Arabella B. Buckley.

The Fairy-Land of Science online

. (page 9 of 14)
Online LibraryArabella B. BuckleyThe Fairy-Land of Science → online text (page 9 of 14)
Font size
QR-code for this ebook


produce a musical sound. We can also prove by this experiment
that the quicker the blows are, the higher the note will be. I
pull the string gently at first, and then quicker and quicker,
and you will notice that the note grows sharper and sharper, till
the movement begins to slacken, when the note goes down again.
This is because the more rapidly the air is hit, the shorter are
the waves it makes, and short waves give a high note.

Let us examine this with two tuning-forks. I strike one, and it
sounds D, the third space in the treble; I strike the other, and
it sounds G, the first leger line, five notes above the C. I
have drawn on this diagram (Fig. 35), an imaginary picture of
these two sets of waves. You see that the G fork makes three
waves, while the C fork makes only two. Why is this? Because
the prong of the G fork moves three times backwards and forwards
while the prong of the C fork only moves twice; therefore the G
fork does not crowd so many atoms together before it draws back,
and the waves are shorter. These two notes, C and G, are a fifth
of an octave apart; if we had two forks, of which one went twice
as fast as the other, making four waves while the other made two,
then that note would be an octave higher.

So we see that all the sounds we hear, - the warning noises which
keep us from harm, the beautiful musical notes with all the tunes
and harmonies that delight us, even the power of hearing the
voices of those we love, and learning from one another that which
each can tell, - all these depend upon the invisible waves of
air, even as the pleasures of light depend on the waves of ether.
It is by these sound-waves that nature speaks to us, and in all
her movements there is a reason why her boice is sharp or tender,
loud or gentle, awful or loving. Take for instance the brook we
spoke of at the beginning of the lecture. Why does it sing so
sweetly, while the wide deep river makes no noise? Because the
little brook eddies and purls round the stones, hitting them as
it passes; sometimes the water falls down a large stone, and
strikes against the water below; or sometimes it grates the
little pebbles together as they lie in its bed. Each of these
blows makes a small globe of sound-waves, which spread and spread
till they fall on your ear, and because they fall quickly and
regularly, they make a low, musical note. We might almost fancy
that the brook wished to show how joyfully it flows along,
recalling Shelley's beautiful lines:-

"Sometimes it fell
Among the moss with hollow harmony,
Dark and profound; now on the polished stones
It danced; like childhood laughing as it went."

The broad deep river, on the contrary, makes none of these
cascades and commotions. The only places against which it rubs
are the banks and the bottom; and here you can sometimes hear it
grating the particles of sand against each other if you listen
very carefully. But there is another reason why falling water
makes a sound, and often even a loud roaring noise in the
cataract and in the breaking waves of the sea. You do not only
hear the water dashing against the rocky ledges or on the beach,
you also hear the bursting of innumerable little bladders of air
which are contained in the water. As each of these bladders is
dashed on the ground, it explodes and sends sound-waves to your
ear. Listen to the sea some day when the waves are high and
stormy, and you cannot fail to be struck by the irregular bursts
of sound.

The waves, however, do not only roar as they dash on the ground;
have you never noticed how they seem to scream as they draw back
down the beach? Tennyson calls it,

"The scream of the madden'd beach dragged down by the wave;" and
it is caused by the stones grating against each other as the
waves drag them down. Dr. Tyndall tells us that it is possible
to know the size of the stones by the kind of noise they make.
If they are large, it is a confused noise, when smaller, a kind
of scream; while a gravelly beach will produce a mere hiss.

Who could be dull by the side of a brook, a waterfall, or the
sea, while he can listen for sounds like these, and picture to
himself how they are being made? You may discover a number of
other causes of sound made by water, if you once pay attention to
them.

Nor is it only water that sings to us. Listen to the wind, how
sweetly it sighs among the leaves. There we hear it, because it
rubs the leaves together, and they produce the sound-waves. But
walk against the wind some day and you can hear it whistling in
your own ear, striking against the curved cup, and then setting
up a succession of waves in the hearing canal of the ear itself.

Why should it sound in one particular tone when all kinds of
sound-waves must be surging about in the disturbed air?

This glass jar will answer our question roughly. If I strike my
tuning-fork and hold it over the jar, you cannot hear it, because
the sound is feeble, but if I fill the jar gently with water,
when the water rises to a certain point you will hear a loud
clear note, because the waves of air in the jar are exactly the
right length to answer to the note of the fork. If I now blow
across the mouth of the jar you hear the same note, showing that
a cavity of a particular length will only sound to the waves
which fit it. do you see now the reason why pan-pipes give
different sounds, or even the hole at the end of a common key
when you blow across it? Here is a subject you will find very
interesting if you will read about it, for I can only just
suggest it to you here. But now you will see that the canal of
your ear also answers only to certain waves, and so the wind
sings in your ear with a real if not a musical note.

Again, on a windy night have you not heard the wind sounding a
wild, sad note down a valley? Why do you think it sounds so much
louder and more musical here than when it is blowing across the
plain? Because air in the valley will only answer to a certain
set of waves, and, like the pan-pipe, gives a particular note as
the wind blows across it, and these waves go up and down the
valley in regular pulses, making a wild howl. You may hear the
same in the chimney, or in the keyhole; all these are waves set
up in the hole across which the wind blows. Even the music in
the shell which you hold to your ear is made by the air in the
shell pulsating to and fro. And how do you think it is set
going? By the throbbing of the veins in your own ear, which
causes the air in the shell to vibrate.

Another grand voice of nature is the thunder. People often have
a vague idea that thunder is produced by the clouds knocking
together, which is very absurd, if you remember that clouds are
but water-dust. The most probable explanation of thunder is much
more beautiful than this. You will remember from Lecture III
that heat forces the air-atoms apart. Now, when a flash of
lightning crosses the sky it suddenly expands the air all round
it as it passes, so that globe after globe of sound-waves is
formed at every point across which the lightning travels. Now
light, you remember, travels so wonderfully rapidly (192,000
miles in a second) that a flash of lightning is seen by us and is
over in a second, even when it is two or three miles long. But
sound comes slowly, taking five seconds to travel half a mile,
and so all the sound-waves at each point of the two or three
miles fall on our ear one after the other, and make the rolling
thunder. Sometimes the roll is made even longer by the echo, as
the sound-waves are reflected to and fro by the clouds on their
road; and in the mountains we know how the peals echo and re-echo
till they die away.

We might fill up far more than an hour in speaking of those
voices which come to us as nature is at work. Think of the
patter of the rain, how each drop as it hits the pavement sends
circles of sound-waves out on all sides; or the loud report which
falls on the ear of the Alpine traveller as the glacier cracks on
its way down the valley; or the mighty boom of the avalanche as
the snow slides in huge masses off the side of the lofty
mountain. Each and all of these create their sound-waves, large
or small, loud or feeble, which make their way to your ear, and
become converted into sound.

We have, however, only time now just to glance at life-sounds, of
which there are so many around us. Do you know why we hear a
buzzing, as the gnat, the bee, or the cockchafer fly past? Not
by the beating of their wings against the air, as many people
imagine, and as is really the case with humming birds, but by the
scraping of the under-part of their hard wings against the edges
of their hind legs, which are toothed like a saw. The more
rapidly their wings move the stronger the grating sound becomes,
and you will now see why in hot, thirsty weather the buzzing of
the gnat is so loud, for the more thirsty and the more eager he
becomes, the wilder his movements will be.

Some insects, like the drone-fly (Eristalis tenax), force the air
through the tiny air-passages in their sides, and as these
passages are closed by little plates, the plates vibrate to and
fro and make sound-waves. Again, what are those curious sounds
you may hear sometimes if you rest your head on a trunk in the
forest? They are made by the timber-boring beetles, which saw
the wood with their jaws and make a noise in the world, even
though they have no voice.

All these life-sounds are made by creatures which do not sing or
speak; but the sweetest sounds of all in the woods are the voices
of the birds. All voice-sounds are made by two elastic bands or
cushions, called vocal chords, stretched across the end of the
tube or windpipe through which we breathe, and as we send the air
through them we tighten or loosen them as we will, and so make
them vibrate quickly or slowly and make sound-waves of different
lengths. But if you will try some day in the woods you will find
that a bird can beat you over and over again in the length of his
note; when you are out of breath and forced to stop he will go on
with his merry trill as fresh and clear as if he had only just
begun. This is because birds can draw air into the whole of
their body, and they have a large stock laid up in the folds of
their windpipe, and besides this the air-chamber behind their
elastic bands or vocal chords has two compartments where we have
only one, and the second compartment has special muscles by which
they can open and shut it, and so prolong the trill.

Only think what a rapid succession of waves must quiver through
the air as a tiny lark agitates his little throat and pours forth
a volume of song! The next time you are in the country in the
spring, spend half an hour listening to him, and try and picture
to yourself how that little being is moving all the atmosphere
round him. Then dream for a little while about sound, what it
is, how marvellously it works outside in the world, and inside in
your ear and brain; and then, when you go back to work again, you
will hardly deny that it is well worth while to listen sometimes
to the voices of nature and ponder how it is that we hear them.



Week 19

LECTURE VII THE LIFE OF A PRIMROSE

When the dreary days of winter and the early damp days of spring
are passing away, and the warm bright sunshine has begun to pour
down upon the grassy paths of the wood, who does not love to go
out and bring home posies of violets, and bluebells, and
primroses? We wander from one plant to another picking a flower
here and a bud there, as they nestle among the green
leaves, and we make our rooms sweet and gay with the tender and
lovely blossoms. But tell me, did you ever stop to think, as you
added flower after flower to your nosegay, how the plants which
bear them have been building up their green leaves and their
fragile buds during the last few weeks? If you had visited the
same spot a month before, a few (of) last year's leaves,
withered and dead, would have been all that you would have found.
And now the whole wood is carpeted with delicate green leaves,
with nodding bluebells, and pale-yellow primroses, as if a fairy
had touched the ground and covered it with fresh young life. And
our fairies have been at work here; the fairy "Life," of whom we
know so little, though we love her so well and rejoice in the
beautiful forms she can produce; the fairy sunbeams with their
invisible influence kissing the tiny shoots and warming them into
vigour and activity; the gentle rain-drops, the balmy air, all
these have been working, while you or I passed heedlessly by;
and now we come and gather the flowers they have made, and too
often forget to wonder how these lovely forms have sprung up
around us.

Our work during the next hour will be to consider this question.
You were asked last week to bring with you to-day a primrose-
flower, or a whole plant if possible, in order the better to
follow out with me the "Life of a Primrose." (To enjoy this
lecture, the reader ought to have, if possible, a primrose-
flower, an almond soaked for a few minutes in hot water, and a
piece of orange.) This is a very different kind of subject from
those of our former lectures. There we took world-
wide histories; we travelled up to the sun, or round the earth,
or into the air; now I only ask you to fix your attention on one
little plant, and inquire into its history.

There is a beautiful little poem by Tennyson, which says -

"Flower in the crannied wall,
I pluck you out of the crannies;
Hold you here, root and all, in my hand,
Little flower; but if I could understand
What you are, root and all, and all in all,
I should know what God and man is."

We cannot learn all about this little flower, but we can learn
enough to understand that it has a real separate life of its
own, well worth knowing. For a plant is born, breathes, sleeps,
feeds, and digests just as truly as an animal does, though in a
different way. It works hard both for itself to get its food,
and for others in making the air pure and fit for animals to
breathe. It often lays by provision for the winter. It sends
young plants out, as parents send their children, to fight for
themselves in the world; and then, after living sometimes to a
good old age, it dies, and leaves its place to others.

We will try to follow out something of this life to-day; and
first, we will begin with the seed.

I have here a packet of primrose-seeds, but they are so small
that we cannot examine them; so I have also had given to each
one of you an almond-kernel, which is the seed of the almond-
tree, and which has been soaked, so that it splits in half
easily. From this we can learn about seeds in general, and then
apply it to the primrose.

If you peel the two skins off your almond-seed (the
thick, brown, outside skin, and the thin, transparent one under
it), the two halves of the almond will slip apart quite easily.
One of these halves will have a small dent at the pointed end,
while in the other half you will see a little lump, which fitted
into the dent when the two halves were joined. This little lump
(a b, Fig. 37) is a young plant, and the two halves of the
almond are the seed leaves which hold the plantlet, and feed it
till it can feed itself. The rounded end of the plantlet (b)
sticking out of the almond, is the beginning of the root, while
the other end (a) will in time become the stem. If you look
carefully, you will see two little points at this end, which are
the tips of future leaves. Only think how minute this plantlet
must be in a primrose, where the whole seed is scarcely larger
than a grain of sand! Yet in this tiny plantlet lies hid the
life of the future plant.

When a seed falls into the ground, so long as the earth is cold
and dry, it lies like a person in a trance, as if it were dead;
but as soon as the warm, damp spring comes, and the busy little
sun-waves pierce down into the earth, they wake up the plantlet
and make it bestir itself. They agitate to and fro the particles
of matter in this tiny body, and cause them to seek out for
other particles to seize and join to themselves.

But these new particles cannot come in at the roots,
for the seed has none; nor through the leaves, for they have not
yet grown up; and so the plantlet begins by helping itself to
the store of food laid up in the thick seed-leaves in which it
is buried. Here it finds starch, oils, sugar, and substances
called albuminoids, - the sticky matter which you notice in
wheat-grains when you chew them is one of the albuminoids. This
food is all ready for the plantlet to use, and it sucks it in,
and works itself into a young plant with tiny roots at one end,
and a growing shoot, with leaves, at the other.

But how does it grow? What makes it become larger? To answer this
you must look at the second thing I asked you to bring - a piece
of orange. If you take the skin off a piece of orange, you will
see inside a number of long-shaped transparent bags, full of
juice. These we call cells, and the flesh of all plants and
animals is made up of cells like these, only of various shapes.
In the pith of elder they are round, large, and easily seen (a,
Fig. 39); in the stalks of plants they are long, and lap over
each other (b, Fig. 39), so as to give the stalk strength to
stand upright. Sometimes many cells growing one on the top of
the other break into one tube and make vessels. But whether
large or small, they are all bags growing one against the other.

In the orange-pulp these cells contain only sweet juice, but in
other parts of the orange-tree or any other plant
they contain a sticky substance with little grains in it. This
substance is called "protoplasm," or the first form of life, for
it is alive and active, and under a microscope you may see in a
living plant streams of the little grains moving about in the
cells.

Now we are prepared to explain how our plant grows. Imagine the
tiny primrose plantlet to be made up of cells filled with active
living protoplasm, which drinks in starch and other food from
the seed-leaves. In this way each cell will grow too full for
its skin, and then the protoplasm divides into two parts and
builds up a wall between them, and so one cell becomes two. Each
of these two cells again breaks up into two more, and so the
plant grows larger and larger, till by the time it has used up
all the food in the seed-leaves, it has sent roots covered with
fine hairs downwards into the earth, and a shoot with beginnings
of leaves up into the air.

Sometimes the seed-leaves themselves come above the ground, as in
the mustard-plant, and sometimes they are left empty behind,
while the plantlet shoots through them.

And now the plant can no longer afford to be idle and
live on prepared food. It must work for itself. Until now it has
been taking in the same kind of food that you and I do; for we
too find many seeds very pleasant to eat and useful to nourish
us. But now this store is exhausted. Upon what then is the plant
to live? It is cleverer than we are in this, for while we cannot
live unless we have food which has once been alive, plants can
feed upon gases and water and mineral matter only. Think over the
substances you can eat or drink, and you will find they are
nearly all made of things which have been alive: meat,
vegetables, bread, beer, wine, milk; all these are made from
living matter, and though you do take in such things as water
and salt, and even iron and phosphorus, these would be quite
useless if you did not eat and drink prepared food which your
body can work into living matter.

But the plant as soon as it has roots and leaves begins to make
living matter out of matter that has never been alive. Through
all the little hairs of its roots it sucks in water, and in this
water are dissolved more or less of the salts of ammonia,
phosphorus, sulphur, iron, lime, magnesia, and even silica, or
flint. In all kinds of earth there is some iron, and we shall see
presently that this is very important to the plant.

Suppose, then, that our primrose has begun to drink in water at
its roots. How is it to get this water up into the stem and
leaves, seeing that the whole plant is made of closed bags or
cells? It does it in a very curious way, which you can prove for
yourselves. Whenever two fluids, one thicker than the other,
such as treacle and water for example, are only separated by a
skin or any porous substance, they will always mix, the thinner
one oozing through the skin into the thicker one. If you tie a
piece of bladder over a glass tube, fill the tube half-full of
treacle, and then let the covered end rest in a bottle of water,
in a few hours the water will get in to the treacle and the
mixture will rise up in the tube till it flows over the top. Now,
the saps and juices of plants are thicker than water, so, directly
the water enters the cells at the root it oozes up into the cells
above, and mixes with the sap. Then the matter in those cells
becomes thinner than in the cells above, so it too oozes up, and
in this way cell by cell the water is pumped up into the leaves.

When it gets there it finds our old friends the sun-beams hard at
work. If you have ever tried to grow a plant in a cellar, you
will know that in the dark its leaves remain white and sickly.
It is only in the sunlight that a beautiful delicate green tint
is given to them, and you will remember from Lecture II. that
this green tint shows that the leaf has used all the sun-waves
except those which make you see green; but why should it do this
only when it has grown up in the sunshine?

The reason is this: when the sunbeam darts into the leaf and sets
all its particles quivering, it divides the protoplasm into two
kinds, collected into different cells. One of these remains
white, but the other kind, near the surface, is altered by the
sunlight and by the help of the iron brought in by the water.
This particular kind of protoplasm, which is called "chlorophyll,"
will have nothing to do with the green waves and throws them back,
so that every little grain of this protoplasm looks green and
gives the leaf its green colour.

It is these little green cells that by the help of the sun-waves
digest the food of the plant and turn the water and gases into
useful sap and juices. We saw in Lecture III. that when we
breathe-in air, we use up the oxygen in it and send back out of
our mouths carbonic acid, which is a gas made of oxygen and
carbon.

Now, every living things wants carbon to feed upon, but plants
cannot take it in by itself, because carbon is solid (the
blacklead in your pencils is pure carbon), and a plant cannot
eat, it can only drink-in fluids and gases. Here the little
green cells help it out of its difficulty. They take in or
absorb out of the air carbonic acid gas which we have given out
of our mouths and then by the help of the sun-waves they tear
the carbon and oxygen apart. Most of the oxygen they throw back
into the air for us to use, but the carbon they keep.

If you will take some fresh laurel-leaves and put them into a
tumbler of water turned upside-down in a saucer of water, and
set the tumbler in the sunshine, you will soon see little bright
bubbles rising up and clinging to the glass. These are bubbles
of oxygen gas, and they tell you that they have been set free by
the green cells which have torn from them the carbon of the
carbonic acid in the water.

But what becomes of the carbon? And what use is made of the water
which we have kept waiting all this time in the leaves? Water,
you already know, is made of hydrogen and oxygen, but perhaps
you will be surprised when I tell you that starch, sugar, and
oil, which we get from plants, are nothing more than hydrogen
and oxygen in different quantities joined to carbon.

It is very difficult at first to picture such a black thing as
carbon making part of delicate leaves and beautiful flowers, and
still more of pure white sugar. But we can make an experiment by
which we can draw the hydrogen and oxygen out of common loaf
sugar, and then you will see the carbon stand out in all its
blackness. I have here a plate with a heap of white sugar in it.
I pour upon it first some hot water to melt and warm it, and then
some strong sulphuric acid. This acid does nothing more than
simply draw the hydrogen and oxygen out. See! in a few moments a
black mass of carbon begins to rise, all of which has come out of
the white sugar you saw just now. *(The common dilute sulphuric
acid of commerce is not strong enough for this experiment, but


1 2 3 4 5 6 7 9 11 12 13 14

Online LibraryArabella B. BuckleyThe Fairy-Land of Science → online text (page 9 of 14)