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.