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.