Fig. 40.
I have told you that the light of a flame is due to solid matter in the flame;[B] further, that the amount of light is due to the amount of solid matter. And now I want to show you that the kind of light is due to the kind of solid matter in the flame. Here are some pieces of cotton wadding, which I am about to saturate with alcoholic solutions of different kinds of solid matter. For instance, I have in one bottle an alcoholic solution of a lithium salt, in another of a barium, in a third of a strontium, and so on. I will set fire to all these solutions, and you see how vastly different the colours are, the colour of the flames being dependent on the various forms of solid matter that I have introduced into them.
[B] I have not forgotten Frankland's experiments on this subject, but the lectures did not admit of dealing with exceptional cases.
Thus I have shown you that the heat of our flame is due to the clashing of the two gases, and the light of the flame to the solid matter in the flame, and the kind of light to the kind of solid matter.
Well, there is another point to which I desire to refer. Light is the paint which colours bodies. You know that ordinary white light is made up of a series of beautiful colours (the spectrum), which I show you here. If I take all these spectrum or rainbow colours which are painted on this glass I can, as you see, recompose them into white light by rotating the disc with sufficient rapidity that they may get mixed together on the little screen at the back of your eye. White light then is a mixture of a number of colours.
Just ask yourselves this question. Why is this piece of ribbon white? The white light falls upon it. White light is made up of all those colours you saw just now upon the screen. The light is reflected from this ribbon exactly as it fell upon the ribbon. The whole of those colours come off together, and that ribbon is white because the whole of the colours of the spectrum are reflected at the same moment. Why is that ribbon green? The white light falls upon the ribbon—the violet, the indigo, the red, the blue, the orange, and the yellow, are absorbed by the dye of the ribbon, and you do not see them. The ribbon, as it were, drinks in all these colours, but it cannot drink in the green. And reflecting the green of the spectrum, you see that ribbon green because the ribbon is incapable of absorbing the green of the white light. Why is this ribbon red? For the same reason. It can absorb the green which the previous piece of ribbon could not absorb, but it cannot absorb the red. The fact is, colour is not an inherent property of a body. If you ask me why that ribbon is green, and why this ribbon is red, the real answer is, that the red ribbon has absorbed every colour except the red, and the green ribbon every colour except the green, not because they are of themselves red and green but because they have the power of reflecting those colours from their surfaces.
This then is the consummated work of our tinder-box. Our tinder-box set fire to the match, and the match set fire to the candle, whilst the heat and the light of the candle are the finished work of the candle that the tinder-box lighted.
The clock warns me that I must bring to an end my story of a tinder-box. To be sure, the tinder-box is a thing of the past, but I hope its story has not been altogether without teaching. Let me assure you that the failure, if failure there be, is not the fault of the story, but of the story-teller. If some day, my young friends, you desire to be great philosophers—and such desire is a high and holy ambition—be content in the first instance to listen to the familiar stories told you by the commonest of common things. There is nothing, depend upon it, too little to learn from. In time you will rise to higher efforts of thought and intellectual activity, but you will be primed for those efforts by the grasp you have secured in your studies of every-day phenomena.
"Great things are made of little things,
And little things go lessening, till at last
Comes God behind them."