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delicate and subtle, that not only is it invisible, but it can
pass through solid bodies such as glass, ice, or even wood or
brick walls. This substance we call "ether." I cannot give you
here the reasons why we must assume that it is throughout all
space; you must take this on the word of such men as Sir John
Herschel or Professor Clerk-Maxwell, until you can study the
question for yourselves.

Now if you can imagine this ether filling every corner of space,
so that it is everywhere and passes through everything, ask
yourselves, what must happen when a great commotion is going on
in one of the large bodies which float in it? When the atoms of
the gases round the sun are clashing violently together to make
all its light and heat, do you not think they must shake this
ether all around them? And then, since the ether stretches on
all sides from the sun to our earth and all other planets, must
not this quivering travel to us, just as the quivering of the
boards would from me to you? Take a basin of water to represent
the ether, and take a piece of potassium like that which we used
in our last lecture, and hold it with a pair of nippers in the
middle of the water. You will see that as the potassium hisses
and the flame burns round it, they will make waves which will
travel all over the water to the edge of the basin,, and you can
imagine how in the same way waves travel over the ether from the
sun to us.

Straight away from the sun on all sides, never stopping, never
resting, but chasing after each other with marvellous quickness,
these tiny waves travel out into space by night and by day. When
our spot of the earth where England lies is turned away from them
and they cannot touch us, then it is night for us, but directly
England is turned so as to face the sun, then they strike on the
land, and the water, and warm it; or upon our eyes, making the
nerves quiver so that we see light. Look up at the sun and
picture to yourself that instead of one great blow from a fist
causing you to see starts for a moment, millions of tiny blows
from these sun-waves are striking every instant on you eye; then
you will easily understand that his would cause you to see a
constant blaze of light.

But when the sun is away, if the night is clear we have light
from the starts. Do these then too make waves all across the
enormous distance between them and us? Certainly they do, for
they too are suns like our own, only they are so far off that the
waves they send are more feeble, and so we only notice them when
the sun's stronger waves are away.

But perhaps you will ask, if no one has ever seen these waves not
the ether in which they are made, what right have we to say they
are there? Strange as it may seem, though we cannot see them we
have measured them and know how large they are, and how many can
go into an inch of space. For as these tiny waves are running on
straight forward through the room, if we put something in their
way, they will have to run round it; and if you let in a very
narrow ray of light through a shutter and put an upright wire in
the sunbeam, you actually make the waves run round the wire just
as water runs round a post in a river; and they meet behind the
wire, just as the water meets in a V shape behind the post. Now
when they meet, they run up against each other, and here it is we
catch them. Fir if they meet comfortably, both rising up in a
good wave, they run on together and make a bright line of light;
but if they meet higgledy-piggledy, one up and the other down,
all in confusion, they stop each other, and then there is no
light but a line of darkness. And so behind your piece of wire
you can catch the waves on a piece of paper, and you will find
they make dark and light lines one side by side with the other,
and by means of these bands it is possible to find out how large
the waves must be. This question is too difficult for us to work
it out here, but you can see that large waves will make broader
light and dark bands than small ones will, and that in this way
the size of the waves may be measured.

And now how large do you think they turn out to be? so very,
very tiny that about fifty thousand waves are contained in a
single inch of space! I have drawn on the board the length of an
inch, and now I will measure the same space in the air between my
finger and thumb. Within this space at this moment there are
fifty thousand tiny waves moving up and down. I promised you we
would find in science things as wonderful as in fairy tales. Are
not these tiny invisible messengers coming incessantly from the
sun as wonderful as any fairies? and still more so when, as we
shall see presently, they are doing nearly all the work of our
world.

We must next try to realize how fast these waves travel. You
will remember that an express train would take 171 years to reach
us from the sun; and even a cannon-ball would take from ten to
thirteen years to come that distance. Well, these tiny waves
take only seven minutes and a half to come the whole 91 millions
of miles. The waves which are hitting your eye at this moment
are caused by a movement which began at the sun only 7 1/2
minutes ago. And remember, this movement is going on
incessantly, and these waves are always following one after the
other so rapidly that they keep up a perpetual cannonade upon the
pupil of your eye. So fast do they come that about 608 billion
waves enter your eye in one single second.* I do not ask you to
remember these figures; I only ask you to try and picture to
yourselves these infinitely tiny and active invisible messengers
from the sun, and to acknowledge that light is a fairy thing.
(*Light travels at the rate of 190,000 miles, or 12,165,120,000
inches in a second. Taking the average number of wave-lengths in
an inch at 50,000, then 12,165,120,000 X 50,000 =
608,256,000,000,000.)

But we do not yet know all about our sunbeams. See, I have here
a piece of glass with three sides, called a prism. If I put it
in the sunlight which is streaming through the window, what
happens? Look! on the table there is a line of beautiful
colours. I can make it long or short, as I turn the prism, but
the colours always remain arranged in the same way. Here at my
left hand is the red, beyond it orange, then yellow, green, blue,
indigo or deep blue, and violet, shading one into the other all
along the line. We have all seen these colours dancing on the
wall when the sun has been shining brightly on the cut-glass
pendants of the chandelier, and you may see them still more
distinctly if you let a ray of light into a darkened room, and
pass it through the prism as in the diagram (Fig. 7). What are
these colours? Do they come from the glass? No; for you will
remember to have seen them in the rainbow, and in the soap-
bubble, and even in a drop of dew or the scum on the top of a
pond. This beautiful coloured line is only our sunbeam again,
which has been split up into many colours by passing through the
glass, as it is in the rain-drops of the rainbow and the bubbles
of the scum of the pond.



Week 5

Till now we have talked of the sunbeam as if it were made of only
one set of waves of different sizes, all travelling along
together from the sun. These various waves have been measured,
and we know that the waves which make up red light are larger and
more lazy than those which make violet light, so that there are
only thirty-nine thousand red waves in an inch, while there are
fifty-seven thousand violet waves in the same space.

How is it then, that if all these different waves making
different colours, hit on our eye, they do not always make us see
coloured light? Because, unless they are interfered with, they
all travel along together, and you know that all colours, mixed
together in proper proportion, make white.

I have here a round piece of cardboard, painted with the seven
colours in succession several times over. When it is still you
can distinguish them all apart, but when I whirl it quickly round
- see! - the cardboard looks quite white, because we see them all
so instantaneously that they are mingled together. In the same
way light looks white to you, because all the different coloured
waves strike on your eye at once. You can easily make on of
these card for yourselves only the white will always look dirty,
because you cannot get the colours pure.

Now, when the light passes through the three-sided glass or
prism, the waves are spread out, and the slow, heavy, red waves
lag behind and remain at the lower end R of the coloured line on
the wall (Fig. 7), while the rapid little violet waves are bent
more out of their road and run to V at the farther end of the
line; and the orange, yellow, green, blue, and indigo arrange
themselves between, according to the size of their waves.

And now you are very likely eager to ask why the quick waves
should make us see one colour, and the slow waves another. This
is a very difficult question, for we have a great deal still to
learn about the effect of light on the eye. But you can easily
imagine that colour is to our eye much the same as music is to
our ear. You know we can distinguish different notes when the
air-waves play slowly or quickly upon the drum of the ear (as we
shall see in Lecture VI) and somewhat in the same way the tiny
waves of the ether play on the retina or curtain at the back of
our eye, and make the nerves carry different messages to the
brain: and the colour we see depends upon the number of waves
which play upon the retina in a second.

Do you think we have now rightly answered the question - What is
a sunbeam? We have seen that it is really a succession of tiny
rapid waves, travelling from the sun to us across the invisible
substance we call "ether", and keeping up a constant cannonade
upon everything which comes in their way. We have also seen
that, tiny as these waves are, they can still vary in size, so
that one single sunbeam is made up of myriads of different-sized
waves, which travel all together and make us see white light;
unless for some reason they are scattered apart, so that we see
them separately as red, green, blue, or yellow. How they are
scattered, and many other secrets of the sun-waves, we cannot
stop to consider not, but must pass on to ask -

What work do the sunbeams do for us?

They do two things - they give us light and heat. It is by means
of them alone that we see anything. When the room was dark you
could not distinguish the table, the chairs, or even the walls of
the room. Why? Because they had no light-waves to send to your
eye. But as the sunbeams began to pour in at the window, the
waves played upon the things in the room, and when they hit them
they bounded off them back to your eye, as a wave of the sea
bounds back from a rock and strikes against a passing boat.
Then, when they fell upon your eye, they entered it and excited
the retina and the nerves, and the image of the chair or the
table was carried to your brain. Look around at all the things
in this room. Is it not strange to think that each one of them
is sending these invisible messengers straight to your eye as you
look at it; and that you see me, and distinguish me from the
table, entirely by the kind of waves we each send to you?

Some substances send back hardly any waves of light, but let them
all pass through them, and thus we cannot see them. A pane of
clear glass, for instance, lets nearly all the light-waves pass
through it, and therefore you often cannot see that the glass is
there, because no light-messengers come back to you from it.
Thus people have sometimes walked up against a glass door and
broken it, not seeing it was there. Those substances are
transparent which, for some reason unknown to us, allow the ether
waves to pass through them without shaking the atoms of which the
substance is made. In clear glass, for example, all the light-
waves pass through without affecting the substance of the glass;
while in a white wall the larger part of the rays are reflected
back to your eye, and those which pass into the wall, by giving
motion to its atoms lose their own vibrations.

Into polished shining metal the waves hardly enter at all, but
are thrown back from the surface; and so a steel knife or a
silver spoon are very bright, and are clearly seen. Quicksilver
is put at the back of looking-glasses because it reflects so many
waves. It not only sends back those which come from the sun, but
those, too, which come from your face. So, when you see yourself
in a looking-glass, the sun-waves have first played on your face
and bounded off from it to the looking-glass; then, when they
strike the looking-glass, they are thrown back again on to the
retina of your eye, and you see your own face by means of the
very waves you threw off from it an instant before.

But the reflected light-waves do more for us than this. They not
only make us see things, but they make us see them in different
colours. What, you will ask, is this too the work of the
sunbeams? Certainly; for if the colour we see depends on the
size of the waves which come back to us, then we must see things
coloured differently according to the waves they send back. For
instance, imagine a sunbeam playing on a leaf: part of its waves
bound straight back from it to our eye and make us see the
surface of the leaf, but the rest go right into the leaf itself,
and there some of them are used up and kept prisoners. The red,
orange, yellow, blue, and violet waves are all useful to the
leaf, and it does not let them go again. But it cannot absorb
the green waves, and so it throws them back, and they travel to
your eye and make you see a green colour. So when you say a leaf
is green, you mean that the leaf does not want the green waves of
the sunbeam, but sends them back to you. In the same way the
scarlet geranium rejects the red waves; this table sends back
brown waves; a white tablecloth sends back nearly the whole of
the waves, and a black coat scarcely any. This is why, when
there is very little light in the room, you can see a white
tablecloth while you would not be able to distinguish a black
object, because the few faint rays that are there, are all sent
back to you from a white surface.

Is it not curious to think that there is really no such thing as
colour in the leaf, the table, the coat, or the geranium flower,
but we see them of different colours because, for some reason,
they send back only certain coloured waves to our eye?

Wherever you look, then, and whatever you see, all the beautiful
tints, colours, lights, and shades around you are the work of the
tiny sun-waves.

Again, light does a great deal of work when it falls upon plants.
Those rays of light which are caught by the leaf are by no means
idle; we shall see in Lecture VII that the leaf uses them to
digest its food and make the sap on which the plant feeds.



Week 6

We all know that a plant becomes pale and sickly if it has not
sunlight, and the reason is, that without these light-waves it
cannot get food out of the air, nor make the sap and juices which
it needs. When you look at plants and trees growing in the
beautiful meadows; at the fields of corn, and at the lovely
landscape, you are looking on the work of the tiny waves of
light, which never rest all through the day in helping to give
life to every green thing that grows.

So far we have spoken only of light; but hold your hand in the
sun and feel the heat of the sunbeams, and then consider if the
waves of heat do not do work also. There are many waves in a
sunbeam which move too slowly to make us see light when they hit
our eye, but we can feel them as heat, though we cannot see them
as light. The simplest way of feeling heat-waves is to hold a
warm iron near your face. You know that no light comes from it,
yet you can feel the heat-waves beating violently against your
face and scorching it. Now there are many of these dark heat-
rays in a sunbeam, and it is they which do most of the work in
the world.

In the first place, as they come quivering to the earth, it is
they which shake the water-drops apart, so that these are carried
up in the air, as we shall see in the next lecture. And then
remember, it is these drops, falling again as rain, which make
the rivers and all the moving water on the earth. So also it is
the heat-waves which make the air hot and light, and so cause it
to rise and make winds and air-currents, and these again give
rise to ocean-currents. It is these dark rays, again, which
strike upon the land and give it the warmth which enables plants
to grow. It is they also which keep up the warmth in our own
bodies, both by coming to us directly from the sun, and also in a
very roundabout way through plants. You will remember that
plants use up rays of light and heat in growing; then either we
eat the plants, or animals eat the plants and we eat the animals;
and when we digest the food, that heat comes back in our bodies,
which the plants first took from the sunbeam. Breathe upon your
hand, and feel how hot your breath is; well, that heat which you
feel, was once in a sunbeam, and has travelled from it through
the food you have eaten, and has now been at work keeping up the
heat of your body.

But there is still another way in which these plants may give out
the heat-waves they have imprisoned. You will remember how we
learnt in the first lecture that coal is made of plants, and that
the heat they give out is the heat these plants once took in.
Think how much work is done by burning coals. Not only are our
houses warmed by coal fires and lighted by coal gas, but our
steam-engines and machinery work entirely by water which has been
turned into steam by the heat of coal and coke fire; and our
steamboats travel all over the world by means of the same power.
In the same way the oil of our lamps comes either from olives,
which grow on trees; or from coal and the remains of plants and
animals in the earth. Even our tallow candles are made of mutton
fat, and sheep eat grass; as so, turn which way we will, we find
that the light and heat on our earth, whether it comes from
fires, or candles, or lamps, or gas, and whether it moves
machinery, or drives a train, or propels a ship, is equally the
work of the invisible waves of ether coming from the sun, which
make what we call a sunbeam.

Lastly, there are still some hidden waves which we have not yet
mentioned, which are not useful to us either as light or heat,
and yet they are not idle.

Before I began this lecture, I put a piece of paper, which had
been dipped in nitrate of silver, under a piece of glass; and
between it and the glass I put a piece of lace. Look what the
sun has been doing while I have been speaking. It has been
breaking up the nitrate of silver on the paper and turning it
into a deep brown substance; only where the threads of the lace
were, and the sun could not touch the nitrate of silver, there
the paper has remained light-coloured, and by this means I have a
beautiful impression of the lace on the paper. I will now dip
the impression into water in which some hyposulphite of soda is
dissolved, and this will "fix" the picture, that is, prevent the
sun acting upon it any more; then the picture will remain
distinct, and I can pass it round to you all. Here, again,
invisible waves have been at work, and this time neither as light
nor as heat, but as chemical agents, and it is these waves which
give us all our beautiful photographs. In any toyshop you can
buy this prepared paper, and set the chemical waves at work to
make pictures. Only you must remember to fix it in the solution
afterwards, otherwise the chemical rays will go on working after
you have taken the lace away, and all the paper will become brown
and your picture will disappear.

And now, tell me, may we not honestly say, that the invisible
waves which make our sunbeams, are wonderful fairy messengers as
they travel eternally and unceasingly across space, never
resting, never tiring in doing the work of our world? Little as
we have been able to learn about them in one short hour, do they
not seem to you worth studying and worth thinking about, as we
look at the beautiful results of their work? The ancient Greeks
worshipped the sun, and condemned to death one of their greatest
philosophers, named Anaxagoras, because he denied that it was a
god. We can scarcely wonder at this when we see what the sun
does for our world; but we know that it is a huge globe made of
gases and fiery matter and not a god. We are grateful for the
sun instead of to him, and surely we shall look at him with new
interest, now that we can picture his tiny messengers, the
sunbeams, flitting over all space, falling upon our earth, giving
us light to see with, and beautiful colours to enjoy, warming the
air and the earth, making the refreshing rain, and, in a word,
filling the world with life and gladness.



Week 7

LECTURE III The Aerial Ocean in Which We Live

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


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