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 ':ome 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 therh 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 lookingglasses 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 lookingglass ; 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 Hght-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 w^ant the green waves of the sunbeam, but sends them back to you. In the same way the scarlet geranium rejects the red waves i 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.

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 heatrays 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 •hat 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 fires ; 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 ; and 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 h'ght 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

Fig. 9.

Piece of lace photographed during the lecture.

to fix it in the solution afterwards, otherwise the chemical rays will goon 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 0/

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 hig-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.

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 summer-

time 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.

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