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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
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