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 aëriform. 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
aëriform state.
MRS. B.