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.