[A] I need scarcely say, that whatever is of any value in the following remarks is derived from that charming book of Professor Tyndall's, Heat a Mode of Motion.
Heat then is generated by mechanical force; it is a mode of motion. There was an old theory that heat was material. There was heat, for instance, you were told, in this nail. Suppose I hammer it, it will get hot, and at the same time I shall reduce by hammering the bulk of the iron nail. A pint pot will not hold so much as a quart pot. The nail (you were told) cannot hold so much heat when it occupies a less bulk as it did when it occupied a larger bulk. Therefore if I reduce the bulk of the nail I squeeze out some of the heat. That was the old theory. One single experiment knocked it on the head. It was certain, that in water there is a great deal more entrapped heat—"latent heat" it was called—than there is in ice. If you take two pieces of ice and rub them together, you will find the ice melts—the solid ice changes (that is to say) into liquid water. Where did the heat come from to melt the ice? You could not get the heat from the ice, because it was not there, there being admittedly more latent heat in the water than in the ice. The explanation is certain—the heat was the result of the friction. And now let me go to my hammer and nail. I wish to see whether I can make this nail hot by hammering. It is quite cold at the present time. I hope to make the nail hot enough by hammering it to fire that piece of phosphorus (Fig. 24). One or two sharp blows with the hammer suffice, and as you see the thing is done—I have fired the phosphorus. But follow the precise details of the experiment. It was I who gave motion to the hammer. I brought that hammer on to that nail. Where did the motion go to that I gave the hammer? It went into the nail, and it is that very motion that made the nail hot, and it was that heat which lighted the phosphorus. It was I who fired the phosphorus: do not be mistaken, I fired the phosphorus. It was my arm that gave motion to the hammer. It was my force that was communicated to the hammer. It was I who made the hammer give the motion to the nail. It was I myself that fired the phosphorus.
Fig. 24.
I want you then to realize this great fact, that when I hold the steel and strike it with the flint, and get sparks, I first of all knock off a minute fragment of iron by the blow that I impart to it, whilst the force I use in striking the blow actually renders the little piece of detached iron red-hot. What a wonderful thought this is! Look at the sun, the great centre of heat! It looks as if it were a blazing ball of fire in the heavens. Where does the heat of the sun come from? It seems bold to suggest that the heat is produced by the impact of meteorites on the sun. Just as I, for instance, take a hammer and heat the nail by the dash of the hammer on it, so the dash of these meteorites on the sun are supposed to produce the heat so essential to our life and comfort.
Fig. 25.
Let us take another step forward in the story of our tinder-box. Having produced a red-hot spark and set fire to my tinder, I want you to see what I do next. I set to work to blow upon my lighted tinder. You remember, by the bye, that Latin motto of our school-books—alĕre flammam, nourish the flame. When I blow on the tinder my object is to nourish the flame. Here is a pair of common kitchen bellows (Fig. 25); when the fire is low the cook blows the fire to make it burn up. What is the object of this blowing operation? It is to supply a larger quantity of atmospheric oxygen to the almost lifeless fire than it would otherwise obtain. Oxygen is the spark's nourishment and life, and the more it gets the better it thrives. Oxygen is an extremely active agent in nourishing flame. If, for instance, I take a little piece of carbon and merely set fire to one small corner of it, and then introduce it into this jar of oxygen, see how brilliantly it burns; you notice how rapidly the carbon is becoming consumed (Fig. 26). In the tinder-box I blow on the tinder to supply a larger amount of oxygen to my spark. A thing to burn under ordinary conditions must have oxygen, and the more oxygen it gets the better it burns. It does not follow that the supply of oxygen to a burning body must necessarily come directly from the air. Here, for instance, I have a squib. I will fire it and put it under water (Fig. 27). You see it goes on burning whether it is in the water or out of it, because one of the materials of which the squib is composed supplies the oxygen. The oxygen is actually locked up inside the squib. When then I blow upon my tinder, my object is to supply more oxygen to it than it would get under ordinary conditions. And, as you see, the more I blow, within certain limits, the more the spark spreads, until now the whole of my tinder has become red-hot. But my time is gone, and we must leave the rest of our story for the next lecture.
Fig. 26.