composed—oxygen, which burns our candles, our phosphorus, or anything
else; and this other substance—nitrogen—which will not burn them. This
other part of the air is by far the larger proportion, and it is a very
curious body, when we come to examine it; it is remarkably curious, and
yet you say, perhaps, that it is very uninteresting. It is uninteresting
in some respects because of this—that it shews no brilliant effects of
combustion. If I test it with a taper as I do oxygen and hydrogen, it
does not burn like hydrogen, nor does it make the taper burn like oxygen.
Try it in any way I will, it does neither the one thing nor the other: it
will not take fire; it will not let the taper burn; it puts out the
combustion of everything. There is nothing that will burn in it in common
circumstances. It has no smell; it is not sour; it does not dissolve in
water; it is neither an acid nor an alkali; it is as indifferent to all
our organs as it is possible for a thing to be. And you might say, “It is
nothing; it is not worth chemical attention; what does it do in the air?”
Ah! then come our beautiful and fine results shewn us by an observant
philosophy. Suppose, in place of having nitrogen, or nitrogen and oxygen,
we had pure oxygen as our atmosphere; what would become of us? You know
very well that a piece of iron lit in a jar of oxygen goes on burning to
the end. When you see a fire in an iron grate, imagine where the grate
would go to if the whole of the atmosphere were oxygen. The grate would
burn up more powerfully than the coals—for the iron of the grate itself
is even more combustible than the coals which we burn in it. A fire put
into the middle of a locomotive would be a fire in a magazine of fuel, if
the atmosphere were oxygen. The nitrogen lowers it down and makes it
moderate and useful for us, and then, with all that, it takes away with it
the fumes that you have seen produced from the candle, disperses them
throughout the whole of the atmosphere, and carries them away to places
where they are wanted to perform a great and glorious purpose of good to
man, for the sustenance of vegetation; and thus does a most wonderful
work, although you say, on examining it, “Why, it is a perfectly
indifferent thing.” This nitrogen in its ordinary state is an inactive
element; no action short of the most intense electric force, and then in
the most infinitely small degree, can cause the nitrogen to combine
directly with the other element of the atmosphere, or with other things
round about it; it is a perfectly indifferent, and therefore to say, a
safe substance.
But before I take you to that result, I must tell you about the atmosphere
itself. I have written on this diagram the composition of one hundred
parts of atmospheric air:—
Bulk. Weight.
Oxygen, . . . . . 20 22.3
Nitrogen, . . . . 80 77.7
——— —————
100 100.0
It is a true analysis of the atmosphere, so far as regards the quantity of
oxygen and the quantity of nitrogen present. By our analysis, we find that
5 pints of the atmosphere contain only 1 pint of oxygen, and 4 pints, or 4
parts, of nitrogen by bulk. That is our analysis of the atmosphere. It
requires all that quantity of nitrogen to reduce the oxygen down, so as to
be able to supply the candle properly with fuel, so as to supply us with
an atmosphere which our lungs can healthily and safely breathe; for it is
just as important to make the oxygen right for us to breathe, as it is to
make the atmosphere right for the burning of the fire and the candle.
But now for this atmosphere. First of all, let me tell you the weight of
these gases. A pint of nitrogen weighs 10⁴⁄₁₀ grains, or a cubic foot
weighs 1⅙ ounce. That is the weight of the nitrogen. The oxygen is
heavier: a pint of it weighs 11⁹⁄₁₀ grains, and a cubic foot weighs 1¾
ounce. A pint of air weighs about 10⁷⁄₁₀ grains, and a cubic foot 1⅕
ounce.
[Illustration: Fig. 25.]
You have asked me several times, and I am very glad you have, “How do you
weigh gases?” I will shew you; it is very simple, and easily done. Here is
a balance, and here a copper bottle, made as light as we can consistent
with due strength, turned very nicely in the lathe, and made perfectly
air-tight, with a stop-cock, which we can open and shut, which at present
is open, and therefore allows the bottle to be full of air. I have here a
nicely-adjusted balance, in which I think the bottle, in its present
condition, will be balanced by the weight on the other side. And here is a
pump by which we can force the air into this bottle, and with it we will
force in a certain number of volumes of air, as measured by the pump.
[Twenty measures were pumped in.] We will shut that in and put it in the
balance. See how it sinks: it is much heavier than it was. By what? By
the air that we have forced into it by the pump. There is not a greater
_bulk_ of air, but there is the same bulk of _heavier_ air, because we
have forced in air upon it. And that you may have a fair notion in your
mind as to how much this air measures, here is a jar full of water. We
will open that copper vessel into this jar, and let the air return to its
former state. All I have to do now is to screw them tightly together, and
to turn the taps, when there, you see, is the bulk of the twenty pumps of
air which I forced into the bottle; and to make sure that we have been
quite correct in what we have been doing, we will take the bottle again to
the balance, and, if it is now counterpoised by the original weight, we
shall be quite sure we have made our experiment correctly.
[Illustration: Fig. 26.]
It is balanced; so, you see, we can find out the weight of the extra
volumes of air forced in, in that way, and by that means we are able to
ascertain that a cubic foot of air weighs 1⅕ ounce. But that small
experiment will by no means convey to your mind the whole literal truth of
this matter. It is wonderful how it accumulates when you come to larger
volumes. This bulk of air [a cubic foot] weighs 1⅕ ounce. What do you
think of the contents of that box above there, which I have had made for
the purpose? The air which is within that box weighs one pound—a full
pound; and I have calculated the weight of the air in this room,—you
would hardly imagine it, but it is above a ton. So rapidly do the weights
rise up, and so important is the presence of the atmosphere, and of the
oxygen and the nitrogen in it, and the use it performs in conveying things
to and fro from place to place, and carrying bad vapours to places where
they will do good instead of harm.
Having given you that little illustration with respect to the weight of
the air, let me shew you certain consequences of it. You have a right to
them, because you would not understand so much without it. Do you remember
this kind of experiment? Have you ever seen it? Suppose I take a pump
somewhat similar to the one I had a little while ago to force air into the
bottle, and suppose I place it in such a manner that by certain
arrangements I can apply my hand to it: my hand moves about in the air so
easily that it seems to feel nothing, and I can hardly get velocity enough
by any motion of my own in the atmosphere to make sure that there is much
resistance to it.
[Illustration: Fig. 27.]
But, when I put my hand here [on the air-pump receiver, which was
afterwards exhausted], you see what happens. Why is my hand fastened to
this place, and why am I able to pull this pump about? And see! how is it
that I can hardly get my hand away? Why is this? It is the weight of the
air—the weight of the air that is above. I have another experiment here,
which I think will explain to you more about it. When the air is pumped
from underneath the bladder which is stretched over this glass, you will
see the effect in another shape: the top is quite flat at present, but I
will make a very little motion with the pump, and now look at it—see how
it has gone down, see how it is bent in. You will see the bladder go in
more and more, until at last I expect it will be driven in and broken by
the force of the atmosphere pressing upon it.
[Illustration: Fig. 28.]
[The bladder at last broke with a loud report.] Now, that was done
entirely by the weight of the air pressing on it, and you can easily
understand how that is. The particles that are piled up in the atmosphere
stand upon each other, as these five cubes do. You can easily conceive
that four of these five cubes are resting upon the bottom one, and if I
take that away, the others will all sink down. So it is with the
atmosphere: the air that is above is sustained by the air that is beneath;
and when the air is pumped away from beneath them, the change occurs which
you saw when I placed my hand on the air-pump, and which you saw in the
case of the bladder, and which you shall see better here. I have tied over
this jar a piece of sheet india-rubber, and I am now about to take away
the air from the inside of the jar; and if you will watch the
india-rubber—which acts as a partition between the air below and the air
above—you will see, when I pump, how the pressure shews itself. See where
it is going to—I can actually put my hand into the jar; and yet this
result is only caused by the great and powerful action of the air above.
How beautifully it shews this curious circumstance!
Here is something that you can have a pull at, when I have finished
to-day. It is a little apparatus of two hollow brass hemispheres, closely
fitted together, and having connected with it a pipe and a cock, through
which we can exhaust the air from the inside; and although the two halves
are so easily taken apart, while the air is left within, yet you will see,
when we exhaust it by-and-by, no power of any two of you will be able to
pull them apart. Every square inch of surface that is contained in the
area of that vessel sustains fifteen pounds by weight, or nearly so, when
the air is taken out; and you may try your strength presently in seeing
whether you can overcome that pressure of the atmosphere.
Here is another very pretty thing—the boys’ sucker, only refined by the
philosopher. We young ones have a perfect right to take toys, and make
them into philosophy, inasmuch as now-a-days we are turning philosophy
into toys. Here is a sucker, only it is made of india-rubber: if I clap it
upon the table, you see at once it holds. Why does it hold? I can slip it
about, and yet if I try to pull it up, it seems as if it would pull the
table with it I can easily make it slip about from place to place; but
only when I bring it to the edge of the table can I get it off. It is only
kept down by the pressure of the atmosphere above. We have a couple of
them; and if you take these two and press them together, you will see how
firmly they stick. And, indeed, we may use them as they are proposed to be
used, to stick against windows, or against walls, where they will adhere
for an evening, and serve to hang anything on that you want. I think,
however, that you boys ought to be shewn experiments that you can make at
home; and so here is a very pretty experiment in illustration of the
pressure of the atmosphere. Here is a tumbler of water. Suppose I were to
ask you to turn that tumbler upside-down, so that the water should not
fall out, and yet not be kept in by your hand, but merely by using the
pressure of the atmosphere. Could you do that? Take a wine-glass, either
quite full or half-full of water, and put a flat card on the top, turn it
upside-down, and then see what becomes of the card and of the water. The
air cannot get in because the water by its capillary attraction round the
edge keeps it out.
I think this will give you a correct notion of what you may call the
materiality of the air; and when I tell you that the box holds a pound of
it, and this room more than a ton, you will begin to think that air is
something very serious. I will make another experiment, to convince you of
this positive resistance. There is that beautiful experiment of the
popgun, made so well and so easily, you know, out of a quill, or a tube,
or anything of that kind,—where we take a slice of potato, for instance,
or an apple, and take the tube and cut out a pellet, as I have now done,
and push it to one end. I have made that end tight; and now I take another
piece and put it in: it will confine the air that is within the tube
perfectly and completely for our purpose; and I shall now find it
absolutely impossible by any force of mine to drive that little pellet
close up to the other. It cannot be done. I may press the air to a certain
extent, but if I go on pressing, long before it comes to the second, the
confined air will drive the front one out with a force something like that
of gunpowder; for gunpowder is in part dependent upon the same action that
you see here exemplified.
I saw the other day an experiment which pleased me much, as I thought it
would serve our purpose here. (I ought to have held my tongue for four or
five minutes before beginning this experiment, because it depends upon my
lungs for success.) By the proper application of air I expect to be able
to drive this egg out of one cup into the other by the force of my breath;
but if I fail, it is in a good cause; and I do not promise success,
because I have been talking more than I ought to do to make the experiment
succeed.
[The Lecturer here tried the experiment, and succeeded in blowing the egg
from one egg-cup to the other.]
You see that the air which I blow goes downwards between the egg and the
cup, and makes a blast under the egg, and is thus able to lift a heavy
thing—for a full egg is a very heavy thing for air to lift. If you want
to make the experiment, you had better boil the egg quite hard first, and
then you may very safely try to blow it from one cup to the other, with a
little care.
I have now kept you long enough upon this property of the weight of the
air, but there is another thing I should like to mention. You saw the way
in which, in this popgun, I was able to drive the second piece of potato
half or two-thirds of an inch before the first piece started, by virtue of
the elasticity of the air—just as I pressed into the copper bottle the
particles of air by means of the pump. Now, this depends upon a wonderful
property in the air, namely, its elasticity; and I should like to give you
a good illustration of this. If I take anything that confines the air
properly, as this membrane, which also is able to contract and expand so
as to give us a measure of the elasticity of the air, and confine in this
bladder a certain portion of air; and then, if we take the atmosphere off
from the outside of it, just as in these cases we put the pressure on—if
we take the pressure off, you will see how it will then go on expanding
and expanding, larger and larger, until it will fill the whole of this
bell-jar, shewing you that wonderful property of the air, its elasticity,
its compressibility, and expansibility, to an exceedingly large extent,
and which is very essential for the purposes and services it performs in
the economy of creation.
We will now turn to another very important part of our subject,
remembering that we have examined the candle in its burning, and have
found that it gives rise to various products. We have the products, you
know, of soot, of water, and of something else which you have not yet
examined. We have collected the water, but have allowed the other things
to go into the air. Let us now examine some of these other products.
Here is an experiment which I think will help you in part in this way. We
will put our candle there, and place over it a chimney, thus. I think my
candle will go on burning, because the air-passage is open at the bottom
and the top. In the first place, you see the moisture appearing—that you
know about. It is water produced from the candle by the action of the air
upon its hydrogen. But, besides that, something is going out at the top:
it is not moisture—it is not water—it is not condensable; and yet, after
all, it has very singular properties. You will find that the air coming
out of the top of our chimney is nearly sufficient to blow the light out I
am holding to it; and if I put the light fairly opposed to the current, it
will blow it quite out. You will say that is as it should be; and I am
supposing that you think it ought to do so, because the nitrogen does not
support combustion, and ought to put the candle out, since the candle will
not burn in nitrogen.
[Illustration: Fig. 29.]
But is there nothing else there than nitrogen? I must now anticipate—that
is to say, I must use my own knowledge to supply you with the means that
we adopt for the purpose of ascertaining these things, and examining such
gases as these. I will take an empty bottle—here is one—and if I hold it
over this chimney, I shall get the combustion of the candle below sending
its results into the bottle above; and we shall soon find that this bottle
contains, not merely an air that is bad as regards the combustion of a
taper put into it, but having other properties.
Let me take a little quick-lime and pour some common water on to it—the
commonest water will do. I will stir it a moment, then pour it upon a
piece of filtering paper in a funnel, and we shall very quickly have a
clear water proceeding to the bottle below, as I have here. I have plenty
of this water in another bottle; but, nevertheless, I should like to use
the lime-water that was prepared before you, so that you may see what its
uses are. If I take some of this beautiful clear lime-water, and pour it
into this jar, which has collected the air from the candle, you will see a
change coming about. Do you see that the water has become quite milky?
Observe, that will not happen with air merely. Here is a bottle filled
with air; and if I put a little lime-water into it, neither the oxygen nor
the nitrogen, nor anything else that is in that quantity of air, will make
any change in the lime-water. It remains perfectly clear, and no shaking
of that quantity of lime-water with that quantity of air in its common
state will cause any change; but if I take this bottle with the
lime-water, and hold it so as to get the general products of the candle in
contact with it, in a very short time we shall have it milky. There is the
chalk, consisting of the lime which we used in making the lime-water,
combined with something that came from the candle—that other product
which we are in search of, and which I want to tell you about to-day. This
is a substance made visible to us by its action, which is not the action
of the lime-water either upon the oxygen or upon the nitrogen, nor upon
the water itself, but it is something new to us from the candle. And then
we find this white powder, produced by the lime-water and the vapour from
the candle, appears to us very much like whitening or chalk, and, when
examined, it does prove to be exactly the same substance as whitening or
chalk. So we are led, or have been led, to observe upon the various
circumstances of this experiment, and to trace this production of chalk to
its various causes, to give us the true knowledge of the nature of this
combustion of the candle—to find that this substance, issuing from the
candle, is exactly the same as that substance which would issue from a
retort, if I were to put some chalk into it with a little moisture, and
make it red-hot: you would then find that exactly the same substance would
issue from it as from the candle.
But we have a better means of getting this substance, and in greater
quantity, so as to ascertain what its general characters are. We find this
substance in very great abundance in a multitude of cases where you would
least expect it. All limestones contain a great deal of this gas which
issues from the candle, and which we call _carbonic acid_. All chalks, all
shells, all corals contain a great quantity of this curious air. We find
it fixed in these stones; for which reason Dr. Black called it “fixed
air”—finding it in these fixed things like marble and chalk. He called it
fixed air, because it lost its quality of air, and assumed the condition
of a solid body. We can easily get this air from marble. Here is a jar
containing a little muriatic acid, and here is a taper which, if I put it
into that jar, will shew only the presence of common air. There is, you
see, pure air down to the bottom; the jar is full of it Here is a
substance—marble[17], a very beautiful and superior marble—and if I put
these pieces of marble into the jar, a great boiling apparently goes on.
That, however, is not steam—it is a gas that is rising up; and if I now
search the jar by a candle, I shall have exactly the same effect produced
upon the taper as I had from the air which issued from the end of the
chimney over the burning candle. It is exactly the same action, and caused
by the very same substance that issued from the candle; and in this way we
can get carbonic acid in great abundance—we have already nearly filled
the jar. We also find that this gas is not merely contained in marble.
Here is a vessel in which I have put some common whitening—chalk, which
has been washed in water and deprived of its coarser particles, and so
supplied to the plasterer as whitening. Here is a large jar containing
this whitening and water, and I have here some strong sulphuric acid,
which is the acid you might have to use if you were to make these
experiments (only, in using this acid with limestone, the body that is
produced is an insoluble substance, whereas the muriatic acid produces a
soluble substance that does not so much thicken the water). And you will
seek out a reason why I take this kind of apparatus for the purpose of
shewing this experiment. I do it because you may repeat in a small way
what I am about to do in a large one. You will have here just the same
kind of action; and I am evolving in this large jar carbonic acid, exactly
the same in its nature and properties as the gas which we obtained from
the combustion of the candle in the atmosphere. And no matter how
different the two methods by which we prepare this carbonic acid, you will
see, when we get to the end of our subject, that it is all exactly the
same, whether prepared in the one way or in the other.
We will now proceed to the next experiment with regard to this gas. What
is its nature? Here is one of the vessels full, and we will try it, as we
have done so many other gases, by combustion. You see it is not
combustible, nor does it support combustion. Neither, as we know, does it
dissolve much in water, because we collect it over water very easily.
Then, you know that it has an effect, and becomes white in contact with
lime-water; and when it does become white in that way, it becomes one of
the constituents to make carbonate of lime or limestone.
The next thing I must shew you is, that it really does dissolve a little
in water, and therefore that it is unlike oxygen and hydrogen in that
respect I have here an apparatus by which we can produce this solution. In
the lower part of this apparatus is marble and acid, and in the upper part
cold water. The valves are so arranged that the gas can get from one to
the other. I will set it in action now, and you can see the gas bubbling
up through the water, as it has been doing all night long, and by this
time we shall find that we have this substance dissolved in the water. If
I take a glass and draw off some of the water, I find that it tastes a
little acid to the mouth: it is impregnated with carbonic acid; and if I
now apply a little lime-water to it, that will give us a test of its
presence. This water will make the lime-water turbid and white, which is
proof of the presence of carbonic acid.
Then it is a very weighty gas—it is heavier than the atmosphere. I have
put their respective weights at the lower part of this table, along with,
for comparison, the weights of the other gases we have been examining:—
Pint. Cubic Foot.
Hydrogen, . . . . ¾ grains. ¹⁄₁₂ ounce.
Oxygen, . . . . 11⁹⁄₁₀ „ 1½ „
Nitrogen, . . . . 10⅒ „ 1¼ „
Air,. . . . . . 10⁷⁄₁₆ „ 1⅜ „
Carbonic acid, . . 16⅓ „ 1⁹⁄₁₀ „
A pint of it weighs 16⅓ grains, and a cubic foot weighs 1⁹⁄₁₀ ounce,
almost two ounces. You can see by many experiments that this is a heavy
gas. Suppose I take a glass containing nothing else but air, and from this
vessel containing the carbonic acid I attempt to pour a little of this gas
into that glass; I wonder whether any has gone in or not. I cannot tell by
the appearance, but I can in this way [introducing the taper]. Yes, there
it is, you see; and if I were to examine it by lime-water, I should find
it by that test also. I will take this little bucket, and put it down into
the well of carbonic acid—indeed, we too often have real wells of
carbonic acid—and now, if there is any carbonic acid, I must have got to
it by this time, and it will be in this bucket, which we will examine with
a taper. There it is, you see; it is full of carbonic acid.
[Illustration: Fig. 30.]
There is another experiment by which I will shew you its weight. I have
here a jar suspended at one end of a balance—it is now equipoised; but
when I pour this carbonic acid into the jar on the one side which now
contains air, you will see it sink down at once, because of the carbonic
acid that I pour into it. And now, if I examine this jar with the lighted
taper, I shall find that the carbonic acid has fallen into it, and it no
longer has any power of supporting the combustion. If I blow a
soap-bubble, which of course will be filled with air, and let it fall into
this jar of carbonic acid, it will float.
[Illustration: Fig. 31.]
But I shall first of all take one of these little balloons filled with
air. I am not quite sure where the carbonic acid is; we will just try the
depth, and see whereabouts is its level. There, you see, we have this
bladder floating on the carbonic acid; and if I evolve some more of the
carbonic acid, the bladder will be lifted up higher. There it goes—the
jar is nearly full; and now I will see whether I can blow a soap-bubble on
that, and float it in the same way. [The Lecturer here blew a soap-bubble,
and allowed it to fall into the jar of carbonic acid, when it floated in
it midway.] It is floating, as the balloon floated, by virtue of the
greater weight of the carbonic acid than of the air. And now, having so
far given you the history of the carbonic acid—as to its sources in the
candle, as to its physical properties and weight—when we next meet I
shall shew you of what it is composed, and where it gets its elements
from.
LECTURE VI.
CARBON OR CHARCOAL—COAL-GAS—RESPIRATION AND ITS ANALOGY TO THE BURNING
OF A CANDLE—CONCLUSION.