you; for caloric is invisible; it affects only the sense of feeling; it
was the light which dazzled you.
CAROLINE.
True; but light and caloric are such constant companions, that it is
difficult to separate them, even in idea.
MRS. B.
The easier it is to confound them, the more careful you should be in
making the distinction.
CAROLINE.
But why has the water now risen, and filled part of the receiver?
MRS. B.
Indeed, Caroline, I did not suppose you would have asked such a
question! I dare say, Emily, you can answer it.
EMILY.
Let me reflect . . . . . . The oxygen has combined with the wire; the
caloric has escaped; consequently nothing can remain in the receiver,
and the water will rise to fill the vacuum.
CAROLINE.
I wonder that I did not think of that. I wish that we had weighed the
wire and the oxygen gas before combustion; we might then have found
whether the weight of the oxyd was equal to that of both.
MRS. B.
You might try the experiment if you particularly wished it; but I can
assure you, that, if accurately performed, it never fails to show that
the additional weight of the oxyd is precisely equal to that of the
oxygen absorbed, whether the process has been a real combustion, or a
simple oxygenation.
CAROLINE.
But this cannot be the case with combustions in general; for when any
substance is burnt in the common air, so far from increasing in weight,
it is evidently diminished, and sometimes entirely consumed.
MRS. B.
But what do you mean by the expression _consumed_? You cannot suppose
that the smallest particle of any substance in nature can be actually
destroyed. A compound body is decomposed by combustion; some of its
constituent parts fly off in a gaseous form, while others remain in a
concrete state; the former are called the _volatile_, the latter the
_fixed products_ of combustion. But if we collect the whole of them, we
shall always find that they exceed the weight of the combustible body,
by that of the oxygen which has combined with them during combustion.
EMILY.
In the combustion of a coal fire, then, I suppose that the ashes are
what would be called the fixed product, and the smoke the volatile
product?
MRS. B.
Yet when the fire burns best, and the quantity of volatile products
should be the greatest, there is no smoke; how can you account for that?
EMILY.
Indeed I cannot; therefore I suppose that I was not right in my
conjecture.
MRS. B.
Not quite: ashes, as you supposed, are a fixed product of combustion;
but smoke, properly speaking, is not one of the volatile products, as it
consists of some minute undecomposed particles of the coals that are
carried off by the heated air without being burnt, and are either
deposited in the form of soot, or dispersed by the wind. Smoke,
therefore, ultimately, becomes one of the _fixed_ products of
combustion. And you may easily conceive that the stronger the fire is,
the less smoke is produced, because the fewer particles escape
combustion. On this principle depends the invention of Argand’s Patent
Lamps; a current of air is made to pass through the cylindrical wick of
the lamp, by which means it is so plentifully supplied with oxygen, that
scarcely a particle of oil escapes combustion, nor is there any smoke
produced.
EMILY.
But what then are the volatile products of combustion?
MRS. B.
Various new compounds, with which you are not yet acquainted, and which
being converted by caloric either into vapour or gas, are invisible; but
they can be collected, and we shall examine them at some future period.
CAROLINE.
There are then other gases, besides the oxygen and nitrogen gases.
MRS. B.
Yes, several: any substance that can assume and maintain the form of an
elastic fluid at the temperature of the atmosphere, is called a gas. We
shall examine the several gases in their respective places; but we must
now confine our attention to those that compose the atmosphere.
I shall show you another method of decomposing the atmosphere, which is
very simple. In breathing, we retain a portion of the oxygen, and expire
the nitrogen gas; so that if we breathe in a closed vessel, for a
certain length of time, the air within it will be deprived of its oxygen
gas. Which of you will make the experiment?
CAROLINE.
I should be very glad to try it.
MRS. B.
Very well; breathe several times through this glass tube into the
receiver with which it is connected, until you feel that your breath is
exhausted.
CAROLINE.
I am quite out of breath already!
MRS. B.
Now let us try the gas with a lighted taper.
EMILY.
It is very pure nitrogen gas, for the taper is immediately extinguished.
MRS. B.
That is not a proof of its being pure, but only of the absence of
oxygen, as it is that principle alone which can produce combustion,
every other gas being absolutely incapable of it.
EMILY.
In the methods which you have shown us, for decomposing the atmosphere,
the oxygen always abandons the nitrogen; but is there no way of taking
the nitrogen from the oxygen, so as to obtain the latter pure from the
atmosphere?
MRS. B.
You must observe, that whenever oxygen is taken from the atmosphere, it
is by decomposing the oxygen gas; we cannot do the same with the
nitrogen gas, because nitrogen has a stronger affinity for caloric than
for any other known principle: it appears impossible therefore to
separate it from the atmosphere by the power of affinities. But if we
cannot obtain the oxygen gas, by this means, in its separate state, we
have no difficulty (as you have seen) to procure it in its gaseous form,
by taking it from those substances that have absorbed it from the
atmosphere, as we did with the oxyd of manganese.
EMILY.
Can atmospherical air be recomposed, by mixing due proportions of oxygen
and nitrogen gases?
MRS. B.
Yes: if about one part of oxygen gas be mixed with about four parts of
nitrogen gas, atmospherical air is produced.*
[Footnote *: The proportion of oxygen in the atmosphere varies
from 21 to 22 per cent.]
EMILY.
The air, then, must be an oxyd of nitrogen?
MRS. B.
No, my dear; for there must be a chemical combination between oxygen and
nitrogen in order to produce an oxyd; whilst in the atmosphere these two
substances are separately combined with caloric, forming two distinct
gases, which are simply mixed in the formation of the atmosphere.
I shall say nothing more of oxygen and nitrogen at present, as we shall
continually have occasion to refer to them in our future conversations.
They are both very abundant in nature; nitrogen is the most plentiful in
the atmosphere, and exists also in all animal substances; oxygen forms a
constituent part, both of the animal and vegetable kingdoms, from which
it may be obtained by a variety of chemical means. But it is now time to
conclude our lesson. I am afraid you have learnt more to-day than you
will be able to remember.
CAROLINE.
I assure you that I have been too much interested in it, ever to forget
it. In regard to nitrogen there seems to be but little to remember; it
makes a very insignificant figure in comparison to oxygen, although it
composes a much larger portion of the atmosphere.
MRS. B.
Perhaps this insignificance you complain of may arise from the compound
nature of nitrogen, for though I have hitherto considered it as a simple
body, because it is not known in any natural process to be decomposed,
yet from some experiments of Sir H. Davy, there appears to be reason for
suspecting that nitrogen is a compound body, as we shall see afterwards.
But even in its simple state, it will not appear so insignificant when
you are better acquainted with it; for though it seems to perform but a
passive part in the atmosphere, and has no very striking properties,
when considered in its separate state, yet you will see by-and-bye what
a very important agent it becomes, when combined with other bodies. But
no more of this at present; we must reserve it for its proper place.
CONVERSATION VII.
ON HYDROGEN.
CAROLINE.
The next simple bodies we come to are CHLORINE and IODINE. Pray what
kinds of substances are these; are they also invisible?
MRS. B.
No; for chlorine, in the state of gas, has a distinct greenish colour,
and is therefore visible; and iodine, in the same state, has a beautiful
claret-red colour. The knowledge of these two bodies, however, and the
explanation of their properties, imply various considerations, which you
would not yet be able to understand; we shall therefore defer their
examination to some future conversation, and we shall pass on to the
next simple substance, HYDROGEN, which we cannot, any more than oxygen,
obtain in a visible or palpable form. We are acquainted with it only in
its gaseous state, as we are with oxygen and nitrogen.
CAROLINE.
But in its gaseous state it cannot be called a simple substance, since
it is combined with heat and electricity?
MRS. B.
True, my dear; but as we do not know in nature of any substance which is
not more or less combined with caloric and electricity, we are apt to
say that a substance is in its pure state when combined with those
agents only.
Hydrogen was formerly called _inflammable air_, as it is extremely
combustible, and burns with a great flame. Since the invention of the
new nomenclature, it has obtained the name of hydrogen, which is derived
from two Greek words, the meaning of which is, _to produce water_.
EMILY.
And how does hydrogen produce water?
MRS. B.
By its combustion. Water is composed of eighty-five parts, by weight, of
oxygen, combined with fifteen parts of hydrogen; or of two parts, by
bulk of hydrogen gas, to one part of oxygen gas.
CAROLINE.
Really! is it possible that water should be a combination of two gases,
and that one of these should be inflammable air! Hydrogen must be a most
extraordinary gas that will produce both fire and water.
EMILY.
But I thought you said that combustion could take place in no gas but
oxygen?
MRS. B.
Do you recollect what the process of combustion consists in?
EMILY.
In the combination of a body with oxygen, with disengagement of light
and heat.
MRS. B.
Therefore when I say that hydrogen is combustible, I mean that it has an
affinity for oxygen; but, like all other combustible substances, it
cannot burn unless supplied with oxygen, and also heated to a proper
temperature.
CAROLINE.
The simply mixing fifteen parts of hydrogen, with eighty-five parts of
oxygen gas, will not, therefore, produce water?
MRS. B.
No; water being a much denser fluid than gases, in order to reduce these
gases to a liquid, it is necessary to diminish the quantity of caloric
or electricity which maintains them in an elastic form.
EMILY.
That I should think might be done by combining the oxygen and hydrogen
together; for in combining they would give out their respective
electricities in the form of caloric, and by this means would be
condensed.
CAROLINE.
But you forget, Emily, that in order to make the oxygen and hydrogen
combine, you must begin by elevating their temperature, which increases,
instead of diminishing, their electric energies.
MRS. B.
Emily is, however, right; for though it is necessary to raise their
temperature, in order to make them combine, as that combination affords
them the means of parting with their electricities, it is eventually the
cause of the diminution of electric energy.
CAROLINE.
You love to deal in paradoxes to-day, Mrs. B. --Fire, then, produces
water?
MRS. B.
The combustion of hydrogen gas certainly does; but you do not seem to
have remembered the theory of combustion so well as you thought you
would. Can you tell me what happens in the combustion of hydrogen gas?
CAROLINE.
The hydrogen combines with the oxygen, and their opposite electricities
are disengaged in the form of caloric. --Yes, I think I understand it
now--by the loss of this caloric, the gases are condensed into a liquid.
EMILY.
Water, then, I suppose, when it evaporates and incorporates with the
atmosphere, is decomposed and converted into hydrogen and oxygen gases?
MRS. B.
No, my dear--there you are quite mistaken: the decomposition of water is
totally different from its evaporation; for in the latter case (as you
should recollect) water is only in a state of very minute division; and
is merely suspended in the atmosphere, without any chemical combination,
and without any separation of its constituent parts. As long as these
remain combined, they form WATER, whether in a state of liquidity, or in
that of an elastic fluid, as vapour, or under the solid form of ice.
In our experiments on latent heat, you may recollect that we caused
water successively to pass through these three forms, merely by an
increase or diminution of caloric, without employing any power of
attraction, or effecting any decomposition.
CAROLINE.
But are there no means of decomposing water?
MRS. B.
Yes, several: charcoal, and metals, when heated red hot, will attract
the oxygen from water, in the same manner as they will from the
atmosphere.
CAROLINE.
Hydrogen, I see, is like nitrogen, a poor dependant friend of oxygen,
which is continually forsaken for greater favourites.
MRS. B.
The connection, or friendship, as you choose to call it, is much more
intimate between oxygen and hydrogen, in the state of water, than
between oxygen and nitrogen, in the atmosphere; for, in the first case,
there is a chemical union and condensation of the two substances; in the
latter, they are simply mixed together in their gaseous state. You will
find, however, that, in some cases, nitrogen is quite as intimately
connected with oxygen, as hydrogen is. --But this is foreign to our
present subject.
EMILY.
Water, then, is an oxyd, though the atmospherical air is not?
MRS. B.
It is not commonly called an oxyd, though, according to our definition,
it may, no doubt, be referred to that class of bodies.
CAROLINE.
I should like extremely to see water decomposed.
MRS. B.
I can gratify your curiosity by a much more easy process than the
oxydation of charcoal or metals: the decomposition of water by these
latter means takes up a great deal of time, and is attended with much
trouble; for it is necessary that the charcoal or metal should be made
red hot in a furnace, that the water should pass over them in a state of
vapour, that the gas formed should be collected over the water-bath, &c.
In short, it is a very complicated affair. But the same effect may be
produced with the greatest facility, by the action of the Voltaic
battery, which this will give me an opportunity of exhibiting.
CAROLINE.
I am very glad of that, for I longed to see the power of this apparatus
in decomposing bodies.
MRS. B.
For this purpose I fill this piece of glass-tube (PLATE VIII. fig. 1.)
with water, and cork it up at both ends; through one of the corks I
introduce that wire of the battery which conveys the positive
electricity; and the wire which conveys the negative electricity is made
to pass through the other cork, so that the two wires approach each
other sufficiently near to give out their respective electricities.
[Illustration: Plate VIII. Vol. I. p. 206
Fig. 1. Apparatus for the decomposition of water by the Voltaic
Battery.
Fig. 2. Apparatus for decomposing water by Voltaic Electricity
& obtaining the gasses separate.
Fig. 3. Apparatus for preparing & collecting hydrogen gas.
Fig. 4. Receiver full of hydrogen gas inverted over water.
Fig. 5 Slow combustion of hydrogen gas.
Fig. 6. Apparatus for illustrating the formation of water by the
combustion of hydrogen gas.
Fig. 7. Apparatus for producing harmonic sounds by the combustion
of hydrogen gas.]
CAROLINE.
It does not appear to me that you approach the wires so near as you did
when you made the battery act by itself.
MRS. B.
Water being a better conductor of electricity than air, the two wires
will act on each other at a greater distance in the former than in the
latter.
EMILY.
Now the electrical effect appears: I see small bubbles of air emitted
from each wire.
MRS. B.
Each wire decomposes the water, the positive by combining with its
oxygen which is negative, the negative by combining with its hydrogen
which is positive.
CAROLINE.
That is wonderfully curious! But what are the small bubbles of air?
MRS. B.
Those that appear to proceed from the positive wire, are the result of
the decomposition of the water by that wire. That is to say, the
positive electricity having combined with some of the oxygen of the
water, the particles of hydrogen which were combined with that portion
of oxygen are set at liberty, and appear in the form of small bubbles of
gas or air.
EMILY.
And I suppose the negative fluid having in the same manner combined with
some of the hydrogen of the water, the particles of oxygen that were
combined with it, are set free, and emitted in a gaseous form.
MRS. B.
Precisely so. But I should not forget to observe, that the wires used in
this experiment are made of platina, a metal which is not capable of
combining with oxygen; for otherwise the wire would combine with the
oxygen, and the hydrogen alone would be disengaged.
CAROLINE.
But could not water be decomposed without the electric circle being
completed? If, for instance, you immersed only the positive wire in the
water, would it not combine with the oxygen, and the hydrogen gas be
given out?
MRS. B.
No; for as you may recollect, the battery cannot act unless the circle
be completed; since the positive wire will not give out its electricity,
unless attracted by that of the negative wire.
CAROLINE.
I understand it now. --But look, Mrs. B., the decomposition of the water
which has now been going on for some time, does not sensibly diminish
its quantity--what is the reason of that?
MRS. B.
Because the quantity decomposed is so extremely small. If you compare
the density of water with that of the gases into which it is resolved,
you must be aware that a single drop of water is sufficient to produce
thousands of such small bubbles as those you now perceive.
CAROLINE.
But in this experiment, we obtain the oxygen and hydrogen gases mixed
together. Is there any means of procuring the two gases separately?
MRS. B.
They can be collected separately with great ease, by modifying a little
the experiment. Thus if instead of one tube, we employ two, as you see
here, (c, d, PLATE VIII. fig. 2.) both tubes being closed at one end,
and open at the other; and if after filling these tubes with water, we
place them standing in a glass of water (e), with their open end
downwards, you will see that the moment we connect the wires (a, b)
which proceed upwards from the interior of each tube, the one with one
end of the battery, and the other with the other end, the water in the
tubes will be decomposed; hydrogen will be given out round the wire in
the tube connected with the positive end of the battery, and oxygen in
the other; and these gases will be evolved, exactly in the proportions
which I have before mentioned, namely, two measures of hydrogen for one
of oxygen. We shall now begin the experiment, but it will be some time
before any sensible quantity of the gases can be collected.
EMILY.
The decomposition of water in this way, slow as it is, is certainly very
striking; but I confess that I should be still more gratified, if you
could shew it us on a larger scale, and by a quicker process. I am sorry
that the decomposition of water by charcoal or metals is attended with
so much inconvenience.
MRS. B.
Water may be decomposed by means of metals without any difficulty; but
for this purpose the intervention of an acid is required. Thus, if we
add some sulphuric acid (a substance with the nature of which you are
not yet acquainted) to the water which the metal is to decompose, the
acid disposes the metal to combine with the oxygen of the water so
readily and abundantly, that no heat is required to hasten the process.
Of this I am going to shew you an instance. I put into this bottle the
water that is to be decomposed, as also the metal that is to effect that
decomposition by combining with the oxygen, and the acid which is to
facilitate the combination of the metal and the oxygen. You will see
with what violence these will act on each other.
CAROLINE.
But what metal is it that you employ for this purpose?
MRS. B.
It is iron; and it is used in the state of filings, as these present a
greater surface to the acid than a solid piece of metal. For as it is
the surface of the metal which is acted upon by the acid, and is
disposed to receive the oxygen produced by the decomposition of the
water, it necessarily follows that the greater is the surface, the more
considerable is the effect. The bubbles which are now rising are
hydrogen gas----
CAROLINE.
How disagreeably it smells!
MRS. B.
It is indeed unpleasant, though, I believe, not particularly hurtful. We
shall not, however, suffer any more to escape, as it will be wanted for
experiments. I shall, therefore, collect it in a glass-receiver, by
making it pass through this bent tube, which will conduct it into the
water-bath. (PLATE VIII. fig. 3.)
EMILY.
How very rapidly the gas escapes! it is perfectly transparent, and
without any colour whatever. --Now the receiver is full----
MRS. B.
We shall, therefore, remove it, and substitute another in its place. But
you must observe, that when the receiver is full, it is necessary to
keep it inverted with the mouth under water, otherwise the gas would
escape. And in order that it may not be in the way, I introduce within
the bath, under the water, a saucer, into which I slide the receiver, so
that it can be taken out of the bath and conveyed any where, the water
in the saucer being equally effectual in preventing its escape as that
in the bath. (PLATE VIII. fig. 4.)
EMILY.
I am quite surprised to see what a large quantity of hydrogen gas can be
produced by such a small quantity of water, especially as oxygen is the
principal constituent of water.
MRS. B.
In weight it is; but not in volume. For though the proportion, by
weight, is nearly six parts of oxygen to one of hydrogen, yet the
proportion of the volume of the gases, is about one part of oxygen to
two of hydrogen; so much heavier is the former than the latter.
CAROLINE.
But why is the vessel in which the water is decomposed so hot? As the
water changes from a liquid to a gaseous form, cold should be produced
instead of heat.
MRS. B.
No; for if one of the constituents of water is converted into a gas, the
other becomes solid in combining with the metal.
EMILY.
In this case, then, neither heat nor cold should be produced?
MRS. B.
True: but observe that the sensible heat which is disengaged in this
operation, is not owing to the decomposition of the water, but to an
extrication of heat produced by the mixture of water and sulphuric acid.
I will mix some water and sulphuric acid together in this glass, that
you may feel the surprising quantity of heat that is disengaged by their
union--now take hold of the glass----
CAROLINE.
Indeed I cannot; it feels as hot as boiling water. I should have
imagined there would have been heat enough disengaged to have rendered
the liquid solid.
MRS. B.
As, however, it does not produce that effect, we cannot refer this heat
to the modification called latent heat. We may, however, I think,
consider it as heat of capacity, as the liquid is condensed by its loss;
and if you were to repeat the experiment, in a graduated tube, you would
find that the two liquids, when mixed, occupy considerably less space
than they did separately. --But we will reserve this to another
opportunity, and attend at present to the hydrogen gas which we have
been producing.
If I now set the hydrogen gas, which is contained in this receiver, at
liberty all at once, and kindle it as soon as it comes in contact with
the atmosphere, by presenting it to a candle, it will so suddenly and
rapidly decompose the oxygen gas, by combining with its basis, that an
explosion, or a _detonation_ (as chemists commonly call it), will be
produced. For this purpose, I need only take up the receiver, and
quickly present its open mouth to the candle---- so . . . .
CAROLINE.
It produced only a sort of hissing noise, with a vivid flash of light.
I had expected a much greater report.
MRS. B.