platina, are incapable of being oxydated by the greatest heat that we
can produce by the common method. It is from this circumstance, that
they have been called perfect metals. Even these, however, have an
affinity for oxygen; but their oxydation or combustion can be performed
only by means of acids or by electricity. The spark given out by the
Voltaic battery produces at the point of contact a greater degree of
heat than any other process; and it is at this very high temperature
only that the affinity of these metals for oxygen will enable them to
act on each other.
I am sorry that I cannot show you the combustion of the perfect metals
by this process, but it requires a considerable Voltaic battery. You
will see these experiments performed in the most perfect manner, when
you attend the chemical lectures of the Royal Institution. But in the
mean time I can, without difficulty, show you an ingenious apparatus
lately contrived for the purpose of producing intense heats, the power
of which nearly equals that of the largest Voltaic batteries. It simply
consists, you see, in a strong box, made of iron or copper, (PLATE X.
fig. 2.) to which may be adapted this air-syringe or condensing-pump,
and a stop-cock terminating in a small orifice similar to that of a
blow-pipe. By working the condensing syringe, up and down in this
manner, a quantity of air is accumulated in the vessel, which may be
increased to almost any extent; so that if we now turn the stop-cock,
the condensed air will rush out, forming a jet of considerable force;
and if we place the flame of a lamp in the current, you will see how
violently the flame is driven in that direction.
CAROLINE.
It seems to be exactly the same effect as that of a blow-pipe worked by
the mouth, only much stronger.
EMILY.
Yes; and this new instrument has this additional advantage, that it does
not fatigue the mouth and lungs like the common blow-pipe, and requires
no art in blowing.
MRS. B.
Unquestionably; but yet this blow-pipe would be of very limited utility,
if its energy and power could not be greatly increased by some other
contrivance. Can you imagine any mode of producing such an effect?
EMILY.
Could not the reservoir be charged with pure oxygen, instead of common
air, as in the case of the gas-holder?
MRS. B.
Undoubtedly; and this is precisely the contrivance I allude to. The
vessel need only be supplied with air from a bladder full of oxygen,
instead of the air of the room, and this, you see, may be easily done by
screwing the bladder on the upper part of the syringe, so that in
working the syringe the oxygen gas is forced from the bladder into the
condensing vessel.
CAROLINE.
With the aid of this small apparatus, therefore, we could obtain the
same effects as those we have just produced with the gas-holder, by
means of a column of water forcing the gas out of it?
MRS. B.
Yes; and much more conveniently so. But there is a mode of using this
apparatus by which more powerful effects still may be obtained. It
consists in condensing in the reservoir, not oxygen alone, but a mixture
of oxygen and hydrogen in the exact proportion in which they unite to
produce water; and then kindling the jet formed by the mixed gases. The
heat disengaged by this combustion, without the help of any lamp, is
probably the most intense known; and various effects are said to have
been obtained from it which exceed all expectation.
CAROLINE.
But why should we not try this experiment?
MRS. B.
Because it is not exempt from danger; the combustion (notwithstanding
various contrivances which have been resorted to with a view to prevent
accident) being apt to penetrate into the inside of the vessel, and to
produce a dangerous and violent explosion. --We shall, therefore, now
proceed in our subject.
CAROLINE.
I think you said the oxyds of metals could be restored to their metallic
state?
MRS. B.
Yes; this is called _reviving_ a metal. Metals are in general capable of
being revived by charcoal, when heated red hot, charcoal having a
greater attraction for oxygen than the metals. You need only, therefore,
decompose, or unburn the oxyd, by depriving it of its oxygen, and the
metal will be restored to its pure state.
EMILY.
But will the carbon, by this operation, be burnt, and be converted into
carbonic acid?
MRS. B.
Certainly. There are other combustible substances to which metals at a
high temperature will part with their oxygen. They will also yield it to
each other, according to their several degrees of attraction for it; and
if the oxygen goes into a more dense state in the metal which it enters,
than it existed in that which it quits, a proportional disengagement of
caloric will take place.
CAROLINE.
And cannot the oxyds of gold, silver, and platina, which are formed by
means of acids or of the electric fluid, be restored to their metallic
state?
MRS. B.
Yes, they may; and the intervention of a combustible body is not
required; heat alone will take the oxygen from them, convert it into a
gas, and revive the metal.
EMILY.
You said that rust was an oxyd of iron; how is it, then, that water, or
merely dampness, produces it, which, you know, it very frequently does
on steel grates, or any iron instruments?
MRS. B.
In that case the metal decomposes the water, or dampness (which is
nothing but water in a state of vapour), and obtains the oxygen from it.
CAROLINE.
I thought that it was necessary to bring metals to a very high
temperature to enable them to decompose water.
MRS. B.
It is so, if it is required that the process should be performed
rapidly, and if any considerable quantity is to be decomposed. Rust, you
knew, is sometimes months in forming, and then it is only the surface of
the metal that is oxydated.
EMILY.
Metals, then, that do not rust, are incapable of spontaneous oxydation,
either by air or water?
MRS. B.
Yes; and this is the case with the perfect metals, which, on that
account, preserve their metallic lustre so well.
EMILY.
Are all metals capable of decomposing water, provided their temperature
be sufficiently raised?
MRS. B.
No; a certain degree of attraction is requisite, besides the assistance
of heat. Water, you recollect, is composed of oxygen and hydrogen; and,
unless the affinity of the metal for oxygen be stronger than that of
hydrogen, it is in vain that we raise its temperature, for it cannot
take the oxygen from the hydrogen. Iron, zinc, tin, and antimony, have a
stronger affinity for oxygen than hydrogen has, therefore these four
metals are capable of decomposing water. But hydrogen having an
advantage over all the other metals with respect to its affinity for
oxygen, it not only withholds its oxygen from them, but is even capable,
under certain circumstances, of taking the oxygen from the oxyds of
these metals.
EMILY.
I confess that I do not quite understand why hydrogen can take oxygen
from those metals that do not decompose water.
CAROLINE.
Now I think I do perfectly. Lead, for instance, will not decompose
water, because it has not so strong an attraction for oxygen as hydrogen
has. Well, then, suppose the lead to be in a state of oxyd; hydrogen
will take the oxygen from the lead, and unite with it to form water,
because hydrogen has a stronger attraction for oxygen, than oxygen has
for lead; and it is the same with all the other metals which do not
decompose water.
EMILY.
I understand your explanation, Caroline, very well; and I imagine that
it is because lead cannot decompose water that it is so much employed
for pipes for conveying that fluid.
MRS. B.
Certainly; lead is, on that account, particularly appropriate to such
purposes; whilst, on the contrary, this metal, if it was oxydable by
water, would impart to it very noxious qualities, as all oxyds of lead
are more or less pernicious.
But, with regard to the oxydation of metals, the most powerful mode of
effecting it is by means of acids. These, you know, contain a much
greater proportion of oxygen than either air or water; and will, most of
them, easily yield it to metals. Thus, you recollect, the zinc plates of
the Voltaic battery are oxydated by the acid and water, much more
effectually than by water alone.
CAROLINE.
And I have often observed that if I drop vinegar, lemon, or any acid on
the blade of a knife, or on a pair of scissars, it will immediately
produce a spot of rust.
EMILY.
Metals have, then, three ways of obtaining oxygen; from the atmosphere,
from water, and from acids.
MRS. B.
The two first you have already witnessed, and I shall now show you how
metals take the oxygen from an acid. This bottle contains nitric acid;
I shall pour some of it over this piece of copper-leaf . . . . . . .
CAROLINE.
Oh, what a disagreeable smell!
EMILY.
And what is it that produces the effervescency and that thick yellow
vapour?
MRS. B.
It is the acid, which being abandoned by the greatest part of its
oxygen, is converted into a weaker acid, which escapes in the form of
gas.
CAROLINE.
And whence proceeds this heat?
MRS. B.
Indeed, Caroline, I think you might now be able to answer that question
yourself.
CAROLINE.
Perhaps it is that the oxygen enters into the metal in a more solid
state than it existed in the acid, in consequence of which caloric is
disengaged.
MRS. B.
If the combination of the oxygen and the metal results from the union of
their opposite electricities, of course caloric must be given out.
EMILY.
The effervescence is over; therefore I suppose that the metal is now
oxydated.
MRS. B.
Yes. But there is another important connection between metals and acids,
with which I must now make you acquainted. Metals, when in the state of
oxyds, are capable of being dissolved by acids. In this operation they
enter into a chemical combination with the acid, and form an entirely
new compound.
CAROLINE.
But what difference is there between the _oxydation_ and the
_dissolution_ of the metal by an acid?
MRS. B.
In the first case, the metal merely combines with a portion of oxygen
taken from the acid, which is thus partly deoxygenated, as in the
instance you have just seen; in the second case, the metal, after being
previously oxydated, is actually dissolved in the acid, and enters into
a chemical combination with it, without producing any further
decomposition or effervescence. --This complete combination of an oxyd
and an acid forms a peculiar and important class of compound salts.
EMILY.
The difference between an oxyd and a compound salt, therefore, is very
obvious: the one consists of a metal and oxygen; the other of an oxyd
and an acid.
MRS. B.
Very well: and you will be careful to remember that the metals are
incapable of entering into this combination with acids, unless they are
previously oxydated; therefore, whenever you bring a metal in contact
with an acid, it will be first oxydated and afterwards dissolved,
provided that there be a sufficient quantity of acid for both
operations.
There are some metals, however, whose solution is more easily
accomplished, by diluting the acid in water; and the metal will, in this
case, be oxydated, not by the acid, but by the water, which it will
decompose. But in proportion as the oxygen of the water oxydates the
surface of the metal, the acid combines with it, washes it off, and
leaves a fresh surface for the oxygen to act upon: then other coats of
oxyd are successively formed, and rapidly dissolved by the acid, which
continues combining with the new-formed surfaces of oxyd till the whole
of the metal is dissolved. During this process the hydrogen gas of the
water is disengaged, and flies off with effervescence.
EMILY.
Was not this the manner in which the sulphuric acid assisted the iron
filings in decomposing water?
MRS. B.
Exactly; and it is thus that several metals, which are incapable alone
of decomposing water, are enabled to do it by the assistance of an acid,
which, by continually washing off the covering of oxyd, as it is formed,
prepares a fresh surface of metal to act upon the water.
CAROLINE.
The acid here seems to act a part not very different from that of a
scrubbing-brush. --But pray would not this be a good method of cleaning
metallic utensils?
MRS. B.
Yes; on some occasions a weak acid, as vinegar, is used for cleaning
copper. Iron plates, too, are freed from the rust on their surface by
diluted muriatic acid, previous to their being covered with tin. You
must remember, however, that in this mode of cleaning metals the acid
should be quickly afterwards wiped off, otherwise it would produce fresh
oxyd.
CAROLINE.
Let us watch the dissolution of the copper in the nitric acid; for I am
very impatient to see the salt that is to result from it. The mixture is
now of a beautiful blue colour; but there is no appearance of the
formation of a salt; it seems to be a tedious operation.
MRS. B.
The crystallisation of the salt requires some length of time to be
completed; if, however, you are so impatient, I can easily show you a
metallic salt already formed.
CAROLINE.
But that would not satisfy my curiosity half so well as one of our own
manufacturing.
MRS. B.
It is one of our own preparing that I mean to show you. When we
decomposed water a few days since, by the oxydation of iron filings
through the assistance of sulphuric acid, in what did the process
consist?
CAROLINE.
In proportion as the water yielded its oxygen to the iron, the acid
combined with the new-formed oxyd, and the hydrogen escaped alone.
MRS. B.
Very well; the result, therefore, was a compound salt, formed by the
combination of sulphuric acid with oxyd of iron. It still remains in the
vessel in which the experiment was performed. Fetch it, and we shall
examine it.
EMILY.
What a variety of processes the decomposition of water, by a metal and
an acid, implies; 1st, the decomposition of the water; 2dly, the
oxydation of the metal; and 3dly, the formation of a compound salt.
CAROLINE.
Here it is, Mrs. B. --What beautiful green crystals! But we do not
perceive any crystals in the solution of copper in nitrous acid?
MRS. B.
Because the salt is now suspended in the water which the nitrous acid
contains, and will remain so till it is deposited in consequence of rest
and cooling.
EMILY.
I am surprised that a body so opake as iron can be converted into such
transparent crystals.
MRS. B.
It is the union with the acid that produces the transparency; for if the
pure metal were melted, and afterwards permitted to cool and
crystallise, it would be found just as opake as before.
EMILY.
I do not understand the exact meaning of _crystallisation_?
MRS. B.
You recollect that when a solid body is dissolved either by water or
caloric it is not decomposed; but that its integrant parts are only
suspended in the solvent. When the solution is made in water, the
integrant particles of the body will, on the water being evaporated,
again unite into a solid mass by the force of their mutual attraction.
But when the body is dissolved by caloric alone, nothing more is
necessary, in order to make its particles reunite, than to reduce its
temperature. And, in general, if the solvent, whether water or caloric,
be slowly separated by evaporation or by cooling, and care taken that
the particles be not agitated during their reunion, they will arrange
themselves in regular masses, each individual substance assuming a
peculiar form or arrangement; and this is what is called
crystallisation.
EMILY.
Crystallisation, therefore, is simply the reunion of the particles of a
solid body that has been dissolved in a fluid.
MRS. B.
That is a very good definition of it. But I must not forget to observe,
that _heat_ and _water_ may unite their solvent powers; and, in this
case, crystallisation may be hastened by cooling, as well as by
evaporating the liquid.
CAROLINE.
But if the body dissolved is of a volatile nature, will it not evaporate
with the fluid?
MRS. B.
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.