ch gives
me very little light. Here, you see, is an arrangement by which I can
shake a quantity of solid matter (lycopodium) into the non-luminous
alcohol flame. You will observe what a magnificently luminous flame I
produce (Fig. 40).

[Illustration: Fig. 39.]

[Illustration: 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 d