Did you ever sit on the bank of a river in some quiet spot where
the water was deep and clear, and watch the fishes swimming
lazily along? When I was a child this was one of my favourite
occupations in the summertime on the banks of the Thames, and
there was one question which often puzzled me greatly, as I
watched the minnows and gudgeon gliding along through the water.
Why should fishes live in something and be often buffeted about
by waves and currents, while I and others lived on the top of the
earth and not in anything? I do not remember ever asking anyone
about this; and if I had, in those days people did not pay much
attention to children's questions, and probably nobody would have
told me, what I now tell you, that we do live in something quite
as real and often quite as rough and stormy as the water in which
the fishes swim. The something in which we live is air, and the
reason that we do not perceive it, is that we are in it, and that
it is a gas, and invisible to us; while we are above the water in
which the fishes live, and it is a liquid which our eyes can
perceive.
But let us suppose for a moment that a being, whose eyes were so
made that he could see gases as we see liquids, was looking down
from a distance upon our earth. He would see an ocean of air, or
aerial ocean, all round the globe, with birds floating about in
it, and people walking along the bottom, just as we see fish
gliding along the bottom of a river. It is true, he would never
see even the birds come near to the surface, for the highest-
flying bird, the condor, never soars more than five miles from
the ground, and our atmosphere, as we shall see, is at least 100
miles high. So he would call us all deep-air creatures, just as
we talk of deep-sea animals; and if we can imagine that he fished
in this air-ocean, and could pull one of us out of it into space,
he would find that we should gasp and die just as fishes do when
pulled out of the water.
He would also observe very curious things going on in our air-
ocean; he would see large streams and currents of air, which we
call winds, and which would appear to him as ocean-currents do to
us, while near down to the earth he would see thick mists forming
and then disappearing again, and these would be our clouds. From
them he would see rain, hail and snow falling to the earth, and
from time to time bright flashes would shoot across the air-
ocean, which would be our lightning. Nay even the brilliant
rainbow, the northern aurora borealis, and the falling stars,
which seem to us so high up in space, would be seen by him near
to our earth, and all within the aerial ocean.
But as we know of no such being living in space, who can tell us
what takes place in our invisible air, and we cannot see it
ourselves, we must try by experiments to see it with our
imagination, though we cannot with our eyes.
First, then, can we discover what air is? At one time it was
thought that it was a simple gas and could not be separated into
more than one kind. But we are now going to make an experiment
by which it has been shown that air is made of two gases mingled
together, and that one of these gases, called oxygen, is used up
when anything burns, while the other nitrogen is not used, and
only serves to dilute the minute atoms of oxygen. I have here a
glass bell-jar, with a cork fixed tightly in the neck, and I
place the jar over a pan of water, while on the water floats a
plate with a small piece of phosphorus upon it. You will see
that by putting the bell-jar over the water, I have shut in a
certain quantity of air, and my object now is to use up the
oxygen out of this air and leave only nitrogen behind. To do
this I must light the piece of phosphorus, for you will remember
it is in burning that oxygen is used up. I will take the cork
out, light the phosphorus, and cork up the jar again. See! as
the phosphorus burns white fumes fill the jar. These fumes are
phosphoric acid which is a substance made of phosphorous and the
oxygen of the air together.
Now, phosphoric acid melts in water just as sugar does, and in a
few minutes these fumes will disappear. They are beginning to
melt already, and the water from the pan is rising up in the
bell-jar. Why is this? Consider for a moment what we have done.
First, the jar was full of air, that is, of mixed oxygen and
nitrogen; then the phosphorus used up the oxygen making white
fumes; afterwards, the water sucked up these fumes; and so, in
the jar now nitrogen is the only gas left, and the water has
risen up to fill all the rest of the space that was once taken up
with oxygen.
We can easily prove that there is no oxygen now in the jar. I
take out the cork and let a lighted taper down into the gas. If
there were any oxygen the taper would burn, but you see it goes
out directly proving that all the oxygen has been used up by the
phosphorous. When this experiment is made very accurately, we
find that for every pint of oxygen in air there are four pints of
nitrogen, so that the active oxygen-atoms are scattered about,
floating in the sleepy, inactive nitrogen.
It is these oxygen-atoms which we use up when we breathe. If I
had put a mouse under the bell-jar, instead of the phosphorus,
the water would have risen just the same, because the mouse would
have breathed in the oxygen and used it up in its body, joining
it to carbon and making a bad gas, carbonic acid, which would
also melt in the water, and when all the oxygen was used, the
mouse would have died.
Do you see now how foolish it is to live in rooms that are
closely shut up, or to hide your head under the bedclothes when
you sleep? You use up all the oxygen-atoms, and then there are
none left for you to breathe; and besides this, you send out of
your mouth bad fumes, though you cannot see them, and these, when
you breathe them in again, poison you and make you ill.
Perhaps you will say, If oxygen is so useful, why is not the air
made entirely of it? But think for a moment. If there was such
an immense quantity of oxygen, how fearfully fast everything
would burn! Our bodies would soon rise above fever heat from the
quantity of oxygen we should take in, and all fires and lights
would burn furiously. In fact, a flame once lighted would spread
so rapidly that no power on earth could stop it, and everything
would be destroyed. So the lazy nitrogen is very useful in
keeping the oxygen-atoms apart; and we have time, even when a
fire is very large and powerful, to put it out before it has
drawn in more and more oxygen from the surrounding air. Often,
if you can shut a fire into a closed space, as in a closely-shut
room or the hold of a ship, it will go out, because it has used
up all the oxygen in the air.
So, you see, we shall be right in picturing this invisible air
all around us as a mixture of two gases. But when we examine
ordinary air very carefully, we find small quantities of other
gases in it, besides oxygen and nitrogen. First, there is
carbonic acid gas. This is the bad gas which we give out of our
mouths after we have burnt up the oxygen with the carbon of our
bodies inside our lungs; and this carbonic acid is also given out
from everything that burns. If only animals lived in the world,
this gas would soon poison the air; but plants get hold of it,
and in the sunshine they break it up again, as we shall see in
Lecture VII, and use up the carbon, throwing the oxygen back into
the air for us to use. Secondly, there are very small quantities
of ammonia, or the gas which almost chokes you in smelling-salts,
and which, when liquid is commonly called "spirits of hartshorn."
This ammonia is useful to plants, as we shall see by and by.
Lastly, there is a great deal of water in the air, floating about
as invisible vapour or water-dust, and this we shall speak of in
the next lecture. Still, all these gases and vapours in the
atmosphere are in very small quantities, and the bulk of the air
is composed of oxygen and nitrogen.
Having now learned what air is, the next question which presents
itself is, Why does it stay round our earth? You will remember
we saw in the first lecture, that all the little atoms of a gas
are trying to fly away from each other, so that if I turn on this
gas-jet the atoms soon leave it, and reach you at the farther end
of the room, and you can smell the gas. Why, then, do not all
the atoms of oxygen and nitrogen fly away from our earth into
space, and leave us without any air?
Ah! here you must look for another of our invisible forces.
Have you forgotten our giant force, "gravitation," which draws
things together from a distance? This force draws together the
earth and the atoms of oxygen and nitrogen; and as the earth is
very big and heavy, and the atoms of air are light and easily
moved, they are drawn down to the earth and held there by
gravitation. But for all that, the atmosphere does not leave off
trying to fly away; it is always pressing upwards and outwards
with all its might, while the earth is doing its best to hold it
down.
The effect of this is, that near the earth, where the pull
downward is very strong, the air-atoms are drawn very closely
together, because gravitation gets the best of the struggle. But
as we get farther and farther from the earth, the pull downward
becomes weaker, and then the air-atoms spring farther apart, and
the air becomes thinner. Suppose that the lines in this diagram
represent layers of air. Near the earth we have to represent
them as lying closely together, but as they recede from the earth
they are also farther apart.
But the chief reason why the air is thicker or denser nearer the
earth, is because the upper layers press it down. If you have a
heap of papers lying one on the top of the other, you know that
those at the bottom of the heap will be more closely pressed
together than those above, and just the same is the case with the
atoms of the air. Only there is this difference, if the papers
have lain for some time, when you take the top ones off, the
under ones remain close together. But it is not so with the air,
because air is elastic, and the atoms are always trying to fly
apart, so that directly you take away the pressure they spring up
again as far as they can.