Fig. 27.
LECTURE III.
Recall for a few minutes the facts I brought before you in my last lecture. The first point we discussed was the preparation of the tinder. I explained to you that tinder was nothing more than carbon in a finely-divided state. The second point was, that I had to strike the steel with the flint in such manner that a minute particle of the iron should be detached; the force used in knocking it off being sufficient to make the small particle of iron red-hot. This 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 of a solid or a liquid. I will endeavour to make this clear to you by an experiment. I have here, as you see, a wooden stool, and I am about to pour a little water on this stool. I place a glass beaker on the stool, the liquid water only intervening between the stool and the bottom of the glass. You see the glass is perfectly loose, and easily lifted off the stool notwithstanding the layer of water. I will now pour into the beaker a little of a very volatile liquid—i. e. a liquid that is easily converted into a gas—(bisulphide of carbon). I wish somewhat rapidly to effect the change of this liquid bisulphide of carbon into gaseous bisulphide of carbon, and in order to accomplish this object I must have heat. So I take this tube which, as you see, is connected with a pair of bellows, and simply blow on my bisulphide of carbon. This effects the change of the liquid into a gas with great rapidity. Just as I converted my solid sulphur into a gas by the heat of the tinder, so here I am converting this liquid bisulphide of carbon into a gas by the wind from my bellows. But my liquid bisulphide of carbon must get heat somewhere or another in order that the change of the liquid into a gas, that I desire should take place, may be effected; and so, seeing that the water that I have placed between the glass and the stool is the most convenient place from which the liquid can derive the necessary heat, it says, "I will take the heat out of the water." It does so, but in removing the heat from the water it changes the liquid water into solid ice. And see, already the beaker is frozen to the stool, so that I can actually lift up the stool by the beaker (Fig. 28). Understand then why my sulphur match wanted some time and some coaxing before it caught fire, viz. to change this solid sulphur into gaseous sulphur.
Fig. 28.
Fig. 29.