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_.