacity for caloric.

EMILY.

In making hartshorn, then, or impregnating water with ammonia, heat must
be absorbed, and cold produced?

MRS. B.

That effect would take place if it was not counteracted by another
circumstance; the gas is liquefied by incorporating with the water, and
gives out its latent heat. The condensation of the gas more than
counterbalances the expansion of the water; therefore, upon the whole,
heat is produced. --But if you dissolve ammoniacal gas with ice or snow,
cold is produced. --Can you account for that?

EMILY.

The gas, in being condensed into a liquid, must give out heat; and, on
the other hand, the snow or ice, in being rarefied into a liquid, must
absorb heat; so that, between the opposite effects, I should have
supposed the original temperature would have been preserved.

MRS. B.

But you have forgotten to take into the account the rarefaction of the
water (or melted ice) by the impregnation of the gas; and this is the
cause of the cold which is ultimately produced.

CAROLINE.

Is the _sal volatile_ (the smell of which so strongly resembles
hartshorn) likewise a preparation of ammonia?

MRS. B.

It is carbonat of ammonia dissolved in water; and which, in its concrete
state, is commonly called salts of hartshorn. Ammonia is caustic, like
the fixed alkalies, as you may judge by the pungent effects of
hartshorn, which cannot be taken internally, nor applied to delicate
external parts, without being plentifully diluted with water. --Oil and
acids are very excellent antidotes for alkaline poisons; can you guess
why?

CAROLINE.

Perhaps, because the oil combines with the alkali, and forms soap, and
thus destroys its caustic properties; and the acid converts it into a
compound salt, which, I suppose, is not so pernicious as caustic alkali.

MRS. B.

Precisely so.

Ammoniacal gas, if it be mixed with atmospherical air, and a burning
taper repeatedly plunged into it, will burn with a large flame of a
peculiar yellow colour.

EMILY.

But pray tell me, can ammonia be procured from this Lybian salt only?

MRS. B.

So far from it, that it is contained in, and may be extracted from, all
animal substances whatever. Hydrogen and nitrogen are two of the chief
constituents of animal matter; it is therefore not surprising that they
should occasionally meet and combine in those proportions that compose
ammonia. But this alkali is more frequently generated by the spontaneous
decomposition of animal substances; the hydrogen and nitrogen gases that
arise from putrefied bodies combine, and form the volatile alkali.

Muriat of ammonia, instead of being exclusively brought from Lybia, as
it originally was, is now chiefly prepared in Europe, by chemical
processes. Ammonia, although principally extracted from this salt, can
also be produced by a great variety of other substances. The horns of
cattle, especially those of deer, yield it in abundance, and it is from
this circumstance that a solution of ammonia in water has been called
hartshorn. It may likewise be procured from wool, flesh, and bones; in a
word, any animal substance whatever yields it by decomposition.

We shall now lay aside the alkalies, however important the subject may
be, till we treat of their combination with acids. The next time we meet
we shall examine the earths.

CONVERSATION XV.

ON EARTHS.

MRS. B.

The EARTHS, which we are to-day to examine, are nine in number:

  SILEX,
  ALUMINE,
  BARYTES,
  LIME,
  MAGNESIA,
  STRONTITES,
  YTTRIA,
  GLUCINA,
  ZIRCONIA.

The last three are of late discovery; their properties are but
imperfectly known; and, as they have not yet been applied to use, it
will be unnecessary to enter into any particulars respecting them; we
shall confine our remarks, therefore, to the first five. They are
composed, as you have already learnt, of a metallic basis combined with
oxygen; and, from this circumstance, are incombustible.

CAROLINE.

Yet I have seen turf burnt in the country, and it makes an excellent
fire; the earth becomes red hot, and produces a very great quantity of
heat.

MRS. B.

It is not the earth that burns, my dear, but the roots, grass, and other
remnants of vegetables that are intermixed with it. The caloric, which
is produced by the combustion of these substances, makes the earth red
hot, and this being a bad conductor of heat, retains its caloric a long
time; but were you to examine it when cooled, you would find that it had
not absorbed one particle of oxygen, nor suffered any alteration from
the fire. Earth is, however, from the circumstance just mentioned, an
excellent radiator of heat, and owes its utility, when mixed with fuel,
solely to that property. It is in this point of view that Count Rumford
has recommended balls of incombustible substances to be arranged in
fire-places, and mixed with the coals, by which means the caloric
disengaged by the combustion of the latter is more perfectly reflected
into the room, and an expense of fuel is saved.

EMILY.

I expected that the list of earths would be much more considerable. When
I think of the great variety of soils, I am astonished that there is not
a greater number of earths to form them.

MRS. B.

You might, indeed, almost confine that number to four; for barytes,
strontites, and the others of late discovery, act but so small a part in
this great theatre, that they cannot be reckoned as essential to the
general formation of the globe. And you must not confine your idea of
earths to the formation of soil; for rock, marble, chalk, slate, sand,
flint, and all kinds of stones, from the precious jewels to the
commonest pebbles; in a word, all the immense variety of mineral
products, may be referred to some of these earths, either in a simple
state, or combined the one with the other, or blended with other
ingredients.

CAROLINE.

Precious stones composed of earth! That seems very difficult to
conceive.

EMILY.

Is it more extraordinary than that the most precious of all jewels,
diamond, should be composed of carbon? But diamond forms an exception,
Mrs. B.; for, though a stone, it is not composed of earth.

MRS. B.

I did not specify the exception, as I knew you were so well acquainted
with it. Besides, I would call a diamond a mineral rather than a stone,
as the latter term always implies the presence of some earth.

CAROLINE.

I cannot conceive how such coarse materials can be converted into such
beautiful productions.

MRS. B.

We are very far from understanding all the secret resources of nature;
but I do not think the spontaneous formation of the crystals, which we
call precious stones, one of the most difficult phenomena to comprehend.

By the slow and regular work of ages, perhaps of hundreds of ages, these
earths may be gradually dissolved by water, and as gradually deposited
by their solvent in the undisturbed process of crystallisation. The
regular arrangement of their particles, during their reunion in a solid
mass, gives them that brilliancy, transparency, and beauty, for which
they are so much admired; and renders them in appearance so totally
different from their rude and primitive ingredients.

CAROLINE.

But how does it happen that they are spontaneously dissolved, and
afterwards crystallised?

MRS. B.

The scarcity of many kinds of crystals, as rubies, emeralds, topazes,
&c. shows that their formation is not an operation very easily carried
on in nature. But cannot you imagine that when water, holding in
solution some particles of earth, filters through the crevices of hills
or mountains, and at length dribbles into some cavern, each successive
drop may be slowly evaporated, leaving behind it the particle of earth
which it held in solution? You know that crystallisation is more regular
and perfect, in proportion as the evaporation of the solvent is slow and
uniform; nature, therefore, who knows no limit of time, has, in all
works of this kind, an infinite advantage over any artist who attempts
to imitate such productions.

EMILY.

I can now conceive that the arrangement of the particles of earth,
during crystallisation, may be such as to occasion transparency, by
admitting a free passage to the rays of light; but I cannot understand
why crystallised earths should assume such beautiful colours as most of
them do. Sapphire, for instance, is of a celestial blue; ruby, a deep
red; topaz, a brilliant yellow?

MRS. B.

Nothing is more simple than to suppose that the arrangement of their
particles is such, as to transmit some of the coloured rays of light,
and to reflect others, in which case the stone must appear of the colour
of the rays which it reflects. But besides, it frequently happens that
the colour of a stone is owing to a mixture of some metallic matter.

CAROLINE.

Pray, are the different kinds of precious stones each composed of one
individual earth, or are they formed of a combination of several earths?

MRS. B.

A great variety of materials enters into the composition of most of
them; not only several earths, but sometimes salts and metals. The
earths, however, in their simple state, frequently form very beautiful
crystals; and, indeed, it is in that state only that they can be
obtained perfectly pure.

EMILY.

Is not the Derbyshire spar produced by the crystallisation of earths, in
the way you have just explained? I have been in some of the
subterraneous caverns where it is found, which are similar to those you
have described.

MRS. B.

Yes; but this spar is a very imperfect specimen of crystallisation; it
consists of a variety of ingredients confusedly blended together, as you
may judge by its opacity, and by the various colours and appearances
which it exhibits.

But, in examining the earths in their most perfect and agreeable form,
we must not lose sight of that state in which they are commonly found,
and which, if less pleasing to the eye, is far more interesting by its
utility.

All the earths are more or less endowed with alkaline properties; but
there are four, barytes, magnesia, lime, and strontites, which are
called _alkaline earths_, because they possess those qualities in so
great a degree, as to entitle them, in most respects, to the rank of
alkalies. They combine and form compound salts with acids, in the same
way as alkalies; they are, like them, susceptible of a considerable
degree of causticity, and are acted upon in a similar manner by chemical
tests. --The remaining earths, silex and alumine, with one or two others
of late discovery, are in some degree more earthy, that is to say, they
possess more completely the properties common to all the earths, which
are, insipidity, dryness, unalterableness in the fire, infusibility, &c.

CAROLINE.

Yet, did you not tell us that silex, or siliceous earth, when mixed with
an alkali, was fusible, and run into glass?

MRS. B.

Yes, my dear; but the characteristic properties of earths, which I have
mentioned, are to be considered as belonging to them in a state of
purity only; a state in which they are very seldom to be met with in
nature. --Besides these general properties, each earth has its own
specific characters, by which it is distinguished from any other
substance. --Let us therefore review them separately.

SILEX, or SILICA, abounds in flint, sand, sandstone, agate, jasper, &c.;
it forms the basis of many precious stones, and particularly of those
which strike fire with steel. It is rough to the touch, scratches and
wears away metals; it is acted upon by no acid but the fluoric, and is
not soluble in water by any known process; but nature certainly
dissolves it by means with which we are unacquainted, and thus produces
a variety of siliceous crystals, and amongst these _rock crystal_, which
is the purest specimen of this earth. Silex appears to have been
intended by Providence to form the solid basis of the globe, to serve as
a foundation for the original mountains, and give them that hardness and
durability which has enabled them to resist the various revolutions
which the surface of the earth has successively undergone. From these
mountains siliceous rocks have, during the course of ages, been
gradually detached by torrents of water, and brought down in fragments;
these, in the violence and rapidity of their descent, are sometimes
crumbled to sand, and in this state form the beds of rivers and of the
sea, chiefly composed of siliceous materials. Sometimes the fragments
are broken without being pulverised by their fall, and assume the form
of pebbles, which gradually become rounded and polished.

EMILY.

Pray what is the true colour of silex, which forms such a variety of
different coloured substances? Sand is brown, flint is nearly black, and
precious stones are of all colours.

MRS. B.

Pure silex, such as is found only in the chemist's laboratory, is
perfectly white, and the various colours which it assumes, in the
different substances you have just mentioned, proceed from the different
ingredients with which it is mixed in them.

CAROLINE.

I wonder that silex is not more valuable, since it forms the basis of so
many precious stones.

MRS. B.

You must not forget that the value we set upon precious stones depends
in a great measure upon the scarcity with which nature affords them;
for, were those productions either common or perfectly imitable by art,
they would no longer, notwithstanding their beauty, be so highly
esteemed. But the real value of siliceous earth, in many of the most
useful arts, is very extensive. Mixed with clay, it forms the basis of
all the various kinds of earthen ware, from the most common utensils to
the most refined ornaments.

EMILY.

And we must recollect its importance in the formation of glass with
potash.

MRS. B.

Nor should we omit to mention, likewise, many other important uses of
silex, such as being the chief ingredient of some of the most durable
cements, of mortar, &c.

I said before, that siliceous earth combined with no acid but the
fluoric; it is for this reason that glass is liable to be attacked by
that acid only, which, from its strong affinity for silex, forces that
substance from its combination with the potash, and thus destroys the
glass.

We will now hasten to proceed to the other earths, for I am rather
apprehensive of your growing weary of this part of our subject.

CAROLINE.

The history of the earths is not quite so entertaining as that of the
simple substances.

MRS. B.

Perhaps not; but it is absolutely indispensable that you should know
something of them; for they form the basis of so many interesting and
important compounds, that their total omission would throw great
obscurity on our general outline of chemical science. We shall, however,
review them in as cursory a manner as the subject can admit of.

ALUMINE derives its name from a compound salt called _alum_, of which it
forms the basis.

CAROLINE.

But it ought to be just the contrary, Mrs. B.; the simple body should
give, instead of taking, its name from the compound.

MRS. B.

That is true; but as the compound salt was known long before its basis
was discovered, it was very natural that when the earth was at length
separated from the acid, it should derive its name from the compound
from which it was obtained. However, to remove your scruples, we will
call the salt according to the new nomenclature, _sulphat of alumine_.
From this combination, alumine may be obtained in its pure state; it is
then soft to the touch, makes a paste with water, and hardens in the
fire. In nature, it is found chiefly in clay, which contains a
considerable proportion of this earth; it is very abundant in fuller's
earth, slate, and a variety of other mineral productions. There is
indeed scarcely any mineral substance more useful to mankind than
alumine. In the state of clay, it forms large strata of the earth, gives
consistency to the soil of valleys, and of all low and damp spots, such
as swamps and marshes. The beds of lakes, ponds, and springs, are almost
entirely of clay; instead of allowing of the filtration of water, as
sand does, it forms an impenetrable bottom, and by this means water is
accumulated in the caverns of the earth, producing those reservoirs
whence springs issue, and spout out at the surface.

EMILY.

I always thought that these subterraneous reservoirs of water were
bedded by some hard stone, or rock, which the water could not penetrate.

MRS. B.

That is not the case; for in the course of time water would penetrate,
or wear away silex, or any other kind of stone, while it is effectually
stopped by clay, or alumine.

The solid compact soils, such as are fit for corn, owe their consistence
in a great measure to alumine; this earth is therefore used to improve
sandy or chalky soils, which do not retain a sufficient quantity of
water for the purpose of vegetation.

Alumine is the most essential ingredient in all potteries. It enters
into the composition of brick, as well as that of the finest porcelain;
the addition of silex and water hardens it, renders it susceptible of a
degree of vitrification, and makes it perfectly fit for its various
purposes.

CAROLINE.

I can scarcely conceive that brick and china should be made of the same
materials.

MRS. B.

Brick consists almost entirely of baked clay; but a certain proportion
of silex is essential to the formation of earthen or stone ware. In
common potteries sand is used for that purpose; a more pure silex is,
I believe, necessary for the composition of porcelain, as well as a
finer kind of clay; and these materials are, no doubt, more carefully
prepared, and curiously wrought, in the one case than in the other.
Porcelain owes its beautiful semitransparency to a commencement of
vitrification.

EMILY.

But the commonest earthen-ware, though not transparent, is covered with
a kind of glazing.

MRS. B.

That precaution is equally necessary for use as for beauty, as the ware
would be liable to be spoiled and corroded by a variety of substances,
if not covered with a coating of this kind. In porcelain it consists of
enamel, which is a fine white opake glass, formed of metallic oxyds,
sand, salts, and such other materials as are susceptible of
vitrification. The glazing of common earthen-ware is made chiefly of
oxyd of lead, or sometimes merely of salt, which, when thinly spread
over earthen vessels, will, at a certain heat, run into opake glass.

CAROLINE.

And of what nature are the colours which are used for painting
porcelain?

MRS. B.

They are all composed of metallic oxyds, so that these colours, instead
of receiving injury from the application of fire, are strengthened and
developed by its action, which causes them to undergo different degrees
of oxydation.

Alumine and silex are not only often combined by art, but they have in
nature a very strong tendency to unite, and are found combined, in
different proportions, in various gems and other minerals. Indeed, many
of the precious stones, such as ruby, oriental sapphire, amethyst, &c.
consist chiefly of alumine.

We may now proceed to the alkaline earths, I shall say but a few words
on BARYTES, as it is hardly ever used, except in chemical laboratories.
It is remarkable for its great weight, and its strong alkaline
properties, such as destroying animal substances, turning green some
blue vegetable colours, and showing a powerful attraction for acids;
this last property it possesses to such a degree, particularly with
regard to the sulphuric acid, that it will always detect its presence in
any substance or combination whatever, by immediately uniting with it,
and forming a sulphat of barytes. This renders it a very valuable
chemical test. It is found pretty abundantly in nature in the state of
carbonat, from which the pure earth can be easily separated.

The next earth we have to consider is LIME. This is a substance of too
great and general importance to be passed over so slightly as the last.

Lime is strongly alkaline. In nature it is not met with in its simple
state, as its affinity for water and carbonic acid is so great, that it
is always found combined with these substances, with which it forms the
common lime-stone; but it is separated in the kiln from these
ingredients, which are volatilised whenever a sufficient degree of heat
is applied.

EMILY.

Pure lime, then, is nothing but lime-stone, which has been deprived, in
the kiln, of its water and carbonic acid?

MRS. B.

Precisely: in this state it is called _quick-lime_, and it is so
caustic, that it is capable of decomposing the dead bodies of animals
very rapidly, without their undergoing the process of putrefaction.
--I have here some quick lime, which is kept carefully corked up in a
bottle to prevent the access of air; for were it at all exposed to the
atmosphere, it would absorb both moisture and carbonic acid gas from it,
and be soon slaked. Here is also some lime-stone--we shall pour a little
water on each, and observe the effects that result from it.

CAROLINE.

How the quick-lime hisses! It is become excessively hot! --It swells,
and now it bursts and crumbles to powder, while the water appears to
produce no kind of alteration on the lime-stone.

MRS. B.

Because the lime-stone is already saturated with water, whilst the
quick-lime, which has been deprived of it in the kiln, combines with it
with very great avidity, and produces this prodigious disengagement of
heat, the cause of which I formerly explained to you; do you
recollect it?

EMILY.

Yes; you said that the heat did not proceed from the lime, but from the
water which was _solidified_, and thus parted with its heat of
liquidity.

MRS. B.

Very well. If we continue to add successive quantities of water to the
lime after being slaked and crumbled as you see, it will then gradually
be diffused in the water, till it will at length be dissolved in it, and
entirely disappear; but for this purpose it requires no less than 700
times its weight of water. This solution is called _lime-water_.

CAROLINE.

How very small, then, is the proportion of lime dissolved!

MRS. B.

Barytes is still of more difficult solution; it dissolves only in 900
times its weight of water: but it is much more soluble in the state of
crystals. The liquid contained in this bottle is lime-water; it is often
used as a medicine, chiefly, I believe, for the purpose of combining
with, and neutralising, the superabundant acid which it meets with in
the stomach.

EMILY.

I am surprised that it is so perfectly clear; it does not at all partake
of the whiteness of the lime.

MRS. B.

Have you forgotten that, in solutions, the solid body is so minutely
subdivided by the fluid as to become invisible, and therefore will not
in the least degree impair the transparency of the solvent?

I said that the attraction of lime for carbonic acid was so strong, that
it would absorb it from the atmosphere. We may see this effect by
exposing a glass of lime-water to the air; the lime will then separate
from the water, combine with the carbonic acid, and re-appear on the
surface in the form of a white film, which is carbonat of lime, commonly
called _chalk_.

CAROLINE.

Chalk is, then, a compound salt! I never should have supposed that those
immense beds of chalk, that we see in many parts of the country, were a
salt. --Now, the white film begins to appear on the surface of the
water; but it is far from resembling hard solid chalk.

MRS. B.

That is owing to its state of extreme division; in a little time it will
collect into a more compact mass, and subside at the bottom of the
glass.

If you breathe into lime-water, the carbonic acid, which is mixed with
the air that you expire, will produce the same effect. It is an
experiment very easily made; --I shall pour some lime-water into this
glass tube, and, by breathing repeatedly into it, you will soon perceive
a precipitation of chalk--

EMILY.

I see already a small white cloud formed.

MRS. B.

It is composed of minute particles of chalk; at present it floats in the
water, but it will soon subside.

Carbonat of lime, or chalk, you see, is insoluble in water, since the
lime which was dissolved re-appears when converted into chalk; but you
must take notice of a very singular circumstance, which is, that chalk
is soluble in water impregnated with carbonic acid.

CAROLINE.

It is very curious, indeed, that carbonic acid gas should render lime
soluble in one instance, and insoluble in the other!

MRS. B.

I have here a bottle of Seltzer water, which, you know, is strongly
impregnated with carbonic acid:-- let us pour a little of it into a
glass of lime-water. You see that it immediately forms a precipitation
of carbonat of lime?

EMILY.

Yes, a white cloud appears.

MRS. B.

I shall now pour an additional quantity of the Seltzer water into the
lime-water--

EMILY.

How singular! The cloud is re-dissolved, and the liquid is again
transparent.

MRS. B.

All the mystery depends upon this circumstance, that carbonat of lime is
soluble in carbonic acid, whilst it is insoluble in water; the first
quantity of carbonic acid, therefore, which I introduce into the
lime-water, was employed in forming the carbonat of lime, which remained
visible, until an additional quantity of carbonic acid dissolved it.
Thus, you see, when the lime and carbonic acid are in proper proportions
to form chalk, the white cloud appears, but when the acid predominates,
the chalk is no sooner formed than it is dissolved.

CAROLINE.

That is now the case; but let us try whether a further addition of
lime-water will again precipitate the chalk.

EMILY.

It does, indeed! The cloud re-appears, because, I suppose, there is now
no more of the carbonic acid than is necessary to form chalk; and, in
order to dissolve the chalk, a superabundance of acid is required.

MRS. B.

We have, I think, carried this experiment far enough; every repetition
would but exhibit the same appearances.

Lime combines with most of the acids, to which the carbonic (as being
the weakest) readily yields it; but these combinations we shall have an
opportunity of noticing more particularly hereafter. It unites with
phosphorus, and with sulphur, in their simple state; in short, of all
the earths, lime is that which nature employs most frequently, and most
abundantly, in its innumerable combinations. It is the basis of all
calcareous earths and stones; we find it likewise in the animal and the
vegetable creations.

EMILY.

And in the arts is not lime of very great utility?

MRS. B.

Scarcely any substance more so; you know that it is a most essential
requisite in building, as it constitutes the basis of all cements, such
as mortar, stucco, plaister, &c.

Lime is also of infinite importance in agriculture; it lightens and
warms soils that are too cold, and compact, in consequence of too great
a proportion of clay. --But it would be endless to enumerate the various
purposes for which it is employed; and you know enough of it to form
some idea of its importance; we shall, therefore, now proceed to the
third alkaline earth, MAGNESIA.

CAROLINE.

I am already pretty well acquainted with that earth; it is a medicine.

MRS. B.

It is in the state of carbonat that magnesia is usually employed
medicinally; it then differs but little in appearance from its simple
form, which is that of a very fine light white powder. It dissolves in
2000 times its weight of water, but forms with acids extremely soluble
salts. It has not so great an attraction for acids as lime, and
consequently yields them to the latter. It is found in a great variety
of mineral combinations, such as slate, mica, amianthus, and more
particularly in a certain lime stone, which has lately been discovered
by Mr. Tennant to contain it in very great quantities. It does not
attract and solidify water, like lime: but when mixed with water and
exposed to the atmosphere, it slowly absorbs carbonic acid from the
latter, and thus loses its causticity. Its chief use in medicine is,
like that of lime, derived from its readiness to combine with, and
neutralise, the acid which it meets with in the stomach.

EMILY.

Yet, you said that it was taken in the state of carbonat, in which case
it has already combined with an acid?

MRS. B.

Yes; but the carbonic is the last of all the acids in the order of
affinities; it will therefore yield the magnesia to any of the others.
It is, however, frequently taken in its caustic state as a remedy for
flatulence. Combined with sulphuric acid, magnesia forms another and
more powerful medicine, commonly called _Epsom salt_.

CAROLINE.

And properly, _sulphat of magnesia_, I suppose? Pray why was it ever
called Epsom salt?

MRS. B.

Because there is a spring in the neighbourhood of Epsom which contains
this salt in great abundance.

The last alkaline earth which we have to mention is STRONTIAN, or
STRONTITES, discovered by Dr. Hope a few years ago. It so strongly
resembles barytes in its properties, and is so sparingly found in
nature, and of so little use in the arts, that it will not be necessary
to enter into any particulars respecting it. One of the remarkable
characteristic properties of strontites is, that its salts, when
dissolved in spirit of wine, tinge the flame of a deep red, or blood
colour.

CONVERSATION XVI.

ON ACIDS.

MRS. B.

We may now proceed to the acids. Of the metallic oxyds, you have already
acquired some general notions. This subject, though highly interesting
in its details, is not of sufficient importance to our concise view of
chemistry, to be particularly treated of; but it is absolutely necessary
that you should be better acquainted with the acids, and likewise with
their combinations with the alkalies, which form the triple compounds
called NEUTRAL SALTS.

The class of acids is characterised by very distinct properties. They
all change blue vegetable infusions to a red colour: they are all more
or less sour to the taste; and have a general tendency to combine with
the earths, alkalies, and metallic oxyds.

You have, I believe, a clear idea of the nomenclature by which the base
(or radical) of the acid, and the various degrees of acidification, are
expressed?

EMILY.

Yes, I think so; the acid is distinguished by the name of its base, and
its degree of oxydation, that is, the quantity of oxygen it contains, by
the termination of that name in _ous_ or _ic_; thus sulphure_ous_ acid
is that formed by the smallest proportion of oxygen combined with
sulphur; sulphur_ic_ acid that which results from the combination of
sulphur with the greatest quantity of oxygen.

MRS. B.

A still greater latitude may, in many cases, be allowed to the
proportions of oxygen than can be combined with acidifiable radicals;
for several of these radicals are susceptible of uniting with a quantity
of oxygen so small as to be insufficient to give them the properties of
acids; in these cases, therefore, they are converted into oxyds. Such is
sulphur, which by exposure to the atmosphere with a degree of heat
inadequate to produce inflammation, absorbs a small proportion of
oxygen, which colours it red or brown. This, therefore, is the first
degree of oxygenation of sulphur; the 2d converts it into sulphur_ous_
acid; the 3d into the sulphur_ic_ acid; and 4thly, if it was found
capable of combining with a still larger proportion of oxygen, it would
then be termed _super-oxygenated sulphuric acid_.

EMILY.

Are these various degrees of oxygenation common to all the acids?

MRS. B.

No; they vary much in this respect: some are susceptible of only one
degree of oxygenation; others, of two, or three; there are but very few
that will admit of more.

CAROLINE.

The modern nomenclature must be of immense advantage in pointing out so
easily the nature of the acids, and their various degrees of
oxygenation.

MRS. B.

Till lately many of the acids had not been decomposed; but analogy
afforded so strong a proof of their compound nature, that I never could
reconcile myself to classing them with the simple bodies, though this
division has been adopted by several chemical writers. At present there
are only the muriatic and the fluoric acids, which have not had their
bases distinctly separated.

CAROLINE.

We have heard of a great variety of acids; pray how many are there in
all?

MRS. B.

I believe there are reckoned at present thirty-four, and their number is
constantly increasing, as the science improves; but the most important,
and those to which we shall almost entirely confine our attention, are
but few. I shall, however, give you a general view of the whole; and
then we shall more particularly examine those that are the most
essential.

This class of bodies was formerly divided into mineral, vegetable, and
animal acids, according to the substances from which they were commonly
obtained.

CAROLINE.

That, I should think, must have been an excellent arrangement; why was
it altered?

MRS. B.

Because in many cases it produced confusion. In which class, for
instance, would you place carbonic acid?

CAROLINE.

Now I see the difficulty. I should be at a loss where to place it, as
you have told us that it exists in the animal, vegetable, and mineral
kingdoms.

EMILY.

There would be the same objection with respect to phosphoric acid,
which, though obtained chiefly from bones, can also, you said, be found
in small quantities in stones, and likewise in some plants.

MRS. B.

You see, therefore, the propriety of changing this mode of
classification. These objections do not exist in the present
nomenclature; for the composition and nature of each individual acid is
in some degree pointed out, instead of the class of bodies from which it
is extracted; and, with regard to the more general division of acids,
they are classed under these three heads:

First, Acids of known or supposed simple bases, which are formed by the
union of these bases with oxygen. They are the following:

The _Sulphuric_
    _Carbonic_
    _Nitric_
    _Phosphoric_
    _Arsenical_  Acids, of known and simple bases.
    _Tungstenic_
    _Molybdenic_
    _Boracic_
    _Fluoric_
    _Muriatic_

This class comprehends the most anciently known and most important
acids. The sulphuric, nitric, and muriatic were formerly, and are still
frequently, called _mineral acids_.

2dly, Acids that have double or binary radicals, and which consequently
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 aeriform 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