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?