Arabella B. Buckley.

The Fairy-Land of Science online

. (page 5 of 14)
Online LibraryArabella B. BuckleyThe Fairy-Land of Science → online text (page 5 of 14)
Font size
QR-code for this ebook


fire, and in the breath of all living beings; and coming out
again tied fast to atoms of carbon and making carbonic acid.
Then we can turn to trees and plants, and see them tearing these
two apart again, holding the carbon fast and sending the
invisible atoms of oxygen bounding back again into the air, ready
to recommence work. We can picture all these air-atoms, whether
of oxygen or nitrogen, packed close together on the surface of
the earth, and lying gradually farther and farther apart, as they
have less weight above them, till they become so scattered that
we can only detect them as they rub against the flying meteors
which flash into light. We can feel this great weight of air
pressing the limpet on to the rock; and we can see it pressing up
the mercury in the barometer and so enabling us to measure its
weight. Lastly, every breath of wind that blows past us tells us
how this aerial ocean is always moving to and fro on the face of
the earth; and if we think for a moment how much bad air and bad
matter it must carry away, as it goes from crowded cities to be
purified in the country, we can see how, in even this one way
alone, it is a great blessing to us.

Yet even now we have not mentioned many of the beauties of our
atmosphere. It is the tiny particles floating in the air which
scatter the light of the sun so that it spreads over the whole
country and into shady places. The sun's rays always travel
straight forward; and in the moon, where there is no atmosphere,
there is no light anywhere except just where the rays fall. But
on our earth the sun-waves hit against the myriads of particles
in the air and glide off them into the corners of the room or the
recesses of a shady lane, and so we have light spread before us
wherever we walk in the daytime, instead of those deep black
shadows which we can see through a telescope on the face of the
moon.

Again, it is electricity playing in the air-atoms which gives us
the beautiful lightning and the grand aurora borealis, and even
the twinkling of the starts is produced entirely by minute
changes in the air. If it were not for our aerial ocean, the
stars would stare at us sternly, instead of smiling with the
pleasant twinkle-twinkle which we have all learned to love as
little children.

All these questions, however, we must leave for the present; only
I hope you will be eager to read about them wherever you can, and
open your eyes to learn their secrets. For the present we must
be content if we can even picture this wonderful ocean of gas
spread round our earth, and some of the work it does for us.

We said in the last lecture that without the sunbeams the earth
would be cold, dark, and frost-ridden. With sunbeams, but
without air, it would indeed have burning heat, side by side with
darkness and ice, but it could have no soft light. our planet
might look beautiful to others, as the moon does to us, but it
could have comparatively few beauties of its own. With the
sunbeams and the air, we see it has much to make it beautiful.
But a third worker is wanted before our planet can revel in
activity and life. This worker is water; and in the next lecture
we shall learn something of the beauty and the usefulness of the
"drops of water" on their travels.



Week 10

LECTURE IV. A DROP OF WATER ON ITS TRAVELS

We are going to spend an hour to-day in following a drop of water
on its travels. If I dip my finger in this basin of water and
lift it up again, I bring with it a small glistening
drop out of the body of water below, and hold it before you. Tell
me, have you any idea where this drop has been? what changes it
has undergone, and what work it has been doing during all the
long ages that water has lain on the face of the earth? It is a
drop now, but it was not so before I lifted it out of the basin;
then it was part of a sheet of water, and will be so again if I
let it fall. Again, if I were to put this basin on the stove till
all the water had boiled away, where would my drop be then? Where
would it go? What forms will it take before it reappears in the
rain-cloud, the river, or the sparkling dew?

These are questions we are going to try to answer to-day; and
first, before we can in the least understand how water travels,
we must call to mind what we have learnt about the sunbeams and
the air. We must have clearly pictured in our imagination those
countless sun-waves which are for ever crossing space, and
especially those larger and slower undulations, the dark heat-
waves; for it is these, you will remember, which force the air-
atoms apart and make the air light, and it is also these which
are most busy in sending water on its travels. But not these
alone. The sun-waves might shake the water-drops as much as they
liked and turn them into invisible vapour, but they could not
carry them over the earth if it were not for the winds and
currents of that aerial ocean which bears the vapour on its
bosom, and wafts it to different regions of the world.

Let us try to understand how these two invisible workers, the
sun-waves and the air, deal with the drops of water. I
have here a kettle (Fig. 18, p. 76) boiling over a spirit-lamp,
and I want you to follow minutely what is going on in it. First,
in the flame of the lamp, atoms of the spirit drawn up from below
are clashing with the oxygen-atoms in the air. This, as you know,
causes heat-waves and light-waves to move rapidly all round the
lamp. The light-waves cannot pass through the kettle, but the
heat-waves can, and as they enter the water inside they agitate
it violently. Quicker, and still more quickly, the particles of
water near the bottom of the kettle move to and fro and are
shaken apart; and as they become light they rise through the
colder water letting another layer come down to be heated in its
turn. The motion grows more and more violent, making the water
hotter and hotter, till at last the particles of which it is
composed fly asunder, and escape as invisible vapour. If this
kettle were transparent you would not see any steam above the
water, because it is in the form of an invisible gas. But as the
steam comes out of the mouth of the kettle you see a cloud. Why
is this? Because the vapour is chilled by coming out into the
cold air, and its particles are drawn together again into tiny,
tiny drops of water, to which Dr. Tyndall has given the
suggestive name of water-dust. If you hold a plate over the steam
you can catch these tiny drops, though they will run into one
another almost as you are catching them.

The clouds you see floating in the sky are made of exactly the
same kind of water-dust as the cloud from the kettle, and I wish
to show you that this is also really the same as the invisible
steam within the kettle. I will do so by an experiment
suggested by Dr. Tyndall. Here is another spirit-lamp, which I
will hold under the cloud of steam - see! the cloud disappears!
As soon as the water-dust is heated the heat-waves scatter it
again into invisible particles, which float away into the room.
Even without the spirit-lamp, you can convince yourself that
water-vapour may be invisible; for close to the mouth of the
kettle you will see a short blank space before the cloud begins.
In this space there must be steam, but it is still so hot that
you cannot see it; and this proves that heat-waves can so shake
water apart as to carry it away invisibly right before your eyes.

Now, although we never see any water travelling from our earth up
into the skies, we know that it goes there, for it comes down
again in rain, and so it must go up invisibly. But where does the
heat come from which makes this water invisible? Not from below,
as in the case of the kettle, but from above, pouring down from
the sun. Wherever the sun-waves touch the rivers, ponds, lakes,
seas, or fields of ice and snow upon our earth, they
carry off invisible water-vapour. They dart down through the top
layers of the water, and shake the water-particles forcibly
apart; and in this case the drops fly asunder more easily and
before they are so hot, because they are not kept down by a great
weight of water above, as in the kettle, but find plenty of room
to spread themselves out in the gaps between the air-atoms of the
atmosphere.

Can you imagine these water-particles, just above any pond or
lake, rising up and getting entangled among the air-atoms? They
are very light, much lighter than the atmosphere; and so, when a
great many of them are spread about in the air which lies just
over the pond, they make it much lighter than the layer of air
above, and so help it to rise, while the heavier layer of air
comes down ready to take up more vapour.

In this way the sun-waves and the air carry off water everyday,
and all day long, from the top of lakes, rivers, pools, springs,
and seas, and even from the surface of ice and snow. Without any
fuss or noise or sign of any kind, the water of our earth is
being drawn up invisibly into the sky.

It has been calculated that in the Indian Ocean three-quarters of
an inch of water is carried off from the surface of the sea in
one day and night; so that as much as 22 feet, or a depth of
water about twice the height of an ordinary room, is silently and
invisibly lifted up from the whole surface of the ocean in one
year. It is true this is one of the hottest parts of the earth,
where the sun-waves are most active; but even in our
own country many feet of water are drawn up in the summer-time.

What, then, becomes of all this water? Let us follow it as it
struggles upwards to the sky. We see it in our imagination first
carrying layer after layer of air up with it from the sea till it
rises far above our heads and above the highest mountains. But
now, call to mind what happens to the air as it recedes from the
earth. Do you not remember that the air-atoms are always trying
to fly apart, and are only kept pressed together by the weight of
air above them? Well, so this water-laden air rises up, its
particles, no longer so much pressed together, begin to separate,
and as all work requires an expenditure of heat, the air becomes
colder, and then you know at once what must happen to the
invisible vapour, - it will form into tiny water-drops, like the
steam from the kettle. And so, as the air rises and becomes
colder, the vapour gathers into the visible masses, and we can
see it hanging in the sky, and call it clouds. When these clouds
are highest they are about ten miles from the earth, but when
they are made of heavy drops and hang low down, they sometimes
come within a mile of the ground.

Look up at the clouds as you go home, and think that the water of
which they are made has all been drawn up invisibly through the
air. Not, however, necessarily here in London, for we have
already seen that air travels as wind all over the world, rushing
in to fill spaces made by rising air wherever they occur, and so
these clouds may be made of vapour collected in the
Mediterranean, or in the Gulf of Mexico off the coast of America,
or even, if the wind is from the north, of chilly
particles gathered from the surface of Greenland ice and snow,
and brought here by the moving currents of air. Only, of one
thing we may be sure, that they come from the water of our earth.

Sometimes, if the air is warm, these water-particles may travel a
long way without ever forming into clouds; and on a hot,
cloudless day the air is often very full of invisible vapour.
Then, if a cold wind comes sweeping along, high up in the sky,
and chills this vapour, it forms into great bodies of water-dust
clouds, and the sky is overcast. At other times clouds hang
lazily in a bright sky, and these show us that just where they
are (as in Fig. 19) the air is cold and turns the invisible
vapour rising from the ground into visible water-dust, so that
exactly in those spaces we see it as clouds. Such clouds form
often on warm, still summer's day, and they are shaped like
masses of wool, ending in a straight line below. They are not
merely hanging in the sky, they are really resting upon a tall
column of invisible vapour which stretches right up from the
earth; and that straight line under the clouds marks
the place where the air becomes cold enough to turn this
invisible vapour into visible drops of water.



Week 11

And now, suppose that while these or any other kind of clouds are
overhead, there comes along either a very cold wind, or a wind
full of vapour. As it passes through the clouds, it makes them
very full of water, for, if it chills them, it makes the water-
dust draw more closely together; or, if it brings a new load of
water-dust, the air is fuller than it can hold. In either case a
number of water-particles are set free, and our fairy force
"cohesion" seizes upon them at once and forms them into large
water-drops. Then they are much heavier than the air, and so they
can float no longer, but down they come to the earth in a shower
of rain.

There are other ways in which the air may be chilled, and rain
made to fall, as, for example, when a wind laden with moisture
strikes against the cold tops of mountains. Thus the Khasia Hills
in India which face the Bay of Bengal, chill the air which
crosses them on its way from the Indian Ocean. The wet winds are
driven up the sides of the hills, the air expands, and the vapour
is chilled, and forming into drops, falls in torrents of rain.
Sir J. Hooker tells us that as much as 500 inches of rain fell in
these hills in nine months. That is to say, if you could measure
off all the ground over which the rain fell, and spread the whole
nine months' rain over it, it would make a lake 500 inches, or
more than 40 feet deep! You will not be surprised that the
country on the other side of these hills gets hardly any rain,
for all the water has been taken out of the air before
it comes there. Again for example in England, the wind comes to
Cumberland and Westmorland over the Atlantic, full of vapour, and
as it strikes against the Pennine Hills it shakes off its watery
load; so that the lake district is the most rainy in England,
with the exception perhaps of Wales, where the high mountains
have the same effect.

In this way, from different causes, the water of which the sun
has robbed our rivers and seas, comes back to us, after it has
travelled to various parts of the world, floating on the bosom of
the air. But it does not always fall straight back into the
rivers and seas again, a large part of it falls on the land, and
has to trickle down slopes and into the earth, in order to get
back to its natural home, and it is often caught on its way
before it can reach the great waters.

Go to any piece of ground which is left wild and untouched you
will find it covered with grass weeds, and other plants; if you
dig up a small plot you will find innumerable tiny roots creeping
through the ground in every direction. Each of these roots has a
sponge-like mouth by which the plant takes up water. Now, imagine
rain-drops falling on this plot of ground and sinking into the
earth. On every side they will find rootlets thirsting to drink
them in, and they will be sucked up as if by tiny sponges, and
drawn into the plants, and up the stems to the leaves. Here, as
we shall see in Lecture VII., they are worked up into food for
the plant, and only if the leaf has more water than it needs,
some drops may escape at the tiny openings under the
leaf, and be drawn up again by the sun-waves as invisible vapour
into the air.

Again, much of the rain falls on hard rock and stone, where it
cannot sink in, and then it lies in pools till it is shaken apart
again into vapour and carried off in the air. Nor is it idle
here, even before it is carried up to make clouds. We have to
thank this invisible vapour in the air for protecting us from the
burning heat of the sun by day and intolerable frost by night.

Let us for a moment imagine that we can see all that we know
exists between us and the sun. First, we have the fine ether
across which the sunbeams travel, beating down upon our earth
with immense force, so that in the sandy desert they are like a
burning fire. Then we have the coarser atmosphere of oxygen and
nitrogen atoms hanging in this ether, and bending the minute sun-
waves out of their direct path. But they do very little to hinder
them on their way, and this is why in very dry countries the
sun's heat is so intense. The rays beat down mercilessly, and
nothing opposes them. Lastly, in damp countries we have the
larger but still invisible particles of vapour hanging about
among the air-atoms. Now, these watery particles, although they
are very few (only about one twenty-fifth part of the whole
atmosphere), do hinder the sun-waves. For they are very greedy of
heat, and though the light-waves pass easily through them, they
catch the heat-waves and use them to help themselves to expand.
And so, when there is invisible vapour in the air, the sunbeams
come to us deprived of some of their heat-waves, and we
can remain in the sunshine without suffering from the heat.

This is how the water-vapour shields us by day, but by night it
is still more useful. During the day our earth and the air near
it have been storing up the heat which has been poured down on
them, and at night, when the sun goes down, all this heat begins
to escape again. Now, if there were no vapour in the air, this
heat would rush back into space so rapidly that the ground would
become cold and frozen even on a summer's night, and all but the
most hardy plants would die. But the vapour which formed a veil
against the sun in the day, now forms a still more powerful veil
against the escape of the heat by night. It shuts in the heat-
waves, and only allows them to make their way slowly upwards from
the earth - thus producing for us the soft, balmy nights of
summer and preventing all life being destroyed in the winter.

Perhaps you would scarcely imagine at first that it is this screen
of vapour which determines whether or not we shall have dew upon
the ground. Have you ever thought why dew forms, or what power has
been at work scattering the sparkling drops upon the grass?
Picture to yourself that it has been a very hot summer's day, and
the ground and the grass have been well warmed, and that the sun
goes down in a clear sky without any clouds. At once the heat-
waves which have been stored up in the ground, bound back into the
air, and here some are greedily absorbed by the vapour, while
others make their way slowly upwards. The grass, especially, gives
out these heat-waves very quickly, because the blades, being very
thin, are almost all surface. In consequence of this they part
with their heat more quickly than they can draw it up from the
ground, and become cold. Now the air lying just above the grass is
full of invisible vapour, and the cold of the blades, as it
touches them, chills the water- particles, and they are no longer
able to hold apart, but are drawn together into drops on the
surface of the leaves.

We can easily make artificial dew for ourselves. I have here a
bottle of ice which has been kept outside the window. When I
bring it into the warm room a mist forms rapidly outside the
bottle. This mist is composed of water-drops, drawn out of the
air of the room, because the cold glass chilled the air all round
it, so that it gave up its invisible water to form dew-drops.
Just in this same way the cold blades of grass chill the air
lying above them, and steal its vapour.

But try the experiment, some night when a heavy dew is expected,
of spreading a thin piece of muslin over some part of the grass,
supporting it at the four corners with pieces of stick so that it
forms an awning. Though there may be plenty of dew on the grass
all round, yet under this awning you will find scarcely any. The
reason of this is that the muslin checks the heat-waves as they
rise from the grass, and so the grass-blades are not chilled
enough to draw together the water-drops on their surface. If you
walk out early in the summer mornings and look at the fine cobwebs
flung across the hedges, you will see plenty of drops on the
cobwebs themselves sparkling like diamonds; but underneath on the
leaves there will be none, for even the delicate cobweb has been
strong enough to shut in the heat-waves and keep the leaves warm.

Again, if you walk off the grass on to the gravel path, you find
no dew there. Why is this? Because the stones of the gravel can
draw up heat from the earth below as fast as they give it out,
and so they are never cold enough to chill the air which touches
them. On a cloudy night also you will often find little or no dew
even on the grass. The reason of this is that the clouds give
back heat to the earth, and so the grass does not become chilled
enough to draw the water-drops together on its surface. But after
a hot, dry day, when the plants are thirsty and there is little
hope of rain to refresh them, then they are able in the evening
to draw the little drops from the air and drink them in before
the rising sun comes again to carry them away.

But our rain-drop undergoes other changes more strange than
these. Till now we have been imagining it to travel only where
the temperature is moderate enough for it to remain in a liquid
state as water. But suppose that when it is drawn up into the air
it meets with such a cold blast as to bring it to the freezing
point. If it falls into this blast when it is already a drop,
then it will freeze into a hailstone, and often on a hot summer's
day we may have a severe hailstorm, because the rain-drops have
crossed a bitterly cold wind as they were falling, and have been
frozen into round drops of ice.

But if the water-vapour reaches the freezing air while it is still
an invisible gas, and before it has been drawn into a drop, then
its history is very different. The ordinary force of cohesion has
then no power over the particles to make them into watery globes,
but its place is taken by the fairy process of "crystallization,"
and they are formed into beautiful white flakes, to fall in a
snow-shower. I want you to picture this process to yourselves, for
if once you can take an interest in the wonderful power of nature
to build up crystals, you will be astonished how often you will
meet with instances of it, and what pleasure it will add to your
life.

The particles of nearly all substances, when left free and not
hurried, can build themselves into crystal forms. If you melt
salt in water and then let all the water evaporate slowly, you
will get salt-crystals; - beautiful cubes of transparent salt
all built on the same pattern. The same is true of sugar; and if
you will look at the spikes of an ordinary stick of sugar-candy,
such as I have here, you will see the kind of crystals which
sugar forms. You may even pick out such shapes as these
from the common crystallized brown sugar in the sugar basin, or
see them with a magnifying glass on a lump of white sugar.

But it is not only easily melted substances such as sugar and
salt which form crystals. The beautiful stalactite grottos are
all made of crystals of lime. Diamonds are crystals of carbon,
made inside the earth. Rock-crystals, which you know probably
under the name of Irish diamonds, are crystallized quartz; and
so, with slightly different colourings, are agates, opals,
jasper, onyx, cairngorms, and many other precious stones. Iron,
copper, gold, and sulphur, when melted and cooled slowly build
themselves into crystals, each of their own peculiar form, and we
see that there is here a wonderful order, such as we should never
have dreamt of, if we had not proved it. If you possess a
microscope you may watch the growth of crystals yourself by
melting some common powdered nitre in a little water till you
find that no more will melt in it. Then put a few drops of this
water on a warm glass slide and place it under the microscope. As
the drops dry you will see the long transparent needles of nitre
forming on the glass, and notice how regularly these crystals
grow, not by taking food inside like living beings, but by adding
particle to particle on the outside evenly and regularly.



Week 12

Can we form any idea why the crystals build themselves up so
systematically? Dr. Tyndall says we can, and I hope by the help
of these small bar magnets to show you how he explains it. These
little pieces of steel, which I hope you can see lying
on this white cardboard, have been rubbed along a magnet until
they have become magnets themselves, and I can attract and lift
up a needle with any one of them. But if I try to lift one bar


1 2 3 5 7 8 9 10 11 12 13 14

Online LibraryArabella B. BuckleyThe Fairy-Land of Science → online text (page 5 of 14)