Arabella B. Buckley.

The Fairy-Land of Science online

. (page 4 of 14)
Online LibraryArabella B. BuckleyThe Fairy-Land of Science → online text (page 4 of 14)
Font size
QR-code for this ebook


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.



Week 8

I have here an ordinary pop-gun. If I push the cork in very
tight, and then force the piston slowly inwards, I can compress
the air a good deal. Now I am forcing the atoms nearer and
nearer together, but at last they rebel so strongly against being
more crowded that the cork cannot resist their pressure. Out it
flies, and the atoms spread themselves out comfortably again in
the air all around them. Now, just as I pressed the air together
in the pop-gun, so the atmosphere high up above the earth presses
on the air below and keeps the atoms closely packed together.
And in this case the atoms cannot force back the air above them
as they did the cork in the pop-gun; they are obliged to submit
to be pressed together.

Even a short distance from the earth, however, at the top of a
high mountain, the air becomes lighter, because it has less
weight of atmosphere above it, and people who go up in balloons
often have great difficulty in breathing, because the air is so
thin and light. In 1804 a Frenchman, named Gay-Lussac, went up
four miles and a half in a balloon, and brought down some air;
and he found that it was much less heavy than the same quantity
of air taken close down to the earth, showing that it was much
thinner, or rarer, as it is called;* and when, in 1862, Mr.
Glaisher and Mr. Coxwell went up five miles and a half, Mr.
Glaisher's veins began to swell, and his head grew dizzy, and he
fainted. The air was too thin for him to breathe enough in at a
time, and it did not press heavily enough on the drums of his
ears and the veins of his body. He would have died if Mr.
Coxwell had not quickly let off some of the gas in the balloon,
so that it sank down into denser air. (*100 cubic inches near the
earth weighed 31 grains, while the same quantity taken at four
and a half miles up in the air weighed only 12 grains, or two-
fifths of the weight.)

And now comes another very interesting question. If the air gets
less and less dense as it is farther from the earth, where does
it stop altogether? We cannot go up to find out, because we
should die long before we reached the limit; and for a long time
we had to guess about how high the atmosphere probably was, and
it was generally supposed not to be more than fifty miles. But
lately, some curious bodies, which we should have never suspected
would be useful to us in this way, have let us into the secret of
the height of the atmosphere. These bodies are the meteors, or
falling stars.

Most people, at one time or another, have seen what looks like a
star shoot right across the sky, and disappear. On a clear
starlight night you may often see one or more of these bright
lights flash through the air; for one falls on an average in
every twenty minutes, and on the nights of August 9th and
November 13th there are numbers in one part of the sky. These
bodies are not really stars; they are simply stones or lumps of
metal flying through the air, and taking fire by clashing against
the atoms of oxygen in it. There are great numbers of these
masses moving round and round the sun, and when our earth comes
across their path, as it does especially in August and November,
they dash with such tremendous force through the atmosphere that
they grow white-hot, and give out light, and then disappear,
melted into vapour. Every now and then one falls to the earth
before it is all melted away, and thus we learn that these stones
contain tin, iron, sulphur, phosphorus, and other substances.

It is while these bodies are burning that they look to us like
falling stars, and when we see them we know that hey must be
dashing against our atmosphere. Now if two people stand a
certain known distance, say fifty miles, apart on the earth and
observe these meteors and the direction in which they each see
them fall, they can calculate (by means of the angle between the
two directions) how high they are above them when they first see
them, and at that moment they must have struck against the
atmosphere, and even travelled some way through it, to become
white-hot. In this way we have learnt that meteors burst into
light at least 100 miles above the surface of the earth, and so
the atmosphere must be more than 100 miles high.

Our next question is as to the weight of our aerial ocean. You
will easily understand that all this air weighing down upon the
earth must be very heavy, even though it grows lighter as it
ascends. The atmosphere does, in fact, weigh down upon land at
the level of the sea as much as if a 15-pound weight were put
upon every square inch of land. This little piece of linen
paper, which I am holding up, measures exactly a square inch, and
as it lies on the table, it is bearing a weight of 15 lbs. on its
surface. But how, then, comes it that I can lift it so easily?
Why am I not conscious of the weight?

To understand this you must give all your attention, for it is
important and at first not very easy to grasp. you must
remember, in the first place, that the air is heavy because it is
attracted to the earth, and in the second place, that since air
is elastic all the atoms of it are pushing upwards against this
gravitation. And so, at any point in air, as for instance the
place where the paper now is as I hold it up, I feel no pressure
because exactly as much as gravitation is pulling the air down,
so much elasticity is resisting and pushing it up. So the
pressure is equal upwards, downwards, and on all sides, and I can
move the paper with equal ease any way.

Even if I lay the paper on the table this is still true, because
there is always some air under it. If, however, I could get the
air quite away from one side of the paper, then the pressure on
the other side would show itself. I can do this by simply
wetting the paper and letting it fall on the table, and the water
will prevent any air from getting under it. Now see! if I try to
lift it by the thread in the middle, I have great difficulty,
because the whole 15 pounds' weight of the atmosphere is pressing
it down. A still better way of making the experiment is with a
piece of leather, such as the boys often amuse themselves with in
the streets. This piece of leather has been well soaked. I drop
it on the floor and see! it requires all my strength to pull it
up. (In fastening the string to the leather the hole must be
very small and the know as flat as possible, and it is even well
to put a small piece of kid under the knot. When I first made
this experiment, not having taken these precautions, it did not
succeed well, owing to air getting in through the hole.) I now
drop it on this stone weight, and so heavily is it pressed down
upon it by the atmosphere that I can lift the weight without its
breaking away from it.

Have you ever tried to pick limpets off a rock? If so, you know
how tight they cling. the limpet clings to the rock just in the
same way as this leather does to the stone; the little animal
exhausts the air inside it's shell, and then it is pressed
against the rock by the whole weight of the air above.

Perhaps you will wonder how it is that if we have a weight of 15
lbs. pressing on every square inch of our bodies, it does not
crush us. And, indeed, it amounts on the whole to a weight of
about 15 tons upon the body of a grown man. It would crush us if
it were not that there are gases and fluids inside our bodies
which press outwards and balance the weight so that we do not
feel it at all.

This is why Mr. Glaisher's veins swelled and he grew giddy in
thin air. The gases and fluids inside his body were pressing
outwards as much as when he was below, but the air outside did
not press so heavily, and so all the natural condition of his
body was disturbed.

I hope we now realize how heavily the air presses down upon our
earth, but it is equally necessary to understand how, being
elastic, it also presses upwards; and we can prove this by a
simple experiment. I fill this tumbler with water, and keeping a
piece of card firmly pressed against it, I turn the whole upside-
down. When I now take my hand away you would naturally expect
the card to fall, and the water to be spilt. But no! the card
remains as if glued to the tumbler, kept there entirely by the
air pressing upwards against it. (The engraver has drawn the
tumbler only half full of water. The experiment will succeed
quite as well in this way if the tumbler be turned over quickly,
so that part of the air escapes between the tumbler and the card,
and therefore the space above the water is occupied by air less
dense than that outside.)

And now we are almost prepared to understand how we can weigh the
invisible air. One more experiment first. I have here what is
called a U tube, because it is shaped like a large U. I pour
some water in it till it is about half full, and you will notice
that the water stands at the same height in both arms of the
tube, because the air presses on both surfaces alike. Putting my
thumb on one end I tilt the tube carefully, so as to make the
water run up to the end of one arm, and then turn it back again.
But the water does not now return to its even position, it
remains up in the arm on which my thumb rests. Why is this?
Because my thumb keeps back the air from pressing at that end,
and the whole weight of the atmosphere rests on the water at the
other end. And so we learn that not only has the atmosphere real
weight, but we can see the effects of this weight by making it
balance a column of water or any other liquid. In the case of
the wetted leather we felt the weight of the air, here we see its
effects.

Now when we wish to see the weight of the air we consult a
barometer, which works really just in the same way as the water
in this tube. An ordinary upright barometer is simply a straight
tube of glass filled with mercury or quicksilver, and turned
upside-down in a small cup of mercury. The tube is a little more
than 30 inches long, and though it is quite full of mercury
before it is turned up, yet directly it stands in the cup the
mercury falls, till there is a height of about 30 inches between
the surface of the mercury in the cup, and that of the mercury in
the tube. As it falls it leaves an empty space above the mercury
which is called a vacuum, because it has no air in it. Now, the
mercury is under the same conditions as the water was in the U
tube, there is no pressure upon it at the top of the tube, while
there is a pressure of 15 lbs. upon it in the bowl, and therefore
it remains held up in the tube.



Week 9

But why will it not remain more than 30 inches high in the tube?
You must remember it is only kept up in the tube at all by the
air which presses on the mercury in the cup. And that column of
mercury now balances the pressure of the air outside, and presses
down on the mercury in the cup at its mouth just as much as the
air does on the rest. So this cup and tube act exactly like a
pair of scales. The air outside is the thing to be weighed at
one end as it presses on the mercury, the column answers to the
leaden weight at the other end which tells you how heavy the air
is. Now if the bore of this tube is made an inch square, then
the 30 inches of mercury in it weigh exactly 15 lbs, and so we
know that the weight of the air is 15 lbs. upon every square
inch, but if the bore of the tube is only half a square inch, and
therefore the 30 inches of mercury only weigh 7 1/2 lbs. instead
of 15 lbs., the pressure of the atmosphere will also be halved,
because it will only act upon half a square inch of surface, and
for this reason it will make no difference to the height of the
mercury whether the tube be broad or narrow.

But now suppose the atmosphere grows lighter, as it does when it
has much damp in it. The barometer will show this at once,
because there will be less weight on the mercury in the cup,
therefore it will not keep the mercury pushed so high up in the
tube. In other words, the mercury in the tube will fall.

Let us suppose that one day the air is so much lighter that it
presses down only with a weight of 14 1/2 lbs. to the square inch
instead of 15 lbs. Then the mercury would fall to 29 inches,
because each inch is equal to the weight of half a pound. Now,
when the air is damp and very full of water-vapour it is much
lighter, and so when the barometer falls we expect rain.
Sometimes, however, other causes make the air light, and then,
although the barometer is low, no rain comes,

Again, if the air becomes heavier the mercury is pushed up above
30 to 31 inches, and in this way we are able to weigh the
invisible air-ocean all over the world, and tell when it grows
lighter or heavier. This then, is the secret of the barometer.
We cannot speak of the thermometer today, but I should like to
warn you in passing that it has nothing to do with the weight of
the air, but only with heat, and acts in quite a different way.

And now we have been so long hunting out, testing and weighing
our aerial ocean, that scarcely any time is left us to speak of
its movements or the pleasant breezes which it makes for us in
our country walks. Did you ever try to run races on a very windy
day? Ah! then you feel the air strongly enough; how it beats
against your face and chest, and blows down your throat so as to
take your breath away; and what hard work it is to struggle
against it! Stop for a moment and rest, and ask yourself, what
is the wind? Why does it blow sometimes one way and sometimes
another, and sometimes not at all?

Wind is nothing more than air moving across the surface of the
earth, which as it passes along bends the tops of the trees,
beats against the houses, pushes the ships along by their sails,
turns the windmill, carries off the smoke from cities, whistles
through the keyhole, and moans as it rushes down the valley.
What makes the air restless? why should it not lie still all
round the earth?

It is restless because, as you will remember, its atoms are kept
pressed together near the earth by the weight of the air above,
and they take every opportunity, when they can find more room, to
spread out violently and rush into the vacant space, and this
rush we call a wind.

Imagine a great number of active schoolboys all crowded into a
room till they can scarcely move their arms and legs for the
crush, and then suppose all at once a large door is opened. Will
they not all come tumbling out pell-mell, one over the other,
into the hall beyond, so that if you stood in their way you would
most likely be knocked down? Well, just this happens to the air-
atoms; when they find a space before them into which they can
rush, they come on helter-skelter, with such force that you have
great difficulty in standing against them, and catch hold of
something to support you for fear you should be blown down.

But how come they to find any empty space to receive them? To
answer this we must go back again to our little active invisible
fairies the sunbeams. When the sun-waves come pouring down upon
the earth they pass through the air almost without heating it.
But not so with the ground; there they pass down only a short
distance and then are thrown back again. And when these sun-
waves come quivering back they force the atoms of the air near
the earth apart and make it lighter; so that the air close to the
surface of the heated ground becomes less heavy than the air
above it, and rises just as a cork rises in water. You know that
hot air rises in the chimney; for if you put a piece of lighted
paper on the fire it is carried up by the draught of air, often
even before it can ignite. Now just as the hot air rises from
the fire, so it rises from the heated ground up into higher parts
of the atmosphere. and as it rises it leaves only thin air
behind it, and this cannot resist the strong cold air whose atoms
are struggling and trying to get free, and they rush in and fill
the space.

One of the simplest examples of wind is to be found at the
seaside. there in the daytime the land gets hot under the
sunshine, and heats the air, making it grow light and rise.
Meanwhile the sunshine on the water goes down deeper, and so does
not send back so many heat-waves into the air; consequently the
air on the top of the water is cooler and heavier, and it rushes
in from over the sea to fill up the space on the shore left by
the warm air as it rises. This is why the seaside is so pleasant
in hot weather. During the daytime a light sea-breeze nearly
always sets in from the sea to the land.

When night comes, however, then the land loses its heat very
quickly, because it has not stored it up and the land-air grows
cold; but the sea, which has been hoarding the sun-waves down in
its depths, now gives them up to the atmosphere above it, and the
sea-air becomes warm and rises. For this reason it is now the
turn of the cold air from the land to spread over the sea, and
you have a land-breeze blowing off the shore.

Again, the reason why there are such steady winds, called the
trade winds, blowing towards the equator, is that the sun is very
hot at the equator, and hot air is always rising there and making
room for colder air to rush in. We have not time to travel
farther with the moving air, though its journeys are extremely
interesting; but if, when you read about the trade and other
winds, you will always picture to yourselves warm air made light
by the heat rising up into space and cold air expanding and
rushing in to fill its place, I can promise you that you will not
find the study of aerial currents so dry as many people imagine
it to be.

We are now able to form some picture of our aerial ocean. We can
imagine the active atoms of oxygen floating in the sluggish
nitrogen, and being used up in every candle-flame, gas-jet and


1 2 4 6 7 8 9 10 11 12 13 14

Online LibraryArabella B. BuckleyThe Fairy-Land of Science → online text (page 4 of 14)