But now I want to carry you somewhat further in our story. It would not do for me simply to knock off a small piece of iron; I want when I knock it off that it should be red-hot. Stay for a moment and think of this--iron particles knocked off--iron particles made red-hot. All mechanical force generates heat.[A] You remember, in my last lecture, I rubbed together some pieces of wood, and they became sufficiently hot to fire phosphorus. On a cold day you rub your hands together to warm them, and the cabmen buffet themselves. It is the same story--mechanical force generating heat! The bather knows perfectly well that a rough sea is warmer than a smooth sea. Why?--because the mechanical dash of the waves has been converted into heat. Let me remind you of the familiar phrase, "striking a light," when I rub the match on the match-box. "Forgive me urging such simple facts by such simple illustrations and such simple experiments. The facts I am endeavouring to bring before you are illustrations of principles that determine the polity of the whole material universe." Friction produces heat. Here is a little toy (cracker) that you may have seen before (Fig. 23). It is scientific in its way. A small quantity of fulminating material is placed between two pieces of card on which a few fragments of sand have been sprinkled (Fig. 23 _a_). The two ends of the paper (_b b_) are pulled asunder. The friction produces heat, the heat fires the fulminate, and off it goes with a crack. And now put this question to yourselves, What produced the friction? Force. What is more, the amount of heat produced is the exact measure of the amount of force used. Heat is a form of force. I must urge you to realize precisely this energy of force. When you sharpen a knife you put oil upon the hone. Why?--When the carpenter saws a piece of wood he greases the saw. Why?--When you travel by train you see the railway-porter running up and down the platform with a box of yellow grease with which he greases the wheels. Why?--The answer to these questions is not far to seek--it is because you want your knife sharpened; it is because you want the saw to cut; it is because you want the train to travel. The carpenter finds sawing hard work, and he does not want the force of the muscles of his arm--his labour, in short--to be converted into heat, and so he greases the saw, knowing that the more completely he prevents friction, the more wood he will cut. It is the force of steam that makes the engine travel. Steam costs money. The engine-driver does not want that steam-force to be converted into heat, because every degree of heat produced means diminished speed of his train; and so the porter greases the wheels. But as you approach the station the train must be stopped. The steam is turned off, and the guard puts on what he calls "the brake." What is the brake? It is a piece of wood so constructed and placed that it can be made to press upon the wheel.