NARRATION
LECTURING ABOUT METALS
Ms. Hermi :
The metals, which we are now to examine, are bodies of a very different
nature from those which we
have hitherto considered. They do not, like the bases of gases, elude the
immediate observation of our senses; for they are the most brilliant, the most
ponderous, and the most palpable substances in nature.
Jelpapo :
I doubt, however, whether the metals will appear to us so interesting, and give
us so much entertainment as those mysterious elements which conceal themselves
from our view. Besides, they cannot afford so much novelty; they are bodies
with which we are already so well acquainted.
Ms. Hermi :
You are not aware, my dear, of the interesting discoveries which were a few
years ago made by Sir H. Davy respecting this class of bodies. By the aid of
the Voltaic battery, he has obtained from 315a variety
of substances, metals before unknown, the properties of which are equally new
and curious. We shall begin, however, by noticing those metals with which you
profess to be so well acquainted. But the acquaintance, you will soon perceive,
is but very superficial; and I trust that you will find both novelty and
entertainment in considering the metals in a chemical point of view. To treat
of this subject fully, would require a whole course of lectures; for metals
form of themselves a most important branch of practical chemistry. We must,
therefore, confine ourselves to a general view of them. These bodies are seldom
found naturally in their metallic form: they are generally more or less
oxygenated or combined with sulphur, earths, or acids, and are often blended
with each other. They are found buried in the bowels of the earth in most parts
of the world, but chiefly in mountainous districts, where the surface of the globe
has suffered from the earthquakes, volcanos, and other convulsions of nature.
They are spread in strata or beds, called veins, and these veins are composed
of a certain quantity of metal, combined with various earthy substances, with
which they form minerals of different nature and appearance, which are
called ores.
Jelpapo :
I now feel quite at home, for my father has 316a lead-mine
in Yorkshire, and I have heard a great deal about veins of ore, and of
the roasting and smelting of the lead; but,
I confess, that I do not understand in what these operations consist.
Ms Hermi : Roasting is the process by which the volatile parts of the ore are evaporated;
smelting, that by which the pure metal is afterwards separated from the earthy
remains of the ore. This is done by throwing the whole into a furnace, and
mixing with it certain substances that will combine with the earthy parts and
other foreign ingredients of the ore; the metal being the heaviest, falls to
the bottom, and runs out by proper openings in its pure metallic state.
Jelpapo :You
told us in a preceding lesson that metals had a great affinity for oxygen. Do
they not, therefore, combine with oxygen, when strongly heated in the furnace,
and run out in the state of oxyds?
Ms Hermi :
No; for the scoriæ, or oxyd, which soon forms on the surface of the fused
metal, when it is oxydable, prevents the air from having any further influence
on the mass; so that neither combustion nor oxygenation can take place.
Doni :Are all the metals equally
combustible?
Ms Hermi :
No; their attraction for oxygen varies extremely. There are some that will
combine with it only at a very high temperature, or by the assistance of acids;
whilst there are others that oxydate spontaneously and with great rapidity,
even at the lowest temperature; such is in particular manganese, which scarcely
ever exists in the metallic state, as it immediately absorbs oxygen on being
exposed to the air, and crumbles to an oxyd in the course of a few hours.
Jelpapo :Is not that the oxyd from which you
extracted the oxygen gas?
Ms Hermi :It
is: so that, you see, this metal attracts oxygen at a low temperature, and
parts with it when strongly heated.
Doni : Is there any other metal that
oxydates at the temperature of the atmosphere?
Ms Hermi :
They all do, more or less, excepting gold, silver, and platina.
Copper, lead, and iron, oxydate slowly in the air, and cover themselves with a
sort of rust, a process which depends on the gradual conversion of the
surface into an oxyd. This rusty surface preserves the interior metal from
oxydation, as it prevents the air from coming in contact with it. Strictly
speaking, however, the word rust applies only to the oxyd, which forms on the
surface of iron, when exposed to air and moisture, which oxyd appears to be
united with a small portion of carbonic acid.
Jelpapo :
When metals oxydate from the atmosphere without an elevation of temperature,
some light and heat, I suppose, must be disengaged, though not in
sufficient quantities to be sensible
.
Ms Hermi :
Undoubtedly; and, indeed, it is not surprising that in this case the light and
heat should not be sensible, when you consider how extremely slow, and, indeed,
how imperfectly, most metals oxydate by mere exposure to the atmosphere. For
the quantity of oxygen with which metals are capable of combining, generally
depends upon their temperature; and the absorption stops at various points of
oxydation, according to the degree to which their temperature is raised.
Jelpapo :
That seems very natural; for the greater the quantity of caloric introduced
into a metal, the more will its positive electricity be exalted, and
consequently the stronger will be its affinity for oxygen.
Ms Hermi :
Certainly. When the metal oxygenates with sufficient rapidity for light and
heat to become sensible, combustion actually takes place. But this happens only
at very high temperatures, and the product is nevertheless an oxyd; for though,
as I have just said, metals will combine with different proportions of oxygen,
yet with the exception of only five of them, they are not susceptible of
acidification. Metals change colour during the different degrees of oxydation
which they undergo. Lead, when heated in contact with the atmosphere, first
becomes grey; if its temperature be then raised, it turns yellow, and a still
stronger heat changes it to red. Iron becomes successively a green, brown, and
white oxyd. Copper changes from brown to blue, and lastly green.
Jelpapo : Pray, is the white lead with which
houses are painted prepared by oxydating lead?
Ms Hermi :
Not merely by oxydating, but by being also united with carbonic acid. It is a
carbonat of lead. The mere oxyd of lead is called red lead. Litharge is another
oxyd of lead, containing less oxygen. Almost all the metallic oxyds are used as
paints. The various sorts of ochres consist chiefly of iron more or less
oxydated. And it is a remarkable circumstance, that if you burn metals rapidly,
the light or flame they emit during combustion partakes of the colours which
the oxyd successively assumes.
Doni :
How is that accounted for, Mrs. B.? For light, you know, does not proceed from
the burning body, but from the decomposition of the oxygen gas?
Ms Hermi :
The correspondence of the colour of the light with that of the oxyd which emits
it, is, in all probability, owing to some particles of the metal which are
volatilised and carried off by the caloric.
Doni : It is then a sort of metallic
gas.
Jelpapo :
Why is it reckoned so unwholesome to breathe the air of a place in which metals
are melting?
Ms Hermi : Perhaps the notion is too generally
entertained. But it is true with respect to lead, and some other noxious
metals, because, unless care be taken, the particles of the oxyd which are
volatilised by the heat are inhaled in with the breath, and may produce
dangerous effects. I must show you some instances of the combustion of metals;
it would require the heat of a furnace to make them burn in the common air, but
if we supply them with a stream of oxygen gas, we may easily
accomplish it.
Doni : But it will still, I suppose,
be necessary in some degree to raise their temperature?
Ms Hermi : This, as you shall see, is very easily done,
particularly if the experiment be tried upon a small scale.—I begin by
lighting this piece of charcoal with the candle, and then increase the
rapidity of its combustion by
blowing upon it with a blow-pipe.
Jelpapo :That
I do not understand; for it is not every kind of air, but merely oxygen gas,
that produces combustion. Now you said that in breathing we 322inspired, but did not expire oxygen gas. Why, therefore,
should the air which you breathe through the blow-pipe promote the combustion
of the charcoal?
Ms Hermi :
Because the air, which has but once passed through the lungs, is yet but little
altered, a small portion only of its oxygen being destroyed; so that a
great deal more is gained by increasing the rapidity of the current, by means
of the blow-pipe, than is lost in consequence of the air passing once through
the lungs, as you shall see—
Jelpapo : Yes, indeed, it makes the charcoal
burn much brighter.
Ms Hermi : Whilst it is red-hot, I shall drop some iron
filings on it, and supply them with a current of oxygen gas, by means of this
apparatus. which consists simply of a closed tin cylindrical vessel, full of
oxygen gas, with two apertures and stop-cocks, by one of which a stream of
water is thrown into the vessel through a long funnel, whilst by the other the
gas is forced out through a blow-pipe adapted to it, as the water gains
admittance.—Now that I pour water into the funnel, you may hear the gas issuing
from the 323blow-pipe—I bring the charcoal close to
the current, and drop the filings upon it—
Doni : They emit much the same vivid light as the
combustion of the iron wire in oxygen gas.
Ms Hermi :
The process is, in fact, the same; there is only some difference in the mode of
conducting it. Let us burn some tin in the same manner—you see that it is
equally combustible.—Let us now try some copper—
Doni : This burns with a greenish
flame; it is, I suppose, owing to the colour of the oxyd?
Jelpapo :
Pray, shall we not also burn some gold?
Ms Hermi : That is not in our power, at least in this
way. Gold, silver, and 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. 324The 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. 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.
