k of the
tinder-box.

[Illustration: Fig. 35.]

[Illustration: Fig. 36.]

Just let me say a few words about the grand result--the consummated work
of the tinder-box. A flame is a very remarkable thing. It looks solid,
but it is not solid. You will find that the inside of a flame consists
of unburnt gas--gas, that is to say, not in a state of combustion at
all. The only spot where true combustion takes place is the outer
covering of the flame. I will try to show you some experiments
illustrating this. I will take a large flame for this purpose. Here is a
piece of glass tube which I have covered with ordinary white paper.
Holding the covered glass tube in our large flame for a minute or two,
you observe I get two rings of charred paper, corresponding to the outer
envelope of the flame, whilst that portion of the paper between the
black rings has not even been scorched, showing you that it is only the
outer part of the flame that is burning (Fig. 35). The heat of the flame
is at that part where, as I said before, the combustible gases come
into contact--into collision with the atmosphere. So completely is this
true, that if I take a tube, such as I have here, I can easily convey
the unburnt gas in the centre of the flame away from the flame, and set
fire to it, as you see, at the end of the glass tube a long distance
from the flame (Fig. 36). I will place in the centre of my flame some
phosphorus which is at the present moment in a state of active burning,
and observe how instantly the combustion of the phosphorus ceases so
soon as it gets into the centre of the flame. The crucible which
contains it is cooled down immediately, and presents an entirely
different appearance within the flame to what it did outside the flame.
It is a curious way, perhaps you think, to stop a substance burning by
putting it into a flame. Indeed I can put a heap of gunpowder inside a
flame so that the outer envelope of burning gas does not ignite it (Fig.
37). There you see a heap of gunpowder in the centre of our large flame.
The flame is so completely hollow that even it cannot explode the
powder.

[Illustration: Fig. 37.]

[Illustration: Fig. 38.]

I want you, if you will, to go a step further The heat of the flame is
due, as I explained in my last lecture, to the clashing of molecules.
But what is the light of my candle and gas due to? The light is due to
the solid matter in the flame, brought to a state of white heat or
inc