The Story of a Tinder-box

by Charles Meymott Tidy

And here I wish to make a slight digression in my story. I will show you an experiment preparatory to bringing before you the fact I am anxious now to make clear. I have before me a tube, one half of which is brass and the other half wood. I have covered the tube, as you see, with a tightly-fitting piece of white paper. The whole tube, wood and brass, has been treated in exactly the same manner. Now I will set fire to some spirit in the trough I have here, and expose the entire tube to the action of the flame. Notice this very curious result, viz. that the paper covering the brass portion of the tube does not catch fire, whereas the paper covering the wood is rapidly consumed (Fig. 31). You see the exact line that divides wood from brass by the burning of the paper. Well, why is that? Now all of you know that some things conduct heat (i. e. carry away heat) better than other substances. For instance, if you were to put a copper rod and a glass rod into the fire, allowing a part of each to project, the copper rod that projects out of the fire would soon become so very hot that you dare not touch it, owing to the copper conducting the heat from the fire, whereas you would be able to take hold of the projecting end of the glass rod long after the end of the glass exposed to the fire had melted. The fact is, the copper carries heat well, and the glass carries heat badly. Now with the teaching of that experiment before you, you will understand, I hope, the exact object of one or two experiments I am about to show you. Here is a piece of coarse wire gauze—I am about to place it over the flame of this Argand burner. You will notice that it lowers the flame for a moment, but almost immediately the flame dashes through the gauze (Fig. 32 A). Here is another piece of gauze, not quite so coarse as the last. I place this over the flame, and for a moment the flame cannot get through it. There, you see it is through now, but it did not pass with the same readiness that it did in the case of the other piece of gauze, which was coarser. Now, when I take a piece of fine gauze, the flame does not pass through at all until the gauze is nearly red-hot. There is plenty of gas passing all the time. If I take a still finer gauze, I shall find that the flame won't pass even when it is almost red-hot (Fig. 32 B). Plenty of gas is passing through, remember, all the time, but the flame does not pass through. Now why is it that the flame is unable to pass? The reason is this—because the metal gauze has so cooled the flame that the heat on one side is not sufficient to set fire to the gas on the other side. I must have, you see, a certain temperature to fire my gas. When therefore I experiment with a very fine piece of gauze, where I have a good deal of metal and a large conducting surface, there is no possibility of the flame passing. In fact, I have so cooled the flame by the metal gauze that it is no longer hot enough to set fire to the gas on the opposite side. I will give you one or two more illustrations of the same fact. Suppose I put upon this gauze a piece of camphor (camphor being a substance that gives off a heavy combustible vapour when heated), and then heat it, you see the camphor gas burning on the under side of the gauze, but the camphor gas on the upper side is not fired (Fig. 33). Plenty of camphor gas is being given off, but the flame of the burning camphor on the under side is not high enough to set fire to the camphor gas on the upper side, owing to the conducting power of the metal between the flame and the upper gas. There is one other experiment I should like to show you. Upon this piece of metal gauze I have piled up a small heap of gunpowder. I will place a spirit-lamp underneath the gunpowder, as you see I am now doing, and I don't suppose the gunpowder will catch fire. I see the sulphur of the gunpowder at the present moment volatilizing, but the flame, cooled by the action of the metal, is not hot enough to set fire to the gunpowder.

Fig. 33.

Fig. 34.

I showed you the steel and flint lamp—if I may call it a lamp—used by coal-miners at the time of Davy (Fig. 22). Davy set to work to invent a more satisfactory lamp than that, and the result of his experiments was the beautiful miner's lamp which I have here (Fig. 34). I regard this lamp with considerable affection, because I have been down many a coal-mine with it. This is the coal-miner's safety-lamp. The old-fashioned form of it that I have here has been much improved, but it illustrates the principle as well as, if not better than, more elaborate varieties. It is simply an oil flame covered with a gauze shade, exactly like that gauze with which I have been experimenting. I will allow a jet of coal gas to play upon this lamp, but the gas, as you see, does not catch fire. You will notice the oil flame in the lamp elongates in a curious manner. The flame of the lamp cooled by the gauze is not hot enough to set fire to the coal gas, but the appearance of the flame warns the miner, and tells him when there is danger. And that is the explanation of the beautiful miner's safety-lamp invented by Sir Humphry Davy.

Now let me once more put this fact clearly before you, that whether it is the gas flame or our farthing rushlight, whether it is our lamp or our lucifer match, if we have a flame we must have a gas to burn, and having a gas, we must heat it to, and maintain it at, a certain temperature. We have now reached a point where our tinder-box has presented us with flame. A flame is indeed the consummated work of the tinder-box.

Fig. 35.