is spark falling upon the
tinder set fire to it. The next stage of the operation was to blow upon
the tinder, in order, as I said, to nourish the flame; in other words,
to promote combustion by an increased supply of oxygen, just as we use
an ordinary pair of bellows for the purpose of fanning a fire which has
nearly gone out into a blaze.

And now comes the next point in my story of a tinder-box. Having ignited
the tinder I want to set fire to the match. Now I have here some of the
old tinder-box matches, and you will see that they are simply wooden
splints with a little sulphur at the end. Why (you say) use sulphur? For
this reason--the wood is not combustible enough to be fired by the
red-hot tinder. We put therefore upon the wood a substance which is more
combustible than the wood. This sulphur--which most people call
brimstone--has been known from very early times. In the middle ages it
was regarded as the "principle of fire." It is referred to by Moses and
Homer and Pliny. A very distinguished chemist, Geber, describes it as
one of "the principles of nature." Having fired my tinder, as you see,
and blown upon it, I place my sulphur match in contact with the red-hot
tinder. And now I want you to notice that the sulphur match does not
catch fire immediately. It wants, in fact, a little time, and as you see
a little coaxing. Now I have got it alight. But note, it is the sulphur
that at the present moment is burning. The burning sulphur is now
beginning to set fire to the wood. The whole match is well alight now!
But it was the sulphur that caught fire first, and it was the sulphur
that set fire to the wood. A little time was occupied, we said, in
making the sulphur catch fire. Ask yourselves this question--Why was it
that the sulphur took a little time to catch fire? This was the
reason--because before the sulphur could catch fire it was necessary to
change the _solid_ sulphur (the condition in which it was upon the match
end) into _gaseous_ sulphur. The solid sulphur could not catch fire.
Therefore the heat of my tinder during the interval that I was coaxing
the match (as I called it) was being exerted in converting my solid into
gaseous sulphur. When the solid sulphur had had sufficient heat applied
to it to vapourize it, the sulphur gas immediately caught fire. Now
understand, that in order to convert a solid into a liquid, or a liquid
into a gas, heat is always a necessity. I must have heat to produce a
gas out o