that waters have, for some years past, been prepared, impregnated both
with oxygen and hydrogen gases. These are not an imitation of nature,
but are altogether obtained by artificial means. They have been lately
used medicinally, particularly on the continent, where, I understand,
they have acquired some reputation.
EMILY.
If I recollect right, Mrs. B., you told us that carbon was capable of
decomposing water; the affinity between oxygen and carbon must,
therefore, be greater than between oxygen and hydrogen?
MRS. B.
Yes; but this is not the case unless their temperature be raised to a
certain degree. It is only when carbon is red-hot, that it is capable of
separating the oxygen from the hydrogen. Thus, if a small quantity of
water be thrown on a red-hot fire, it will increase rather than
extinguish the combustion; for the coals or wood (both of which contain
a quantity of carbon) decompose the water, and thus supply the fire both
with oxygen and hydrogen gases. If, on the contrary, a large mass of
water be thrown over the fire, the diminution of heat thus produced is
such, that the combustible matter loses the power of decomposing the
water, and the fire is extinguished.
EMILY.
I have heard that fire-engines sometimes do more harm than good, and
that they actually increase the fire when they cannot throw water enough
to extinguish it. It must be owing, no doubt, to the decomposition of
the water by the carbon during the conflagration.
MRS. B.
Certainly. --The apparatus which you see here (PLATE XI. fig. 3.), may
be used to exemplify what we have just said. It consists in a kind of
open furnace, through which a porcelain tube, containing charcoal,
passes. To one end of the tube is adapted a glass retort with water in
it; and the other end communicates with a receiver placed on the
water-bath. A lamp being applied to the retort, and the water made to
boil, the vapour is gradually conveyed through the red-hot charcoal, by
which it is decomposed; and the hydrogen gas which results from this
decomposition is collected in the receiver. But the hydrogen thus
obtained is far from being pure; it retains in solution a minute portion
of carbon, and contains also a quantity of carbonic acid. This renders
it heavier than pure hydrogen gas, and gives it some peculiar
properties; it is distinguished by the name of _carbonated hydrogen
gas_.
CAROLINE.
And whence does it obtain the carbonic acid that is mixed with it?
EMILY.
I believe I can answer that question, Caroline. --From the union of the
oxygen (proceeding from the decomposed water) with the carbon, which,
you know, makes carbonic acid.
CAROLINE.
True; I should have recollected that. --The product of the decomposition
of water by red-hot charcoal, therefore, is carbonated hydrogen gas, and
carbonic acid gas.
MRS. B.
You are perfectly right now.
Carbon is frequently found combined with hydrogen in a state of
solidity, especially in coals, which owe their combustible nature to
these two principles.
EMILY.
Is it the hydrogen, then, that produces the flame of coals?
MRS. B.
It is so; and when all the hydrogen is consumed, the carbon continues to
burn without flame. But again, as I mentioned when speaking of the
gas-lights, the hydrogen gas produced by the burning of coals is not
pure; for, during the combustion, particles of carbon are successively
volatilized with the hydrogen, with which they form what is called a
_hydro-carbonat_, which is the principal product of this combustion.
Carbon is a very bad conductor of heat; for this reason, it is employed
(in conjunction with other ingredients) for coating furnaces and other
chemical apparatus.
EMILY.
Pray what is the use of coating furnaces?
MRS. B.
In most cases, in which a furnace is used, it is necessary to produce
and preserve a great degree of heat, for which purpose every possible
means are used to prevent the heat from escaping by communicating with
other bodies, and this object is attained by coating over the inside of
the furnace with a kind of plaster, composed of materials that are bad
conductors of heat.
Carbon, combined with a small quantity of iron, forms a compound called
plumbago, or black-lead, of which pencils are made. This substance,
agreeably to the nomenclature, is _a carburet of iron_.
EMILY.
Why, then, is it called black-lead?
MRS. B.
It is an ancient name given to it by ignorant people, from its shining
metallic appearance; but it is certainly a most improper name for it, as
there is not a particle of lead in the composition. There is only one
mine of this mineral, which is in Cumberland. It is supposed to approach
as nearly to pure carbon as the best prepared charcoal does, as it
contains only five parts of iron, unadulterated by any other foreign
ingredients. There is another carburet of iron, in which the iron,
though united only to an extremely small proportion of carbon, acquires
very remarkable properties; this is steel.
CAROLINE.
Really; and yet steel is much harder than iron?
MRS. B.
But carbon is not ductile like iron, and therefore may render the steel
more brittle, and prevent its bending so easily. Whether it is that the
carbon, by introducing itself into the pores of the iron, and, by
filling them, makes the metal both harder and heavier; or whether this
change depends upon some chemical cause, I cannot pretend to decide. But
there is a subsequent operation, by which the hardness of steel is very
much increased, which simply consists in heating the steel till it is
red-hot, and then plunging it into cold water.
Carbon, besides the combination just mentioned, enters into the
composition of a vast number of natural productions, such, for instance,
as all the various kinds of oils, which result from the combination of
carbon, hydrogen, and caloric, in various proportions.
EMILY.
I thought that carbon, hydrogen, and caloric, formed carbonated hydrogen
gas?
MRS. B.
That is the case when a small portion of carbonic acid gas is held in
solution by hydrogen gas. Different proportions of the same principles,
together with the circumstances of their union, produce very different
combinations; of this you will see innumerable examples. Besides, we are
not now talking of gases, but of carbon and hydrogen, combined only with
a quantity of caloric sufficient to bring them to the consistency of oil
or fat.
CAROLINE.
But oil and fat are not of the same consistence?
MRS. B.
Fat is only congealed oil; or oil, melted fat. The one requires a little
more heat to maintain it in a fluid state than the other. Have you never
observed the fat of meat turned to oil by the caloric it has imbibed
from the fire?
EMILY.
Yet oils in general, as salad-oil, and lamp-oil, do not turn to fat when
cold?
MRS. B.
Not at the common temperature of the atmosphere, because they retain too
much caloric to congeal at that temperature; but if exposed to a
sufficient degree of cold, their latent heat is extricated, and they
become solid fat substances. Have you never seen salad oil frozen in
winter?
EMILY.
Yes; but it appears to me in that state very different from animal fat.
MRS. B.
The essential constituent parts of either vegetable or animal oils are
the same, carbon and hydrogen; their variety arises from the different
proportions of these substances, and from other accessory ingredients
that may be mixed with them. The oil of a whale, and the oil of roses,
are, in their essential constituent parts, the same; but the one is
impregnated with the offensive particles of animal matter, the other
with the delicate perfume of a flower.
The difference of _fixed oils_, and _volatile_ or _essential oils_,
consists also in the various proportions of carbon and hydrogen. Fixed
oils are those which will not evaporate without being decomposed; this
is the case with all common oils, which contain a greater proportion of
carbon than the essential oils. The essential oils (which comprehend the
whole class of essences and perfumes) are lighter; they contain more
equal proportions of carbon and hydrogen, and are volatilized or
evaporated without being decomposed.
EMILY.
When you say that one kind of oil will evaporate, and the other be
decomposed, you mean, I suppose, by the application of heat?
MRS. B.
Not necessarily; for there are oils that will evaporate slowly at the
common temperature of the atmosphere; but for a more rapid
volatilization, or for their decomposition, the assistance of heat is
required.
CAROLINE.
I shall now remember, I think, that fat and oil are really the same
substances, both consisting of carbon and hydrogen; that in fixed oils
the carbon preponderates, and heat produces a decomposition; while, in
essential oils, the proportion of hydrogen is greater, and heat produces
a volatilization only.
EMILY.
I suppose the reason why oil burns so well in lamps is because its two
constituents are so combustible?
MRS. B.
Certainly; the combustion of oil is just the same as that of a candle;
if tallow, it is only oil in a concrete state; if wax, or spermaceti,
its chief chemical ingredients are still hydrogen and carbon.
EMILY.
I wonder, then, there should be so great a difference between tallow and
wax?
MRS. B.
I must again repeat, that the same substances, in different proportions,
produce results that have sometimes scarcely any resemblance to each
other. But this is rather a general remark that I wish to impress upon
your minds, than one which is applicable to the present case; for tallow
and wax are far from being very dissimilar; the chief difference
consists in the wax being a purer compound of carbon and hydrogen than
the tallow, which retains more of the gross particles of animal matter.
The combustion of a candle, and that of a lamp, both produce water and
carbonic acid gas. Can you tell me how these are formed?
EMILY.
Let me reflect . . . . Both the candle and lamp burn by means of fixed
oil--this is decomposed as the combustion goes on; and the constituent
parts of the oil being thus separated, the carbon unites to a portion of
oxygen from the atmosphere to form carbonic acid gas, whilst the
hydrogen combines with another portion of oxygen, and forms with it
water. --The products, therefore, of the combustion of oils are water
and carbonic acid gas.
CAROLINE.
But we see neither water nor carbonic acid produced by the combustion of
a candle.
MRS. B.
The carbonic acid gas, you know, is invisible, and the water being in a
state of vapour, is so likewise. Emily is perfectly correct in her
explanation, and I am very much pleased with it.
All the vegetable acids consist of various proportions of carbon and
hydrogen, acidified by oxygen. Gums, sugar, and starch, are likewise
composed of these ingredients; but, as the oxygen which they contain is
not sufficient to convert them into acids, they are classed with the
oxyds, and called vegetable oxyds.
CAROLINE.
I am very much delighted with all these new ideas; but, at the same
time, I cannot help being apprehensive that I may forget many of them.
MRS. B.
I would advise you to take notes, or, what would answer better still, to
write down, after every lesson, as much of it as you can recollect. And,
in order to give you a little assistance, I shall lend you the heads or
index, which I occasionally consult for the sake of preserving some
method and arrangement in these conversations. Unless you follow some
such plan, you cannot expect to retain nearly all that you learn, how
great soever be the impression it may make on you at first.
EMILY.
I will certainly follow your advice. --Hitherto I have found that I
recollected pretty well what you have taught us; but the history of
carbon is a more extensive subject than any of the simple bodies we have
yet examined.
MRS. B.
I have little more to say on carbon at present; but hereafter you will
see that it performs a considerable part in most chemical operations.
CAROLINE.
That is, I suppose, owing to its entering into the composition of so
great a variety of substances?
MRS. B.
Certainly; it is the basis, you have seen, of all vegetable matter; and
you will find that it is very essential to the process of animalization.
But in the mineral kingdom also, particularly in its form of carbonic
acid, we shall often discover it combined with a great variety of
substances.
In chemical operations, carbon is particularly useful, from its very
great attraction for oxygen, as it will absorb this substance from many
oxygenated or burnt bodies, and thus deoxygenate, or _unburn_ them, and
restore them to their original combustible state.
CAROLINE.
I do not understand how a body can be _unburnt_, and restored to its
original state. This piece of tinder, for instance, that has been burnt,
if by any means the oxygen were extracted from it, would not be restored
to its former state of linen; for its texture is destroyed by burning,
and that must be the case with all organized or manufactured substances,
as you observed in a former conversation.
MRS. B.
A compound body is decomposed by combustion in a way which generally
precludes the possibility of restoring it to its former state; the
oxygen, for instance, does not become fixed in the tinder, but it
combines with its volatile parts, and flies off in the shape of gas, or
watery vapour. You see, therefore, how vain it would be to attempt the
recomposition of such bodies. But, with regard to simple bodies, or at
least bodies whose component parts are not disturbed by the process of
oxygenation or deoxygenation, it is often possible to restore them,
after combustion, to their original state. --The metals, for instance,
undergo no other alteration by combustion than a combination with
oxygen; therefore, when the oxygen is taken from them, they return to
their pure metallic state. But I shall say nothing further of this at
present, as the metals will furnish ample subject for another morning;
and they are the class of simple bodies that come next under
consideration.
CONVERSATION X.
ON METALS.
MRS. B.
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.
CAROLINE.
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.
MRS. B.
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 a 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_.
CAROLINE.
I now feel quite at home, for my father has a 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.
MRS. B.
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.
EMILY.
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?
MRS. B.
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.
CAROLINE.
Are all the metals equally combustible?
MRS. B.
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.
EMILY.
Is not that the oxyd from which you extracted the oxygen gas?
MRS. B.
It is: so that, you see, this metal attracts oxygen at a low
temperature, and parts with it when strongly heated.
EMILY.
Is there any other metal that oxydates at the temperature of the
atmosphere?
MRS. B.
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.
EMILY.
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.
MRS. B.
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.
EMILY.
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.
MRS. B.
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.
EMILY.
Pray, is the white lead with which houses are painted prepared by
oxydating lead?
MRS. B.
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.
CAROLINE.
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?
MRS. B.
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.
CAROLINE.
It is then a sort of metallic gas.
EMILY.
Why is it reckoned so unwholesome to breathe the air of a place in which
metals are melting?
MRS. B.
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.
CAROLINE.
But it will still, I suppose, be necessary in some degree to raise their
temperature?
MRS. B.
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. (PLATE XII. fig. 1.)
[Illustration: Plate XII.
Apparatus for the combustion of metals by means of oxygen gas.
Fig. 1.
Igniting charcoal with a taper & blow-pipe.
Fig. 2.
Combustion of metals by means of a blow-pipe conveying a stream of
oxygen gas from a gas holder.]
EMILY.
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
inspired, but did not expire oxygen gas. Why, therefore, should the air
which you breathe through the blow-pipe promote the combustion of the
charcoal?
MRS. B.
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--
EMILY.
Yes, indeed, it makes the charcoal burn much brighter.
MRS. B.
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, (PLATE
XII. fig 2.) 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 blow-pipe--I bring the charcoal close
to the current, and drop the filings upon it--
CAROLINE.
They emit much the same vivid light as the combustion of the iron wire
in oxygen gas.
MRS. B.
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--
CAROLINE.
This burns with a greenish flame; it is, I suppose, owing to the colour
of the oxyd?
EMILY.
Pray, shall we not also burn some gold?
MRS. B.